Can endemic diseases be acute?

Diseases can be classified as :endemic ,or pandemic,… etc. An endemic disease is an infectious disease which is generally or constantly found among people in a particular area. Disease conditions are :

  • acute: rapid onset , severe symptoms and brief duration.
  • chronic:gradual onset and long duration

Can an endemic disease be acute? Or is it always chronic ??

Endemic and chronic/acute are unrelated properties of a disease. Endemic is a description of the frequency of a disease in a population or in an area. Chronic/Acute is a description of the state or course of a disease in an individual.

For example, influenza is endemic in North America: the frequency goes up and down, there's always a few people who have it, but most people don't. Periodically the influenza virus mutates, and becomes much more successful at infecting people. The frequency of the disease goes way up, and influenza becomes epidemic. However, regardless of whether the disease is endemic or epidemic, the disease processes is usually acute: it comes on suddenly, it gets worse for a few days, and then it get better (or you die).

Malaria in endemic in much of Africa and SE Asia (it's always around), but the disease can be either acute (you have fever, chills, prostration), or chronic (the parasite lives in your body, and you can be a source of the disease, you don't necessarily have obvious symptoms, but may be suffering long term damage.

Difference Between Endemic and Pandemic

Endemic is the spread of a disease or an infectious agent within a geographic area that is existing perpetually while pandemic is an advanced form of epidemic and it involves the spread of infection or disease at a global level. Pandemic outbreak affects most of the world.

However, there are some major differences between the two that have been discussed below.

Hepatitis E

Incidence and Prevalence and Worldwide Disease Patterns

Worldwide, two geographic patterns can be differentiated: (1) endemic regions or areas of high HEV prevalence, in which major outbreaks and a substantial number of sporadic cases occur and (2) non–endemic regions, in which HEV accounts for a few cases of acute viral hepatitis, mainly among travelers to endemic regions ( Fig. 33-5 ).

HEV in Endemic Regions

In such regions epidemics of hepatitis E occur frequently. These epidemics are usually separated by a few years. Such outbreaks have been observed in the Indian subcontinent, China, Southeast and Central Asia, the Middle East, and the northern and western parts of Africa. In North America (Mexico), two small outbreaks were reported during 1986 to 1987, but no further outbreaks have since been reported. The outbreaks are often large, and affect several hundred to several thousand persons. 1 Most reported outbreaks have been related to consumption of fecally contaminated drinking water. Their time course varies from unimodal outbreaks, which last a few weeks, to prolonged, multipeaked epidemics, with a duration of more than 1 year. The latter represent continuing water contamination. The outbreaks frequently follow heavy rainfall and floods, which create conditions that favor mixing of human excreta with sources of drinking water. Some outbreaks have occurred in hot and dry summer months, possibly a result of diminished water flow in rivers leading to an increased concentration of fecal contaminants. In Southeast Asia, recurrent epidemics have been shown to be associated with disposal of human excreta into rivers and subsequent use of water from the same river for drinking, cooking, and personal hygiene. 57 These practices provide conditions that allow continuous fecal contamination of water. Outbreaks of hepatitis E have occurred in underdeveloped urban areas with leaky water pipes passing through soil that is contaminated with sewage. Intermittent water supply in these areas leads to a negative pressure in pipes during periods of no flow, allowing inward suction of contaminants. 58 Although the dissemination of HEV infection through contamination of food may be possible, few outbreaks related to food-borne transmission have been reported from disease-endemic areas. This may be due to a relatively long incubation period, which makes it difficult to establish a relationship between consumption of a particular food and occurrence of disease. 59

Overall attack rates during hepatitis E outbreaks have ranged from 1% to 15%. The rates are highest among young adults (3% to 30%) the reason for this is not clear. Lower attack rates among children are related probably to a higher proportion of asymptomatic infections than to rarity of infection. Males outnumber females in most outbreaks this may be due either to their greater risk of exposure to HEV infection or to a greater propensity for clinical disease once infected. 59 Hepatitis E outbreaks have characteristically been associated with a high attack rate and mortality among pregnant women. The rate of development of acute liver failure among those with symptomatic hepatitis E was also higher among pregnant women. Once acute liver failure appears, the death rate may be no different among pregnant women with hepatitis E than in those with other causes of severe liver injury (see Pregnant Women later). Because HEV infection during pregnancy is associated with a high attack rate and risk of severe disease, pregnant women are overrepresented among case series of women with liver failure in the endemic areas, and also among fatal cases during hepatitis E outbreaks. 59

In disease-endemic areas, HEV infection accounts for a large proportion of acute sporadic hepatitis in all age groups. In India HEV infection is the most common cause of acute sporadic hepatitis and accounts for up to 70% of such cases among adults. 59 In these regions, patients with sporadic hepatitis E closely resemble those of epidemic hepatitis E in age distribution, severity and duration of illness, propensity for worse prognosis among pregnant women, and absence of chronic sequelae. The route of acquisition of infection in most patients with sporadic hepatitis E is unclear. However, given the high potential for fecal contamination of water and food in these areas, these sources are most likely. HEV genomic sequences could be isolated from nearly 40% of sewage specimens obtained through all seasons from a large drain in a large Indian city. 60 This indicates ubiquitous circulation of HEV in the population, even in the absence of a disease outbreak.

Unlike several other enterically transmitted infections, person-to-person transmission of HEV from either epidemic or sporadic cases is distinctly uncommon. 61 The exact reason for this is unknown. During outbreaks, secondary attack rates among household contacts of patients with hepatitis E have been as low as 0.7% to 2.2%. This is much lower than the 50% to 75% infection rate among susceptible household contacts of hepatitis A cases. Even with multiple cases in one family, the time interval between cases is short, indicating a shared primary water-borne infection rather than person-to-person transmission. 59

Maternal–fetal transmission of HEV infection has been reported (see Modes of Transmission , later). In regions endemic for hepatitis E, the presence of HEV viremia among healthy blood donors and transmission of this infection to transfusion recipients have been documented. However, the contribution of such transmission to the overall disease burden remains unclear.

Reservoirs of HEV in Endemic Regions

In disease-endemic areas, the reservoir of HEV responsible for maintaining the disease in a population has not been clearly determined. Protracted viremia and prolonged fecal shedding of HEV have been suggested. However, viral shedding in feces lasts for a short period, making this possibility unlikely. Another potential reservoir of the virus may be a continuously circulating pool of individuals with subclinical HEV infection. In an experimental cynomolgus macaque model, HEV-infected animals that lacked biochemical evidence of liver injury were found to excrete large amounts of viable and infectious HEV. 62 Similar fecal shedding of the virus by persons with subclinical HEV infection could lead to continuous maintenance of a source of infection in a disease-endemic area, somewhat similar to the situation that existed with poliovirus in areas where it was endemic. This pool of infection could, in turn, lead to periodic contamination of drinking water supplies.

