Outbreak: A Study in Epidemiology
Epidemiologists are the disease detectives of the world. They use scientific methods to determine why some people get sick and others do not, how and why people are sick and how to control illnesses.
- Spread of disease
- Disease prevention
Epidemiologists must have knowledge in psychology, sociology, history, chemistry, ecology, physiology, biology and statistics. There are two general types of epidemiologists. The descriptive epidemiologist investigates incidents of disease outbreak from field reports. They receive reports of unidentified illnesses or outbreaks of known infectious diseases from physicians and hospitals. The descriptive epidemiologist describes the person or people, place and time the event occurred using the mandatory reporting forms submitted by the medical community. These mandatory reporting forms are called PPT forms, where PPT stands for person, place and time. Descriptive epidemiologists often conduct field interviews with victims and their close associates. If descriptive epidemiologists determine that a pattern of disease exists, they notify the analytical epidemiologist.
Analytical epidemiologists visit the outbreak area looking for clues and sources. They verify the descriptive epidemiologist’s findings by checking the diagnosis, time, location and people involved. The analytical epidemiologist develops hypotheses about the cause(s) of the outbreak, then develops a plan to test the hypothesis. As part of their testing, analytical epidemiologists collect samples of likely vectors (hosts) of the disease. These samples are tested in a laboratory facility to determine control, treatment or preventative methods to eliminate the causal agent. The terms causal agent and pathogen are broad terms used to describe any virus, bacteria, prion (protein), protozoa, worm or genetic mutation that causes harm to living things. Some causal agents are contagious while others, like DNA mutations, are inherited or occur spontaneously. The epidemiologist then develops a working hypothesis. This working hypothesis identifies the likely causal agent and any infection control measures. Control measures typically include elimination of the source of the pathogen if possible, methods to block the transmission of the disease and elimination of human susceptibility through medication or vaccination. After the control and prevention measures have been in use for some time, the analytical epidemiologist conducts an evaluation to determine if the control, prevention and ultimately the hypothesis are correct and working to control the disease.
When epidemiologists evaluate the rate of infection for a disease or illness, they consider two different factors—the endemic rate and the epidemic rate. The endemic rate is the usual rate for a disease in a population. Endemic rates vary according to the disease and location. For example, breast cancer occurs more frequently than Lassa Fever in the United States but the opposite is true in portions of West Africa where Lassa Fever is endemic and occasionally epidemic. An epidemic rate occurs when there is a dramatic and rapid increase in the number of cases of illness. An outbreak of food poisoning may be considered an epidemic if caused by contaminated processed food purchased at grocery stores, restaurant contamination or contaminated food at a banquet. Another term used by epidemiologists is emerging infectious disease. This term applies to any infection that is new to a population or has existed but is rapidly increasing in incidence or geographical range.
The Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, is charged with tracking epidemics and emerging infectious diseases throughout the world in an effort to protect U.S. citizens from widespread illness. The CDC currently lists 67 mandatory reporting diseases. These diseases may become epidemics if they are not followed and controlled. Currently, the CDC employs a strategy of surveillance, applied research, prevention and control to prevent emerging diseases from plaguing the United States.
The CDC has determined six causes of epidemics throughout the world. Ecological changes in which people live in closer contact with pathogens is the first cause. The second is due to infestation of humans into more desolate regions of the world. This has also created a population influx in urban areas, creating a dense human population for pathogens to infest. The third cause is international travel and commerce. Faster modes of travel mean infected individuals reach destination before the pathogen has run its course. While more travelers/commerce means more opportunity for hitchhiker pathogens. The shift to globalization has also changed how materials and foods are produced. The mass production and global distribution of materials means a pathogen can infect many countries because it was contained in an item shipped globally. The fifth cause of an epidemic is linked to microbial adaptation. Pathogens mutate and become more virulent or they become resistant to current antibiotics and cause epidemics. The final cause is created by problems within the public health system. As a result of monetary issues, not all humans are immunized and do not have access to clean water or functioning sewer systems. Pathogens are opportunistic parasites that infect hosts whenever the opportunity arises.
