Question 1: What is poliomyelitis?
Question 2: What is the incubation period for poliomyelitis? How will it be used in this investigation?
Question 3: How is poliomyelitis transmitted?
Question 4: On what sources of infection should public health officials focus for the two cases of poliomyelitis in the southwestern region of Trans-Carpathia? Is it possible that one of the children was the source of infection? Do you think that it is likely that the children became infected with poliomyelitis from school?
Question 5: What are the two most common types of epidemiologic studies used to investigate the source of an outbreak (or other public health problem)? Which would you use to investigate the source of the poliomyelitis outbreak in south-western region of Trans- Carpathia? Why?
Question 6: Because the outbreak appeared to affect a small, well-defined group of individuals (i.e., two children in south-western region of Trans-Carpathia), investigators undertook a retrospective cohort study to investigate the source of the poliomyelitis. Investigators first surveyed the students in the children’s school to characterize the illness associated with the outbreak. (Twenty-seven of the 28 students were interviewed.) All ill people experienced severe headache and stiffness. In addition, 87% reported vomiting; 87% reported fatigue; 75% reported dizziness; 75% reported sore throat; and 25% reported fever. Five ill people had stool specimens positive for poliovirus.
Based on this information, investigators defined a case of poliomyelitis for the cohort study as headache and stiffness of at least 3 days duration in someone who attends the elementary school. Laboratory confirmation of poliovirus infection was not required. Of the 27 students who were interviewed, 16 (59%) met the case definition for poliomyelitis. Onsets of illness occurred from May 14 through May 19. (See figure 1).
Why would you question people who did not become ill about possible sources of infection with poliomyelitis?
Question 7: Grocery store owners in the region refused to take calls from investigators or cooperate with the investigation. As a result, a list of food history eaten by the students and their immunization history were obtained from the parents of these students. Parents of the twenty-four students provided information on the source of foods eaten within the past 2 months as well as the polio immunization history. (Four students, including three cases, did not provide the information.) Investigators examined the occurrence of illness among people who ate different food items. Twelve (92%) of 13 students who ate food from the local butcher became ill. Only one (9%) of 11 students who did not eat meat from the local butcher became ill. All but one of the ill students never received polio vaccine. The vaccinated student is related to and visited the butcher one week after his second oral polio vaccine shot and was helping out in the shop. The relative risk for eating meat from this shop was 10.2 (p-value <0.0001). No other exposures were associated with illness. Parents of case-patients reported that they bought their meat from the butcher’s shop.
In your own words, interpret the results of the cohort study given above.
Dr. Genevieve O. Aguolu
Question 8: What problems in study design or execution should you consider when reviewing the results of this study (or any epidemiologic study)?
Question 9: On June 4, before the first investigation had been completed, the region’s department of health was notified of another outbreak of poliomyelitis involving elementary students who attend another school in the same southwestern region of Trans-Carpathia. A second cohort study was undertaken. Nineteen attendees were interviewed. Ten met the case definition for poliomyelitis (i.e., headache and stiffness of at least 3 days duration). Students who ate meat from the local butcher were more likely to become ill than students who did not.
Illness, however, was not associated with eating a particular type of meat. No other exposures were associated with illness. Of the 7 students who reported eating from the butcher, all seven became ill. Of the eight attendees who reported not eating from said butcher, only one became ill (relative risk= 8.0, p- value = 0.001). (Note: Parents of four students, including two cases, could not recall whether their children had eaten meat and were excluded from this analysis.)
Based on the results of the two cohort studies, investigators hypothesized that the imported meat was the source of the poliomyelitis outbreaks in southwestern region of Trans-Carpathia.What additional studies might confirm (or refute) the hypothesis that the meat from the local butcher was the source of the poliomyelitis outbreaks?
Question 10: You are writing a newspaper article about the poliomyelitis outbreaks in the two elementary schools in southwestern region of Trans-Carpathia. It is thought that the poliomyelitis problem is ongoing. Four people are available for interview: the CDC expert on poliomyelitis, one of the parents of the infected students who became ill from the meat from the local butcher, the owner of the elementary school where the first outbreak occurred, and the attorney for the south-western region of Trans-Carpathia Butchers’ Association. Your deadline is looming. You have time to ask each of these people only three questions. What would you ask them?