The issue regarding the existence of an animal reservoir in disease-endemic regions remains unresolved. The zoonotic hypothesis for transmission of HEV is based primarily on the following factors: (1) the high prevalence of anti-HEV antibodies in several animal species (2) the isolation of HEV genomic sequences from pigs and (3) the genomic sequence homology between human and animal HEV isolates. However, most of the supporting genomic data have been from non–endemic regions. In contrast, data from endemic regions are conflicting. Isolates from animals and sporadic human cases have belonged to the same genotype (genotype 4) in China and Vietnam. In India, however, animal isolates have all been members of genotype 4 and human isolates genotype 1. Genotype 1 HEV, which is responsible for the large majority of cases in all endemic countries, has never been isolated from pigs. Also, in experimental studies, genotype 1 virus is unable to infect pigs. Thus zoonotic transmission may not be a major mode of distribution in these areas, in particular for the widely prevalent genotype 1 HEV. 59

Thus it appears that in the regions where hepatitis E is endemic, the infection is acquired from either an environmental or a human reservoir through poor general sanitation, contaminated drinking water supplies, and lack of attention to personal hygiene. Further data are necessary before zoonotic transmission of HEV can be implicated in these regions.

HEV in Non-Endemic Regions

In non-endemic regions, where outbreaks have not been reported, the disease accounts for only a minority of reported cases of acute viral hepatitis. Until a few years ago, most such cases were found to be related to travel to disease-endemic areas. However, in recent years isolated cases or small series of cases related to autochthonous transmission of hepatitis E in these regions have been recorded in the United States, Europe (including the United Kingdom, France, the Netherlands, Austria, Spain, and Greece), and developed countries of Asia-Pacific (Japan, Taiwan, Hong Kong, Australia). In a series of 40 cases with hepatitis E identified in the United Kingdom, the disease showed seasonal variations with peaks in spring and summer and no cases during November and December. 63 In the United Kingdom, the disease appeared to be more common among residents of coastal and estuarine areas. 64 The mode of transmission in most of these cases could not be identified, although zoonotic distribution has been proposed (see HEV Infection as a Zoonosis , later). Table 33-1 shows epidemiologic differences in hepatitis E in disease-endemic and non-endemic regions.

Endemic STDs

In certain areas of the world, there are several STDs that could be considered endemic.   For example, HIV is considered to be endemic in many parts of Africa. At this point, and for the conceivable future, eradication is unlikely.   That's true even as improving treatment is starting to bring the HIV epidemic under control.

Hepatitis B (HBV) is also endemic throughout the world.   Although HBV is not always a sexually transmitted disease, it can be sexually transmitted. Fortunately, there is a vaccine available to prevent hepatitis B transmission. If a person is planning on traveling to an area where hepatitis is endemic, vaccination is usually recommended. Vaccination against hepatitis is also part of the standard childhood immunization schedule in the United States.  

Epidemic, Endemic, Pandemic: What are the Differences?

The novel coronavirus pandemic is the perfect model for understanding what exactly a pandemic is and how it impacts life on a global scale. Since the emergence of COVID-19 in 2020, the public has been bombarded with new language to understand the virus and the subsequent global public health response. This article will uncover the factors that make a pandemic and how it differs from epidemics and endemics.

What is an Epidemic?

The Centers for Disease Control and Prevention (CDC) describes an epidemic as an unexpected increase in the number of disease cases in a specific geographical area. Yellow fever, smallpox, measles, and polio are prime examples of epidemics that occurred throughout American history.

Notably, an epidemic disease doesn't necessarily have to be contagious. For example, West Nile fever and the rapid increase in obesity rates are also considered epidemics.

In broader terms, epidemics can refer to a disease or other specific health-related behavior (e.g., smoking) with rates that are clearly above the expected occurrence in a community or region.

What is a Pandemic?

The World Health Organization (WHO) declares a pandemic when a disease’s growth is exponential. This means growth rate skyrockets, and each day cases grow more than the day prior.

In being declared a pandemic, the virus has nothing to do with virology, population immunity, or disease severity. It means a virus covers a wide area, affecting several countries and populations.

What is an Endemic?

An endemic is a disease outbreak that is consistently present but limited to a particular region. This makes the disease spread and rates predictable.

Malaria, for example, is considered an endemic in certain countries and regions.

What are the Differences Between Pandemics and Epidemics?

The WHO defines pandemics, epidemics, and endemics based on a disease's rate of spread. Thus, the difference between an epidemic and a pandemic isn't in the severity of the disease, but the degree to which it has spread.

A pandemic cuts across international boundaries, as opposed to regional epidemics. This wide geographical reach is what makes pandemics lead to large-scale social disruption, economic loss, and general hardship.

It's important to note that a once-declared epidemic can progress into pandemic status. While an epidemic is large, it is also generally contained or expected in its spread, while a pandemic is international and out of control.

Causes of Disease Outbreaks

Several factors contribute to the outbreak of infectious diseases. Contraction can occur as a result of transmission from people, animals, or even the environment. For example:

    For example, whooping cough occurs in spring, whereas measles tend to appear in the winter season. For example, Minamata is a disease contracted after exposure to mercury.
  • The social aftermath of disasters such as storms, earthquakes, and droughts can lead to high disease transmission.
  • A number of environmental factors such as water supply, food, air quality, and sanitation facilities can catalyze the spread of infectious diseases.

Disease origins can also be unknown. These kinds of diseases could be caused by a variety of factors, including:

  • A new or newly modified pathogen
  • Natural toxins
  • Undetected chemical releases
  • Unknown ionizing radiation over-exposure

The field of epidemiology works to trace these unidentified outbreaks to the source in an effort to protect public health and safety.

Notable Past Pandemics

The current COVID-19 outbreak is not the only disease to have impacted the world on a global scale. Here are just a few examples of past pandemics that have shaped the evolution of outbreaks and human immunity.

The Black Death (1346 - 1353): The Black Death caused an estimated death of 25 million people across the world in the 14th century. According to scientists, the outbreak was caused by a bacteria called Yersinia pestis. The virus lasted for about four years.

American Plagues (16th Century): A cluster of Eurasian diseases brought to the Americas by European explorers, smallpox was one of the chief illnesses of the American Plagues, which contributed to the collapse of the Inca and Aztec civilizations. Some estimates suggest that 90% of the indigenous population in the Western Hemisphere was killed off as a result.

The Flu Pandemic (1889 - 1890): New transportation routes made possible in the industrial age made it easier for influenza viruses to spread widely in the United States and beyond. In the span of months, influenza traveled around the globe, with the earliest cases reported in Russia. The virus spread rapidly throughout St. Petersburg before quickly making its way through Europe and the rest of the world, despite the fact that air travel didn't exist yet, leaving 1 million people in its wake.

Spanish Flu (1918 - 1920): Another massive disease outbreak was the influenza pandemic, popularly called Spanish flu. This viral pandemic began in 1918, immediately following World War 1. Over 50 million deaths were recorded during this outbreak, with the disease lasting only two years.

The Asian Flu (1957 - 1958): The Asian Flu pandemic, which was a blend of avian flu viruses, began in China and eventually claimed more than 1 million lives. The CDC notes that the rapidly-spreading disease was reported in Singapore in February 1957, Hong Kong in April 1957, and the coastal cities of the United States in the summer of 1957. The total death toll was more than 1.1 million worldwide, with 116,000 deaths occurring in the United States.

AIDS Pandemic and Epidemic (1981 - Present): Since it was first identified, AIDS has claimed an estimated 35 million lives. Scientists believe that HIV, the virus that causes AIDS, is likely to have evolved from a virus found in chimpanzees that was transferred to humans in West Africa in the 1920s. By the late 20th century, the virus had made its way around the world. For decades, the disease had no known cure, but medication developed in the 1990s now allows people with the disease to experience a normal life span with regular treatment.

