Epidemiology
Epidemiology, the study of epidemics, is sometimes called the medical aspect of ecology because it is the study of diseases in animal populations, including humans. The epidemiologist is concerned with the interactions of organisms and their environments as related to the presence of disease. Environmental factors of disease include geographical features, climate, and concentration of pathogens in soil and water. Epidemiology determines the numbers of individuals affected by a disease, the environmental circumstances under which the disease may occur, the causative agents, and the transmission of disease.
Epidemiology is commonly thought to be limited to the study of infectious diseases, but that is only one aspect of the medical specialty. The epidemiology of the environment and lifestyles has been studied since Hippocrates's time. More recently, scientists have broadened the worldwide scope of epidemiology to studies of violence, of heart disease due to lifestyle choices, and to the spread of disease because of environmental degradation.
Epidemiologists at the Epidemic Intelligence Service (EIS) of the Centers for Disease Control and Prevention have played important roles in landmark epidemiologic investigations. Those include the identification in 1955 of a lot of poliovirus vaccine, supposedly dead, that was contaminated with live polio virus; an investigation of the definitive epidemic of Legionnaires' disease in 1976; identification of tampons as a risk factor for toxic-shock syndrome; and investigation of the first cluster of cases that came to be called acquired immunodeficiency syndrome (AIDS). EIS officers are increasingly involved in the investigation of noninfectious disease problems, including the risk of injury associated with all-terrain vehicles and cluster deaths related to flour contaminated with parathion.
The epidemiological classification of disease deals with the incidence, distribution, and control of disorders of a population. Using the example of typhoid, a disease spread through contaminated food and water, scientists first must establish that the disease observed is truly caused by Salmonella typhosa, the typhoid organism. Investigators then must know the number of cases, whether the cases were scattered over the course of a year or occurred within a short period, and the geographic distribution. It is critical that the precise locations of the diseased patients be established. In a hypothetical case, two widely separated locations within a city might be found to have clusters of cases of typhoid arising simultaneously. It might be found that each of these clusters revolved around a family unit, suggesting that personal relationships might be important. Further investigation might disclose that all of the infected persons had dined at one time or at short intervals in a specific home, and that the person who had prepared the meal had visited a rural area, suffered a mild attack of the disease, and now was spreading it to family and friends by unknowing contamination of food.
One very real epidemic of cholera in the West African nation of Guinea-Bissau was tracked by CDC researchers using maps, interviews, and old-fashioned footwork door-to-door through the country. An investigator eventually tracked the source of the cholera outbreak to contaminated shellfish.
Epidemic diseases result from an ecological imbalance of some kind. Ecological imbalance, and hence, epidemic disease may be either naturally caused or induced by man. A breakdown in sanitation in a city, for example, offers conditions favorable for an increase in the rodent population, with the possibility that diseases may be introduced into and spread among the human population. In this case, an epidemic would result as much from an alteration in the environment as from the presence of a causative agent. For example, an increase in the number of epidemics of viral encephalitis, a brain disease, in man has resulted from the ecological imbalance of mosquitoes and wild birds caused by man's exploitation of lowland for farming. Driven from their natural habitat of reeds and rushes, the wild birds, important natural hosts for the virus that causes the disease, are forced to feed near farms; mosquitoes transmit the virus from birds to cattle to man.
Lyme disease, which was tracked by epidemiologists from man to deer to the ticks which infest deer, is directly related to environmental changes. The lyme disease spirochete probably has been infecting ticks for a long time; museum specimens of ticks collected on Long Island in the l940s were found to be infected. Since then, tick populations in the Northeast have increased dramatically, triggering the epidemic.
There are more ticks because many of the forests that had been felled in the Northeast have returned to forestland. Deer populations in those areas have exploded, close to concentrated human populations, as have the numbers of Ixodes dammini ticks which feed on deer. The deer do not become ill, but when a tick bite infects a human host, the result can be a devastating disease, including crippling arthritis and memory loss.
Disease detectives, as epidemiologists are called, are taking on new illnesses like heart disease and cancer, diseases that develop over a lifetime. In 1948, epidemiologists enrolled 5,000 people in Framingham, Massachusetts, for a study on heart disease. Every two years the subjects have undergone physicals and answered survey questions. Epidemiologists began to understand what factors put people at risk, such as high blood pressure, elevated cholesterol levels, smoking, and lack of exercise.
CDC epidemiologists are now tracking the pattern of violence, traditionally a matter for police. If a pattern is found, then young people who are at risk can be taught to stop arguments before they escalate to violence, or public health workers can recognize behaviors that lead to spouse abuse, or the warning signs of teenage suicide, for example.
In the 1980s, classic epidemiology discovered that a puzzling array of illnesses was linked, and it came to be known as AIDS. Epidemiologists traced the disease to sexual contact, then to contaminated blood supplies, then proved the AIDS virus could cross the placental barrier, infecting babies born to HIV-infected mothers.
The AIDS virus, called human immunodeficiency virus, may have existed for centuries in African monkeys and apes. Perhaps 40 years ago, this virus crossed from monkey to man, although researchers do not know how or why. African chimpanzees can be infected with HIV, but they don't develop the disease, suggesting that chimps have developed protective immunity. Eventually AIDS, over centuries, probably will develop into a less deadly disease in humans. But before then, researchers fear that new, more deadly, diseases will evolve.
As human communities change and create new ways for diseases to spread, viruses and bacteria constantly evolve as well. Rapidly increasing human populations prove a fertile breeding ground for microbes, and as the planet becomes more crowded, the distances that separate communities become smaller.
Epidemiology has become one of the important sciences in the study of nutritional and biotic diseases around the world. The United Nations supports, in part, a World Health Organization investigation of nutritional diseases.
Epidemiologists have also been called upon in times of natural emergencies. When Mount St. Helens erupted on May 18, 1980, CDC epidemiologists were asked to assist in an epidemiologic evaluation. The agency funded and assisted in a series of studies on the health effects of dust exposure, occupational exposure, and mental health effects of the volcanic eruption.
In 1990, CDC epidemiologists began research for the Department of Energy to study people who have been exposed to radiation. A major task of the study is to quantify exposures based on historical reconstructions of emissions from nuclear plant operations. Epidemiologists have undertaken a major thyroid disease study for those people exposed to radioactive iodine as a result of living near the Hanford Nuclear Reservation in Richland, Washington, during the l940s and l950s.
Resources
Books
Friedman, G. D. Primer of Epidemiology. 3rd ed. New York: McGraw-Hill, 1987.
Goldsmith, J. R., ed. Environmental Epidemiology: Epidemiological Investigations of Community Environmental Problems. St. Louis: CRC Press, 1986.
Kopfler, F. C., and G. Craun, eds. Environmental Epidemiology. Chelsea, MI: Lewis, 1986.
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