There are four types of epidemiological studies to test hypotheses. Three are observational studies and one is a trial study. A trial study is a controlled experiment in which some human test subjects are purposely exposed to the causal agent while others are given a placebo exposure. The test subjects are carefully monitored to see if they develop the disease. It is unethical to perform a trial study on unknowing humans if an exposure is likely to cause disease. If the exposure is likely to prevent the disease human trial studies may be ethical. The three types of observational studies are the cohort study, the case-control study and the cross-sectional study. In the cohort study, the epidemiologist does not assign exposure to the causal agent. The test subjects are exposing themselves during their normal life. The epidemiologist just observes the subjects’ activities including exposure, and tracks whether or not the test subjects develop the disease. In the case-control study, the epidemiologist begins the study with groups of diseased and disease-free test subjects. The epidemiologist then collects data about past exposures through questionnaires and test subject interviews. The cross-sectional study is a “slice in time” study. The epidemiologist questions a statistical sample of people about their exposure to the causal agent during a very specific period of time. The epidemiologist also determines the number of test subjects who have the disease at the time of the study. Each type of study has strengths and limitations. The trial study allows the epidemiologist to have control over the exposure but it may be unethical to perform. The cohort study allows the epidemiologist to acquire accurate exposure data but the study can be very expensive and it can take a lot of time. The case-control study is quick and less expensive, but exposure data may be inaccurate due to memory problems. The cross-sectional study is also quick and less expensive, but some data may falsely link the exposure to the disease. It may actually indicate that people with the disease have a confounding link to the exposure.
All observational studies are flawed in that they may link the disease and exposure by association and not causation. An association means that the disease and exposure are linked in some way that makes them turn up together. A cause is something that produces the disease. There are two associations that concern epidemiologists: confounders and reverse–time order associations. A confounder is another agent that is associated with the exposure being studied and the disease. The confounder may be the causal agent or it may enhance the effects of the agent being studied. For example, asbestos causes lung cancer when it is inhaled into the lungs. Smoking is a confounder. It also causes lung cancer. Smoking and exposure to asbestos increases the risk of developing lung cancer by 50%. Another association problem is termed reversed-time order. In this association, the disease actually causes the exposure. An example of a reversed–time order is an incident of food poisoning in cans of chicken broth. They believed that their illness was caused by the influenza virus. A group became ill after eating contaminated chicken and dumplings made with canned chicken broth. They unknowingly continued their exposure by consuming cans of contaminated chicken broth while they were too ill to eat solid food. The bacteria or toxins produced by bacteria in the broth caused the sickness—not an influenza virus as they had assumed.
After the causal agent of the disease has been established, the epidemiologist must decide upon a strategy to reduce the number and severity of illnesses. The best strategies prevent further outbreaks of the disease, reduce the severity of the illness, or contain those who are currently infectious. Any plan to prevent future outbreaks must take human psychology into account. How easy is the strategy to use? How much does it cost? Do people perceive a risk to contracting the disease? Epidemiologists develop and implement disease prevention strategies with the assistance of doctors and public health officials. The disease prevention strategies are evaluated for effectiveness after a predetermined length of time. Did the disease prevention strategy work? Are there fewer incidences of the illness? Have personal exposures declined? The epidemiologist may need to revise his/her hypothesis and the disease prevention strategy at this point if the number of cases has not decreased significantly.
Determine the cause of an outbreak of conjunctivitis (pink eye) in a high school. You are given cards with the demographic information for the high school students and a sheet of relevant information. Use this information to determine the source(s) of the pink eye outbreak.
Conjunctivitis Information Sheet
Student Demographic Cards
Supplemental Demographic Information Sheet
Although this activity is considered nonhazardous, it is good safety practice to wash hands thoroughly with soap and water before leaving the laboratory.
Review the Conjuctivitis Information provided before beginning the Procedure.
- Calculate the prevalence of conjunctivitis among students at the high school and record the results in the Outbreak Worksheet. Prevalence (P)—the percentage of existing cases in a population. Note: There are 25 student cards given for each grade. There are 75 additional nonexposed, nonill students in each grade
- Formulate a hypothesis as to the source of the epidemic of pink eye at the school. Record the hypothesis on the Outbreak Worksheet.
- Use a 2 x 2 table to determine a relationship between the exposure and the development of pink eye. Based on your hypothesis, count the number of students who were exposed to pink eye and have pink eye in cell a. Cell b students were exposed to pink eye (based on the hypothesis) but do not have the illness. Cell c students were not exposed according to the hypothesis but have pink eye. Cell d students were not exposed and do not have the disease. Record the numbers on the Outbreak Worksheet. Note: All exposed and infected students are listed on the Student Demographic Cards. Only nonexposed, no-disease students are not listed.
- Calculate the risk of infection for an exposed student. Record the result on the Outbreak Worksheet.
- Calculate the risk of infection for a non-exposed student. Record the result on the Outbreak Worksheet. This is the control group.
- Calculate the relative risk of infection for any student. Record the result on the Student Worksheet.
- Determine the likelihood that the exposure is the source of the pink eye.
- If the relative risk is greater than 2.0, there is a strong association that the exposure may cause the illness.
- If the relative risk is less than 0.5, there is a strong association that the exposure may prevent the disease.
- If the relative risk is approximately 1, there is no association between the exposure and the disease and the exposure is likely unrelated to the illness.
- Complete the Outbreak Worksheet.
- Consult your instructor for appropriate disposal procedures.