Explore More: Columbia Public Health faculty have led on every aspect of the global response to HIV, from research into mother-to-child transmission to strengthening treatment and care systems to the history of stigma, advocacy, and coalition-building.

Diseases – Communicable diseases, Endemic diseases

The most common diseases which are endemic in India are as follows:

Communicable Disease- Malaria:

  • Malaria is a very common disease in developing countries. The word malaria is derived from the word ‘mal-aria meaning bad air.
  • Ronald Ross first discovered the transmission of malaria by mosquitoes, while he was working in India (Secunderabad, AP) in 1897. Malaria is one of the most widespread diseases in the world.
  • Each year, there are 300 to 500 million clinical cases of malaria, 90 percent of them in Africa alone.
  • Among all infectious diseases, malaria continues to be one of the biggest contributors to disease burden in terms of deaths and suffering.
  • Malaria kills more than one million children a year in the developing world, accounting for about half of malaria deaths globally.
  • The risk of getting malaria extends to almost the entire population in India (almost 95 percent).
  • The following states that have the highest number of malaria cases are Madhya Pradesh, Maharashtra, Orissa, Karnataka, Rajasthan, Assam, Gujarat and Andhra Pradesh.

Communicable Disease- Typhoid:

  • Typhoid fever is an acute, systemic infection presenting as fever with abdominal symp­toms, caused by Salmonella typhi and paratyphi.
  • Before nineteenth century, typhus and typhoid fever were considered to be the same. Enteric fever is an alternative name for typhoid. Salmonella typhi and paratyphi colonise only humans.
  • The organisms are acquired via ingestion of food or water, contaminated with human excreta from infected persons.
  • Direct person-to-person transmission is rare. Typhoid is a global health problem. It is seen in children older than the age of one.
  • Outbreak of typhoid in developing countries results in high mortality. The recent development of antibiotic resistant organisms is causing much concern.
  • Typhoid fever is more common in the tropics. It tends to occur in places, where the sanitation standards are poor. A bacterial organism called salmonella typhi causes typhoid fever.
  • Salmonella paratyphi can also cause fever and abdominal symptoms. The disease caused by both these entities is called enteric fever.
  • The disease presents with a typical, continuous fever for about three to four weeks, relative bradycardia with abdomi­nal pain (due to enlargement of lymph nodes in the abdomen), and constipation.
  • Geographical Distribution Worldwide, typhoid fever affects about six million people with more than 6, 00,000 deaths a year.
  • Almost 80 percent of cases and deaths occur in Asia, and most others in Africa and Latin America. Among Asian countries, India prob­ably has a large number of these cases.
  • Indian Statistics Typhoid fever is endemic in India.
  • Health surveys conducted by the Central Ministry of Health in the community development areas indicated a morbidity rate varying from 102 to 2,219 per 1, 00,000 population in different parts of the country.
  • A limited study in an urban slum showed 1 percent of children up to 17 years of age suf­fer from typhoid fever every year.
  • Carriers of Typhoid Fever Typhoid infection is mainly acquired from persons who are carriers of the disease.
  • Carriers are the people who continue to excrete salmonella through their urine and feces a year after an attack of typhoid. A chronic carrier state develops in about 2 to 5 percent of the cases.
  • The organisms in such cases make the gall bladder their habitat.

Communicable Disease- Hepatitis:

  • Hepatitis is the inflammation of liver. It can be caused by viruses (five different viruses— termed A, B, C, D and E cause viral Hepatitis), bacterial infections, or continuous expo­sure to alcohol, drugs, or toxic chemicals, such as those found in aerosol sprays and paint thinners, or as a result, of an auto-immune disorder.
  • Hepatitis results in either damage or reduction in the livers ability to perform life-preserving functions, including filtering harmful, infectious agents from blood, storing blood sugar and converting it into usable energy forms, and producing many proteins necessary for life.
  • Symptoms seen in Hepatitis differ according to the cause and the overall health of the infected individual. However, at times, the symptoms can be very mild.
  • The com­monly seen clinical features are general weakness and fatigue, loss of appetite, nausea, fever, abdominal pain and tenderness.
  • The main feature is the presence of jaundice (yel­lowing of skin and eyes that occurs when the liver fails to break-down excess yellow- coloured bile pigments in the blood).
  • Depending on the progress and intensity, Hepatitis can be categorized as acute or chronic. In acute Hepatitis, clinical features often subside without treatment within a few weeks or months. However, about 5 percent of the cases go on to develop into chronic Hepatitis, which may last for years. Chronic Hepatitis slowly leads to progressive liver damage and cirrhosis.
  • Hepatitis A is a self-limiting disease that is found all across the world.
  • It is usually transmitted through oral ingestion of infected material (mainly water), but sometimes transmitted parenterally most cases resemble the symptoms of a mild flu attack and jaundice is mild too.
  • Hepatitis B is an acute vital disease. It primarily spreads parenterally, but sometimes orally as well.
  • However, the main mode of spread is intimate contact and from mother to the new born.
  • Fever, anorexia, nausea, vomiting are the initial symp­toms, and they soon lead to severe jaundice, urticarial skin lesions, arthritis, etc.
  • Some patients become carriers or even remain chronically ill, even though most patients recover in about three to four months.
  • Hepatitis C is a viral disease commonly occurring after transfusion or par­enteral drug abuse.
  • It frequently progresses to a chronic form that is usually asympto­matic, but may involve liver cirrhosis.
  • Hepatitis D or Delta Hepatitis is caused by the Hepatitis D virus.
  • It usually occurs simultaneously with or as a super infection in case of Hepatitis B, thus increasing its severity.
  • Hepatitis E is transmitted by the oral fecal route usually by contaminated water.
  • Chronic infection does not occur but acute infection may be fatal in pregnant women.

Communicable Disease- Jaundice:

  • Jaundice, also known as icterus, is a condition, which is characterized by yellowish dis­colouration of the skin and whites of eyes. It is a symptom or clinical sign, not a disease by itself.
  • The yellow colouration is caused by an excess amount of bile pigment known as bilirubin in the body. Normally, bilirubin is formed by the breakdown of haemoglobin during the destruction of worn-out red blood cells.

Communicable Disease- Leptospirosis:

  • Leptospirosis is a disease caused by a type of bacteria and is associated with animals. It is more common in the tropical countries.
  • The disease is also known as canefield fever cani- cola fever, field-fever, mud fever, seven day fever and swineherd disease. Leptospirosis is caused by different strains of bacteria of the genus Leptospira.
  • Of all the varieties that cause disease, Leptospira icterohaemorrhagiae is the most serious type.
  • If not treated properly, it could lead to serious complications. Leptospirosis is a disease of animals that can spread to humans.
  • Rats are the most common carriers. Soil contaminated with urine of infected animals can also transmit the disease to persons exposed to cattle urine, rat urine or to foetal fluids from cattle.
  • Sewage workers, agricultural workers, butchers, meat inspectors, workers in contact with contaminated waters and veterinarians are generally at risk.
  • Person to person transmission is not possible. Leptospirosis can spread due to con­tact with urine, blood or tissues from infected persons. The organisms enter the body through the breaks in the skin or through mucous membranes.
  • The organisms can also be acquired by drinking contaminated water. Infection is commonly acquired by bath­ing in contaminated water.
  • The organisms multiply in the blood and tissues of the body. Though the organism can affect any organ of the body, the kidney and liver are com­monly involved. The incubation period is usually 10 days. It may vary from 2 to 20 days.

