7.1.3 - Carrying Out Additional Studies

Additional epidemiological studies Section

When analytic epidemiological studies do not confirm your hypotheses, you need to reconsider your hypotheses and look for new vehicles or modes of transmission. This is the time to meet with case-patients to look for common links and to visit their homes to look at the products on their shelves.

An investigation of an outbreak of Salmonella muenchen in Ohio illustrates this point. A case-control study failed to turn up a food source as a common vehicle. Interestingly, people 15 to 35 years of age lived in all of the households with cases, but in only 41% of control households. This difference caused the investigators to consider vehicles of transmission to which young adults might be exposed. By asking about drug use in a second case-control study, the investigators found that illegal use of marijuana was the likely vehicle. Laboratory analysts subsequently isolated the outbreak strain of S. muenchen from several samples of marijuana provided by case-patients.

Even when the analytic study identifies an association between an exposure and a disease, you often will need to refine your hypotheses. Sometimes you will need to obtain more specific exposure histories or a more specific control group. For example, in a large community outbreak of botulism in Illinois, investigators used three sequential case-control studies to identify the vehicle. In the first study, investigators compared exposures of case-patients and controls from the general public and implicated a restaurant. In a second study, they compared the menu items eaten by the case-patients with those eaten by healthy restaurant patrons and identified a specific menu item, a meat and cheese sandwich. In a third study, appeals were broadcast over radio to identify healthy restaurant patrons who had eaten the sandwich. It turned out that controls were less likely than case-patients to have eaten the onions that came with the sandwich. Type A Clostridium botulinum was then identified from a pan of leftover sautéed onions used only to make that particular sandwich .

When an outbreak occurs, whether it is routine or unusual, you should consider what questions remain unanswered about the disease and what kind of study you might use in the particular setting to answer some of these questions. The circumstances may allow you to learn more about the disease, its modes of transmission, the characteristics of the agent, and host factors.

Laboratory and environmental studies Section

While epidemiology can implicate vehicles and guide appropriate public health action, laboratory evidence can clinch the findings. The laboratory was essential in the outbreak of salmonellosis linked to use of contaminated marijuana. The investigation of the outbreak of Legionnaires' disease in Philadelphia mentioned earlier was not considered complete until the new organism was isolated in the laboratory over 6 months after the outbreak actually had occurred . Environmental studies often help explain why an outbreak occurred and may be very important in some settings. For example, in an investigation of an outbreak of shigellosis among swimmers in the Mississippi River, a local sewage plant was identified as the cause of the outbreak..

Implementing Control and Prevention Measures Section

In a real investigation, control and prevention measures should be implemented as soon as possible. Control measures should be aimed at specific links in the chain of infection: the agent, the source, or the reservoir. For example, an outbreak might be controlled by destroying contaminated foods, sterilizing contaminated water, destroying mosquito breeding sites, or requiring an infectious food handler to stay away from work until he or she is well.

In other situations, you might direct control measures at interrupting transmission or exposure. For example, to limit the airborne spread of an infectious agent among residents of a nursing home, you could use the method of "cohorting" by putting infected people together in a separate area to prevent exposure to others. You could instruct people wishing to reduce their risk of acquiring Lyme disease to avoid wooded areas or to wear insect repellent and protective clothing. Finally, in some outbreaks, you would direct control measures at reducing susceptibility. Two such examples are immunization against rubella and malaria chemoprophylaxis (prevention by taking antimalarial medications) for travelers.