Communicable Disease- Diarrhoeal Diseases:

  • The term gastroenteritis’ is most frequently used to describe acute diarrhoea. Diarrhoea is defined as the passage of loose, liquid or watery stools.
  • These liquid stools are usually passed more than three times a day. The attack usually lasts for about 3 to 7 days, but may also last up to 10 to 14 days.
  • Diarrhoea is a major public health problem in developing countries. Diarrhoeal dis­eases cause a heavy economic burden on health services.
  • About 15 percent of all pediatric beds in India are occupied by admissions due to gastroenteritis.
  • In India, diarrhoeal dis­eases are a major public health problem among children under the age of 5 years. In health institutions, up to a third of total pediatric admissions are due to diarrhoeal diseases.
  • Diarrhoea related diseases are a significant cause of mortality in children less than five years of age. Incidence is highest in the age group of 6 to 11 months.
  • The National Diarrhoeal Disease Control Programme has made a significant contribution in averting deaths among children less than five years of age.

Communicable Disease- Amoebiasis:

  • Amoebiasis is an infection caused by a parasite ‘Entamoeba Histolytica. The intestinal disease varies from mild abdominal discomfort and diarrhoea to acute fulminating dys­entery.
  • Extra intestinal amoebiasis includes involvement of the liver (liver abseess), lungs, brain, spleen, skin, etc.
  • Amoebiasis is a common infection of the human gastroin­testinal tract. It has a worldwide distribution.
  • It is a major health problem in the whole of China south-east and west Asia and Latin America, especially Mexico. It is generally agreed that amoebiasis affects about 15 percent of the Indian popula­tion. Amoebiasis has been reported throughout India.

Communicable Disease- Cholera:

  • Cholera is an acute diarrhoeal disease caused by V. Cholera (classical or El T). It is now commonly due to the El T or biotype.
  • The majority of infections are mild or symptomatic. Epidemics of cholera are characteristically abrupt and often create an acute public health problem.
  • They have a high potential to spread fast and cause deaths. The epidemic reaches a peak and subsides gradually as the ‘force of infection declines.
  • Often, when time control measures are instituted, the epidemic has already reached its peak and is waning.

Communicable Disease- Brucellosis:

  • Brucellosis is one of the major bacterial zoonoses, and in humans is also known as undulent fever, Malta fever or Mediterranean fever.
  • It is occasionally transmitted to humans by direct or indirect contact with infected animals.
  • The disease may last for several days, months or occasionally, even years.
  • Brucellosis is both a severe human disease and a disease of animals with serious economic consequences. Brucellosis is a recognized pub­lic health hazard that is found the world over.
  • It is endemic wherever cattle, pigs, goats and sheep are raised in large numbers. The important endemic areas for Brucellosis exist in Mediterranean zones, Europe, Central Asia, Mexico and South America. Animal Brucellosis has been reported from practically every state in India.
  • However, no statisti­cal information is available about the extent of infection in humans in various parts of the country.
  • The prevalence of human Brucellosis is difficult to estimate. Many cases remain undiagnosed either because they are not apparent, or because physicians in many countries are unfamiliar with the disease.

Communicable Disease- Hookworm Infection:

  • Hookworm infection is defined as: ‘any infection caused by Ancylostoma or Necator’.
  • They may occur as single or mixed infections in the same person through various fac­tors, which have to be prevented. Hookworm infection is widely prevalent in India.
  • Necator americanus is predomi­nant in south India, and Ancylostoms duodenal in north India. Recently, another spe­cies, A. ceylanicum has been reported from a village near Calcutta.
  • The heavily infected areas are found in Assam (tea gardens).
  • West Bengal, Bihar, Orissa, Andhra Pradesh, Tamil Nadu, Kerala and Maharashtra. More than 200 million people are estimated to be infected in India.
  • It is believed that 60 to 80 percent of the population of certain areas of West Bengal, Uttar Pradesh, Bihar, Orissa, Punjab, and the eastern coast of Tamil Nadu and Andhra Pradesh are infected with hookworms.

Communicable Disease- Influenza:

  • Influenza is an acute respiratory tract infection caused by influenza virus of which there are three types—A, B and C.
  • All known pandemics were caused by influenza A strains, due to various factors. Influenza is found all over the world.
  • It occurs in all countries and affects millions of people. Outbreaks of influenza A occur virtually every year. Major epidemics occur at intervals of two to three years, and pandemics at intervals of about 10 to 15 years.
  • The first pandemic during the present century occurred in 1918-19, which affected an esti­mated 500 million people and killed more than 20 million.
  • In India alone, over six mil­lion people died during this pandemic. This pandemic was caused by what is now known as the swine influenza virus.
  • Recent pandemics occurred in 1957-58 owing to the influ­enza A (H2N2) and in 1968 owing to the influenza A (H3N2).
  • Outbreaks of influenza B also occur annually with epidemics occurring at intervals of407 years. Influenza brought on by the type C virus occurs sporadically as small outbreaks.
  • The unique features of influenza epidemics are the suddenness with which they arise, and the speed and ease with which they spread.
  • The short incubation period, a large number of subclinical cases, a high proportion of susceptible population, short duration of immunity, and an absence of cross-immunity, all contribute to its rapid spread.
  • The fate of the virus during inter-epidemic periods is also known. Possible expla­nations include transmission of virus to extra-human reservoirs (pigs, horses, birds. etc.,) latent infection or continuous transfer from one human to another. This explains the occurrence of sporadic cases.

Communicable Disease- Filariasis:

  • It is caused by a parasite, which belongs to the nematode family Filariasis. According to WHO reports, an estimated 751 million people are at ‘risk’ for infection, and 120 million have actually been infected.
  • The public health problem of lymphatic filariasis is greatest in China, India and Indonesia. These three countries account for about two-thirds of the estimated world total of persons infected.

Filarial Problem in India:

  • Filariasis is a major public health problem in India. There are an estimated six million attacks of acute filarial disease per year, and at least 45 million persons currently have one or more chronic filarial lesions.
  • Heavily infected areas are found in Uttar Pradesh, Bihar, Andhra Pradesh, Orissa, Tamil Nadu, Kerala and Gujarat.
  • The infection is acquired from a person who has filariasis. The maximum infectivity is when the organisms are circulating the blood.
  • The largest number appears in the blood at night time, and retreats from the blood stream during the day. Their usual habitat is in the lymph nodes.
  • The mosquito feeds on such a person and acquires the filarial parasite. The filarial organism is transmitted when the mosquito bites a person. The parasite is deposited near the site of puncture.
  • It passes through the punctured skin or may penetrate the skin on its own and finally reach the lymphatic system. Filariasis affects all age groups.

Communicable Disease- Tuberculosis:

  • Tuberculosis remains a worldwide public health problem, particularly in the Third World countries. Tuberculosis is India’s biggest public health problem. An estimated that 5, 00,000 deaths annually are reported due to this disease, while a similar number of persons get cured.
  • The population in the Third World countries like India is exposed to tuberculo­sis. The disease, however, does not develop in everyone who is exposed. Poor nutrition, overcrowding, low socio-economic status, are more likely to develop the disease.
  • The prevalence of people who are infected is about 30 percent of the population. The prevalence of infection is more common in the younger population.
  • The vast majority of cases are to be found in rural and semi-urban areas, where more than 80 percent of the country’s population lives. In urban areas, tuberculosis is found more frequently in slum-dwellers and lower socio-economic groups than in well-off groups

Current Burden of Non-Communicable Diseases in India

Medical Definition of Endemic

Endemic: A characteristic of a particular population, environment, or region. Examples of endemic diseases include chicken pox that occurs at a predictable rate among young school children in the United States and malaria in some areas of Africa. The disease is present in a community at all times but in relatively low frequency.

By contrast, there are the related terms "epidemic" and "pandemic":

  • An epidemic is a sudden severe outbreak within a region or a group, as with AIDS in Africa or AIDS in intravenous drug users.
  • A pandemic occurs when an epidemic becomes very widespread and affects a whole region, a continent, or the entire world.

The word "endemic" comes from the Greek "en-", "in" + "demos", "people or population" = "endemos" = "in the population." An endemic is in the people.

By contrast, "epi-" means "upon." An epidemic is visited upon the people. And "pan-" means "all." A pandemic affects all the people.

Key Concepts and Summary

  • Epidemiology is the science underlying public health.
  • Morbidity means being in a state of illness, whereas mortality refers to death both morbidity rates and mortality rates are of interest to epidemiologists.
  • Incidence is the number of new cases (morbidity or mortality), usually expressed as a proportion, during a specified time period prevalence is the total number affected in the population, again usually expressed as a proportion.
  • Sporadic diseases only occur rarely and largely without a geographic focus. Endemic diseases occur at a constant (and often low) level within a population. Epidemic diseases and pandemic diseases occur when an outbreak occurs on a significantly larger than expected level, either locally or globally, respectively.
  • Koch&rsquos postulates specify the procedure for confirming a particular pathogen as the etiologic agent of a particular disease. Koch&rsquos postulates have limitations in application if the microbe cannot be isolated and cultured or if there is no animal host for the microbe. In this case, molecular Koch&rsquos postulates would be utilized.
  • In the United States, the Centers for Disease Control and Prevention monitors notifiable diseases and publishes weekly updates in the Morbidity and Mortality Weekly Report.

The Science of Epidemiology

Epidemiological studies include disease etiology, disease surveillance and screening, biomonitoring, and clinical trials.

Learning Objectives

Discuss the various factors that characterize epidemiology

Key Takeaways

Key Points

  • Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.
  • Epidemiologists employ a range of study designs from the observational to experimental. Its study designs are generally categorized as descriptive, analytical, and experimental.
  • The identification of causal relationships between disease exposures and outcomes is an important aspect of epidemiology.

Key Terms

  • epidemiologist: A scientist (often a medical doctor) who specializes in epidemiology.
  • causal: A cause of something causing.

Major areas of epidemiological study include disease etiology, outbreak investigation, disease surveillance and screening, biomonitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.

Epidemiological studies are aimed, where possible, at revealing unbiased relationships between exposures such as alcohol or smoking, biological agents, stress, or chemicals to mortality or morbidity. Epidemiologists employ a range of study designs from the observational to experimental. Its study designs are generally categorized as descriptive, analytical (aiming to further examine known associations or hypothesized relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions).

In observational studies, nature is allowed to “take its course”, as epidemiologists observe from the sidelines. Observational studies have two components: descriptive or analytical. Descriptive observations pertain to the “who, what, where and when of health-related state occurrence”. On the other hand, analytical observations deal more with the “how” of a health-related event.

World map of people living with HIV/AIDS: This map captures the estimated number of people in the world living with HIV/AIDS in 2008.

Controversially, in experimental studies, the epidemiologist is the one in control of all of the factors relating to the particular case study. Experimental epidemiology contains three case types: randomized control trials (often used for new medicine or drug testing), field trials (conducted on those at a high risk of conducting a disease), and community trials (research on social originating diseases).

The identification of causal relationships between these exposures and outcomes is an important aspect of epidemiology. It is nearly impossible to say with perfect accuracy how even the most simple physical systems behave beyond the immediate future. The complex field of epidemiology, which draws on biology, sociology, mathematics, statistics, anthropology, psychology, and policy only makes analysis even more challenging.

A common theme in much of the epidemiological literature is that “correlation does not imply causation. ” For epidemiologists, the key is in the term inference. Epidemiologists use gathered data and a broad range of biomedical and psychosocial theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal, and about exactly how they are causal.

Classification of Plant Diseases | Botany

Study of plant dis­eases in relation to their occurrence (interaction of populations of plants, pathogens and environment) is known as epidemiology. There may be infectious disease and contagious disease. A disease which spreads slowly and is incited by a transmissible pathogen is referred as infectious disease, and that which spreads rapidly is a contagious disease.

Considering from the point of view of occurrence the diseases are often classified as:

Endemic, epidemic or epiphytotic, and sporadic.

A disease which constantly occurs from year to year in moderate to severe form is an endemic disease. Greeneria fruit rot of grape (Vitis vinifera) is an endemic disease in India caused by Greeneria uvicola. But an epidemic disease or epiphytotic disease is the one whose incidence is periodical in wide areas spreading very fast. The term epi­phytotic is rather used in case of plant disease.

The epiphytotic disease is usually very responsive to variations of environmental conditions which actually control its incidence. Like endemic disease, the pathogen may be more or less constantly present but to ensure disease development environmental conditions must be favourable.

The Epiphytotic Disease incidence is controlled by:

(a) Availability of susceptible host,

(c) Rapidity of dissemination of inoculum and host infection,

(d) Rate of production of inoculum and

(e) Micro- and macro-climatic conditions.

According to their occurrence in crops, the epiphytotic diseases may be of two kinds the diseases ordinarily endemic but assume epidemic proportions from time to time, and those completely newly introduced in the locality—the exotic diseases.

Wart disease of potato caused by Synchytrium endobioticum is an exotic dis­ease in India. In the former kind, the disease incidence depends mainly on the varia­tions of the climatic conditions. Whereas, the latter is resulted either from a new parasite having been introduced from another country, or from the passage of a para­site of some pre-existing plant of a place to a newly introduced host.

Some of the important epiphytotic outbreaks are:

The Irish famine of 1945 and 1946 due to the destruction of the potato crop by the late blight of potato disease the destruction of vineyards in England in 1845 and in France in 1848, by downy mildew disease the devastation of the flourishing coffee plantation of Ceylon caused by coffee rust disease and the destruction of sugarcane industry of the West Indies in 1895 by the rind disease.

Similar situations arose in the United States where almost the entire chestnut trees were completely killed by chestnut blight disease, so also the fast spreading of the Dutch elm disease in the wiping out of elm trees from the country.

The sporadic disease is rather an epiphytotic disease except that its occurrence is at very irregular intervals and in lesser areas. In a sporadic disease, pathogen affects only a few plants in a large population of host plants and others remain unaffected.

(ii) Diseases classified based on the nature of dispersal medium:

Diseases are often classified as: soil-borne disease, seed-borne disease, wind-borne disease, etc. Here importance is given only on the dispersal medium of the pathogen and nothing else.

(iii) Diseases classified according to parts of host affected:

The classi­fication of plant diseases based on parts of the host affected may be outlined as:

Such a classification neither does clearly indicate the nature of the causal agency nor the effect produced on the host tissues and does not have much practical value.

(iv) Diseases classified on the basis of hosts affected:

The classification of plant diseases on the basis of hosts affected is simply a matter of convenience. There is no fundamental principle involved in this system of classification.

Some of these diseases are:

Vegetable diseases or Diseases of vegetable, Fruit disease, Cereal diseases, Timber diseases, Diseases of ornamental plants, Diseases of shade trees, etc.

(v) Diseases classified on the basis of symptoms:

On the basis of symp­toms, plant diseases are classified into three categories:

(a) Diseases resulting in the death of the affected tissues-necrotic

(b) Diseases resulting in the slowing down or stunting of the growth of the plant or plant parts-atrophic and

(c) Diseases resulting in the overgrowth or over development of the plants or organs affected-hypertrophic.

According to symptoms diseases are often designated as blight disease, mildew disease, rust disease, smut disease, rots, cankers, etc. This system of classification gives an idea about the nature of damage done to the host and as such has some practical value.

(vi) Diseases classified on the nature of causal agencies:

The diseases of plants may be attributed to one or more of the following causal agencies:

(e) Parasitic angiosperms (Fig. e.g., Viscum album, Dendrophthae sp., Phoradendror. sp., Orobanche sp., Cuscuta spp., and Striga spp.

(a) Soil-moisture disbalance.

(c) Optimal temperature disbalance.

(d) Light intensity disbalance.

(e) Gas, smoke and other air pollutants.

(f) Careless spraying of chemicals.

The first group comprises of parasitic causal agencies (with the exception of Virus, may be) and the disease produced is the parasitic disease. Whereas, the second group consists of non-parasitic causal agencies and the corresponding disease produced is the non-parasitic disease.

The first group includes a great host of living organisms which live within or are attached to the host plants, affecting their health and frequently causing death.

The extent of their parasitism is extremely variable, which again is dependent upon the nature of host and parasite and their interaction. Some parasites destroy their hosts in very short periods of time while others live upon their hosts for long period, even for many years, before death results.

Still other parasites never cause the death of the host plant, but do reduce its vitality, growth, or fruit production. Some parasites do not require a great amount of food from the host plant but grow in such a manner as to interfere with the performance of its ordinary functions.

Again some of which are of little importance in themselves, open the way for the attacks of organisms which cause other and more destructive diseases.

A parasitic causal agency may also produce conditions suitable for the growth of causal organisms which are in no way the cause of the disease in question. The presence of two or more organisms in or on a diseased plant thus frequently prevents a satisfactory diagnosis and treatment.

Diseases resulting from the second or non-parasitic group of causal agencies are frequently very difficult to diagnose. Often they are of little importance in them­selves, but make a plant so weak as to make it possible for one of the specific organisms of the first group to attack it.

Plant diseases are classified with a view to:

(i) Get a clear idea about the cause or causes of the disease,

(ii) Follow the pattern of disease development, and

(iii) De­termine the extent of damage done to the host.

These facts are needed to diagnose a disease and to recommend control measures of the disease concerned. Of the various ways of classification of plant diseases stated above, only the one classified on the nature of causal agencies gives adequate information about the cause of the disease develop­ment and related facts.

Hence such a classification has the most practical value and is often considered as modern system of classification of plant diseases. Next best is the one based on the nature of symptoms which is also very useful to diagnose a disease.

A. Non-Parasitic Diseases:

Diseases caused by non-parasitic causal agencies are known as non-parasitic diseases or physiological diseases, also known as deficiency diseases. They are diseases having specific symptoms. Although the symptoms and effects of certain diseases are well-known, the factor or combination of factors producing them is still questionable.

Until early part of 1900 the field of non-parasitic diseases did not receive much atten­tion.

It is now well-recognized that much loss is also brought about through the effect of non-parasitic causal agencies, they are precisely soil-moisture disbalance nutri­tional disorders high or low temperature change in intensity and quality of light atmospheric impurities like poisonous gases certain disagreeable chemicals (fungi­cides and insecticides) of unusual doses, etc.

I. Soil-Moisture Dis-Balance:

The general condition of water imbalance in diseased plants is associated with a derangement in its absorption, transport, and/or transpiration and is often accompanied by a disturbance to the various other basic processes, such as carbohydrate and nitrogen metabolism, respiration, and mineral uptake.

It is reasonable to presume that the primary effect of a disease is on the water status of the cells, which reflects in the various derangements in metabolism functions, leading to a progressive degeneration of vital activities, culminating in death. It will not be exaggeration to say that the fire of plant life burns in water.

Different plants react differently with regard to soil-moisture variations. The most apparent effect of water deficiency is loss of form in herbaceous plant tissues due to loss of turgour exhibited by wilting, die-back, etc. Continued shortage of water eventually results in cessation of growth and decrease in the production of auxins.

The physical structure of the soil not only affects its water retaining capacity and aeration but, if sufficiently hard and compact, can retard the growth of roots and so result in stunted and weak plants.

Some of the symptoms due to soil-moisture dis balance are:

This is the most common symptom caused by deficiency of water supply. Wilting is recognizable from drooping of leaves and stem tips. There may be physiological and pathological wilting. The former condition is seen due to inter­ference of water balance caused by irregularity of any of the physiological processes connected with water circulation in the body, such as absorption, conduction and transpiration.

This deficiency can be easily corrected by rectifying the hindrance of absorption mechanism existing in the soil. Whereas, pathological wilting is caused by pathogenic agencies.

This symptom would naturally arise not only from the inter­ference with water uptake consequent to the damage to subterranean portions of the plant, but as a result of the presence of metabolites of certain infecting parasitic fungi which act as wilt toxins.

The disease also arises out of the dysfunctions of root system resulting from injury and foot rot incited by some fungi. Water shortage may also be caused by the dys-functioning of conductive elements (rupturing of xylem elements), brought about by the growth of hyphae and masses of bacteria which also obstruct water flow through the xylem (Fig. 343)

The dying backward from the tip of twigs or branches of trees and shrubs during the growing season is the symptom of plants and their branches caused by definite interference with water supply, either by tissue disintegration or by inducing gum formation in the xylem region.

This is a symptom that is exhibited by the sudden death and brown­ing of large indefinite areas of leaves and fruits (a) due to inadequate supply of water, and (b) due to a sudden and rapid loss of water from the leaves which cannot be replaced by the roots, since the soil moisture is low.

The affected leaves show dead brownish areas on their margins or similar areas between the veins, but the leaves remain alive and do not drop so that the tree is little injured (Fig. 344B & G). The symptom is most pronounced on the side of the tree facing the wind.

Blast is the sudden death of young buds, inflorescences, or young fruits due to water shortage.

The drying and dying of leaves specially of grasses, maize and tobacco, for want of water is firming.

(vi) Blossom-end rot:

Water-soaked spots appear at the junction of the floral parts increasing rapidly ultimately forming depressions turning dark sunken areas which are very suitable for the infection of parasites. This is the blossom-end rot. This happens due to excessive loss of water from the plant body.

An excess of water is unfavourable to trees, since it stimulates the excessive development of tender, succulent tissue, which is more readily invaded by parasitic fungi and more susceptible to extremes of heat and cold.

Roots must get oxygen necessary for their development from the soil air, and, since most trees cannot obtain sufficient oxygen from soil saturated with water which results in the death of the roots by asphyxiation. Bark becomes very soft and spongy and several times thicker than normal, such a condition is commonly known as dropsy.

II. Nutritional Disorders:

Abnormalities of one kind or the other appear in plants due to dither pronounced shortage of supply or in excess of certain essential elements which really upsets the normal nutritive processes. These abnormalities are known as mineral-deficiency or mineral-excess diseases.

These diseases are expressed by some characteristic symptoms like chlorosis, yellowing, defoliation, resetting, dark spots, etc. Here only the most common and widely prevalent and comparatively easily recognizable ones are considered.

Unsuitable chemical composi­tion or pH of the soil can be damaging to plants. Some plants can withstand consi­derable alkalinity or acidity, but most grow best around neutrality or slight acidity. An unfavourable pH results in poor growth and sometimes death.

It also has indirect effects on the availability of mineral elements in the soil to plant roots, as in calcareous soils where it reduces the availability of iron and manganese (lime induced chlorosis). It is sometimes possible to correct high acidity by addition of lime and, less easily, high alkalinity by addition of sulphur, although this may have phytotoxic effects.

The salts responsible for the unfavourable pH can sometimes be washed out by irrigation.

Faulty mineral nutrition can predispose plants to attack by some pathogens. Potato blight caused by Phytophthora infestans is said to be more severe on plants grow­ing in magnesium deficient soil, and excess of nitrogen results in ‘soft’ plants which tend to be more susceptible to pathogens and pests.

(i) Iron-deficiency diseases:

Chlorosis is a most common iron-deficiency disease symptom. It is exhibited by a very acute yellowing (Fig. 344D) of green parts and usually happens when the soil becomes alkaline due to conversion of available ferrous salt to unavailable ferric salts. It is not always due to an absolute deficiency of iron in the soil.

The chief causes of chlorosis are usually due to presence of excess of lime—lime-induced chlorosis or of manganese—manganese-induced chlorosis in the soil.

The lime-induced chlorosis is rather common in vineyards, orchards and in clovers. Whereas, the manganese-induced chlorosis, comparatively less common, is best known in pineapple cultivation as ‘yellows’. This is very well-known pineapple disease.

(ii) Boron-deficiency diseases:

Boron-deficiency, in general, affects storage organs of plants such as, roots, tubers, fruits, and similar other organs. The diseases are recognizable by various symptoms of which more common ones are, heart and dry rot caused by distortion and necrosis of fleshy tissue.

The chief anatomical res­ponse to boron-deficiency is visible in the meristematic regions of certain plants leading to hyperplasia, hypertrophy and abnormal differentiation of parts.

Most common boron-deficiency disease in sugar-beet is heart and dry rot. Besides this, the young unfolded leaves fail to develop normally and ultimately become necrotic and fall off. Similar is the condition of the dormant buds in the axils of older leaves. Distur­bance of normal functioning and development of vascular tissue takes place.

In apples due to boron-deficiency light brown spots of dead tissue appear in the fruits, and in trees rosette and die back. In case of cruciferous plants particularly in cauliflower, leaves become deformed, brown-coloured, sometimes discolouration takes place.

In comparison with this, cabbage is less susceptible to boron-deficiency. In cabbage, discolouration of head and break down in the core take place.

(iii) Potassium-deficiency diseases:

Symptoms of potassium-deficiency dis­eases are very clearly exhibited by the leaves and can be recognised from mottle and chlorosis of leaves particularly near the margins. In some cases the entire leaf turns brown, dies, and becomes brittle. Necrotic spots may also occur in the inner portion of the leaf blade.

Often the first symptom of potassium-deficiency is exhibited by dark- brown colour of the leaflets which is followed by yellowing or bronzing.

(iv) Phosphorus-deficiency disease:

A deficiency of phosphorus in many plants results in abnormally high quantities of anthocyanin’s producing purpling of leaves.

III. High or low temperature effects:

Every growing plant has its own tem­perature range of normal growth. Any variation either above or below of which results in the irregularities, since temperature affects almost every function of life. These irregularities in growth are expressed by the various symptoms of the diseases induced by high or low temperature.

The nature of the symptoms is also dependent on the nature of plants, their respective parts affected and the extent of variations of tem­perature.

Here only few are discussed:

(i) High-temperature effects:

Growing plants when subjected to tempera­tures much higher the optimum are liable to sustain certain injury of cell degeneration which is considered as symptom of the disease induced by high-temperature. The symptom may be visible both in aerial as well as subterranean parts.

Most common high soil temperature disease of potato tuber is the blackheart of potato expressed by blackened centres (Fig. 344A). High temperature above 90°F in transit may also cause blackheart.

Similar symptom may also develop when potatoes are put in poorly ventilated storage places. Specially the tubers at the bottom of deep piles are very commonly liable to face shortage of oxygen developing blackheart. Another common high-temperature injury is sunscald of vegetables, particularly of tomato.

To start with, a water-soaked appearance of tissue is visible which is followed by rapid desi­ccation leading to the formation of sunken areas which ultimately turn grey in green fruits and yellow in ripe ones.

Premature casting of leaves may occur in trees -when subjected to high-temperature. Leaves of the inner crown are first affected, because they are more sensitive to heat, this is known as heat of defoliation. Again bark when suddenly exposed to the summer sun by removal of neighbouring foliages becomes overheated and dries out.

Such heating and drying out of the bark, resulting in open wounds, occurs on certain young trees with smooth, thin bark, known as sunscald also as bark scorch. Sometimes leaves and twigs of Douglas fir are covered with white sugar. Sugar is exuded in solution directly from the leaf tips during the hot, dry, cloudless days of summer—sugar exudation.

The water then evaporates leaving the sugar encrusted on the leaves. The effect of sugar exudation is rather negligible.

(ii) Low-temperature effects:

Potato tubers when stored in cold storage are often liable to freezing injury of temperature range of 0°C. to 5°C. when stored for a long period of time. The freezing injury is seldom externally visible. The freezing injury results in necrosis of cells. Necrosis may be ring necrosis net necrosis and blotch type of necrosis.

The ring necrosis of discoloured tissue occurs in the peripheral region, net necrosis of blackened small areas are rather scattered through­out, and blotch type usually comprises irregular areas of various sizes and colour- opaque grey to black in the tubers.

IV. Effects of Change in Intensity and Quality of Light:

Both reduced or increased intensities and any appreciable change in the quality of light result in the development of various symptoms of the disease.

Reduced light intensity promotes succulent growth along with developing internodes longer than usual and turning weak in physical structure and retards chlorophyll formation as a result of which normal green colour does not develop resulting in the etiolation which is rather a very com­mon symptom.

The change in quality of light for example, when the atmosphere is clear short waves reach the earth with greater intensity inducing sunscald of leaves and fruits of vegetables.

V. Effects of Atmospheric Impurities:

Gas, smoke and other air pollutants of the atmosphere cause serious damage to the plant parts.

Orchards and shade trees are commonly injured by unfavourable conditions of the air or soil arising from industrial processes. Forest trees escape most of these adverse factors. Death or injury- is often by the escape of artificial illuminating gas into the soil or by the escape of steam over-heating the soil, encountered in young succulent plants which are sensi­tive to some of the constituents of illuminating gas.

Forest trees are often affected by smoke. The smoke injury is caused by the gases arising from the incomplete com­bustion of coal, from the smelting of ores containing sulphur, sulphur dioxide frequently causes severe damage, the symptoms consist of a variety of forms from yellowing and reddening of tissues of leaves to defoliation and stunting.

Three stages of smoke injury are recognized:

(i) The acute stage due to high concentration of smoke in the air causes rapid discolouration of the foliage followed by defoliation and in extreme cases by the death of the plant,

(ii) The chronic stage due to small quantities of gas present in the atmosphere produces unhealthy condition of the plant expressed in stunted growth and

(iii) The invisible stage, due to the action of gas in very dilute quantity, is characterized by reduction in growth.

The gaconfines its effect to the chlorophyll-bering or green parts of plants, woody tissues are not directly affected. In addition to direct injury to foliage indirect injury is claimed because of soil changes, such as the acidity of the smoke depleting the soil of calcium carbonate and thus reducing the number and hampering the activity of nitrifying bacteria.

Sometimes gas injuries occur in certain fruits and vegetables when they are fumigated in storage and transit, since fumigation becomes necessary to protect them against certain diseases during storage and as a quarantine require­ment. The symptoms are usually discolouration of pairs, appearance of sunken brown spots, etc.

Again the effects of air pollutants on plants can be seen around the major population and industrial centres. Plants affected by air pollutants generally respond according to the principal toxicant present. Burning, bronzing, silvering, and growth abnormalities and necrosis are common symptoms. Sulphurdioxide causes black tip or mango necrosis.

VI. Effects of Fungicides and Insecticides:

Certain chemicals—fungicides and insecticides when sprayed on plants as a part of plant protection measure, if sufficient care is not taken during use, cause injuries particularly to the leaves.

The injuries are in the nature of discolouration of the affected parts, abnormal growth leading to un­usually rapid development of certain parts of the leaf induced by the careless spraying of pesticide 2, 4—D. Resetting in apples may be caused by parathion burning, spotting discolouration by improper application of Bordeaux mixture sprays.

B. Parasitic Diseases:

Diseases induced by parasitic causal agencies are the parasitic diseases. The development of a parasitic disease induced by parasitic causal agencies particularly by fungi, bacteria, slime molds and viruses has a sequence of events following in cyclic order and as such comprises of disease cycle. The sequence of events of the disease cycle is greatly conditionable.

But the disease cycle basically consists of two phases— the dormant phase and the active phase.

During dormant phase the causal agency (pathogen) remains in dormant condition and overcomes the unfavourable conditions in the perennating organs. These perennating organs may be spores of varied forms, resting vegetative cells, resting mycelium, virus particles, etc.

The perennating organs may remain in or on propagating organs of the host, in diseased plant debris, in soil, in collateral hosts, in insect body, etc. With the advent of favourable environmental conditions the pathogen becomes active and there starts the active phase and the perennating organs produce primary inoculum.

When disease occurs, its quality, its severity and its epidemic potential or how fast it will advance is very much dependent on the interactions among host, pathogen and physical environment.

The process starts with pre-inoculation (pre-penetration) period when the inoculum is produced which is again controlled by pre-inoculation environment such as, condition of the source of inoculum (perennating organs) and its microclimate.

After successful deve­lopment of inoculum, will follow the dispersal of inoculum. The inoculum arrives the host tissue after either being dispersed by some agency or directly coming in contact without the help of any medium.

The density, viability, and potential infectivity of the inoculum are important for successful infection. It is meaningless unless the ino­culum has found susceptible hosts and can penetrate the host and establish infection.

This is the most vital link in the entire process where environmental dependencies are quite strong since most pathogens have an obligate free moisture requirement. This process is also temperature dependent and so the interaction of time span and temperature of moisture on host surfaces become important.

The situation may be further complicated by light effects, by the change that can take place in the infection court and by the age and physiology of hosts.

Precisely speaking, it is the microclimate of the infection court that plays a very important role in the establishment of host infection. The details of development of the disease and the duration of the disease cycle are often extremely variable.

In some case the inoculum (secondary inoculum) produced by the primary infection is dispersed by various agencies and on coming in contact with the same host, produces secondary infection and secondary disease cycle as against the primary disease cycle produced by the primary inoculum developed by the perennating organs of the pathogen. Whereas in others, the secondary disease cycle may be absent.

The general pattern of disease cycle is presented in Figure 345.

Some of the parasitic diseases are as follow:

Some parasitic angiosperms have a wide and apparently unselective host range but in others there seems to be some degree of host specialization. Viscum album is a semi-parasite on trees. It can adversely affect growth and yield as well as bring about premature death of severely infected trees. Dendrophthae sp. and Phoradendron sp. also can cause extensive damage to various trees.

Orobanche sp. damages tobacco crop. Cuscuta spp. can cause severe damage to leguminous plants. Striga spp. are partial root parasites of sugarcane, cereals, maize and millets.

There are large number of parasitic algae of which special mention may be made of the genus Cephaleuros. Different species of Cephaleuros grow as parasites on various angiosperms causing severe damage to leaves and shoots. C. virescens forms the red rust of tea plants causing economic loss. C. mycoidea attacks tea, citrus, and other plants causing red rust disease.

This alga is sometimes epiphytic and sometimes parasitic on the leaves of the tea plant in India, but is usually parasitic on the stems.

In Sudan C. mycoidea has been found to attack coffee, citrus and guava plants. Cephaleuros causes leaf spotting and shedding and the photosynthetic activity of the foliage is reduced. In case of severe infection, branches and twigs may be girdled inducing die back symptom. This disease is spread by air-borne sporangia which produce zoospores which invade the leaf or stem through stomata.

Large number of insects, mites and nematodes feed on various plants, thereby causing injury and symptoms which can resemble those caused by other pathogens. The damaged tissue may subsequently be colonized by fungi and bacteria, so that it is sometimes difficult to find out the primary cause of symptoms.

Many wound parasites enter the injured plants through the injuries caused by insects, mites and nematodes.

Insects damage plants by chewing, sucking, by injecting phytotoxic saliva besides behaving as vectors and inoculators of pathogens, especially viruses.

The injected phytotoxic saliva may diffuse into the tissues beyond the actual point of feeding and cause symptoms similar to those caused by fungi, bacteria and viruses. Such insects are known as toxicogenic insects. Insects also play a great role as disseminators of inoculum. Mites are frequently present on diseased plant tissue and it is sometimes difficult to assess their significance.

Nematodes attack and sometimes kill host plants. More often their attack results in weakening, reduced growth and diminished yield, and it may also enable other more destructive pathogens to enter the host plant.

When feeding on roots, nematodes can cause several types of injury:

Necrotic lesions, induce formation of hypertrophic cells, and suppression of cell division of apical meristem of roots. Nematodes which attack the aerial parts of plants may cause discolouration, neorosis, blotches, spots, distortion, and galls on leaf, stem and seed. Buds, growing points or flower primoidia are attacked by some nematodes, resulting in abnormal growth of the affected plant.

WHO response

WHO&rsquos work on schistosomiasis is part of an integrated approach to the control of neglected tropical diseases. Although medically diverse, neglected tropical diseases share features that allow them to persist in conditions of poverty, where they cluster and frequently overlap.

WHO coordinates the strategy of preventive chemotherapy in consultation with collaborating centres and partners from academic and research institutions, the private sector, nongovernmental organizations, international development agencies, and other United Nations organizations. WHO develops technical guidelines and tools for use by national control programmes.

Working with partners and the private sector, WHO has advocated for increased access to praziquantel and resources for implementation. A significant amount of praziquantel, to treat more than 100 million children of the school age per year, has been pledged by the private sector and development partners.