Epidemiology As A Liberal Art
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Cited References
Analogic Thinking
Insight into many complex situations, in human experience generally and in scientific exploration specifically, comes from analogy with situations that seem roughly comparable. Much of the value of history and literature derives from the analogies with other times and places, factual or otherwise, that illuminate our lives. In many instances, the wise selection of a hypothesis to be tested by the scientific method comes from insightful analogic thinking.
In September 1972, a Peace Corps physician working in the eastern province of Sierra Leone reported several cases of a severe febrile illness that he thought might be Lassa fever (Ref. 6). The high fatality rate and lack of responsiveness to therapy for malaria and typhoid fever were typical of that disease -- a viral infection that had been discovered in 1969 and recognized in Nigeria and Liberia. However, the Sierra Leone cases were scattered over at least two towns, whereas all earlier outbreaks had occurred in hospitals and had shown clustering typical of person-to-person spread.
When testing of serum specimens from the patients in Sierra Leone confirmed the diagnosis of Lassa fever, an investigation was organized to determine how the disease was spreading and -- if it was not from one person to another -- what the ultimate source might be. Because Lassa virus under the electron microscope resembles lymphocytic choriomeningitis virus and other arenaviruses (which also cause chronic infections in particular rodents), the investigators reasoned, by analogy with the spread of the lymphocytic choriomeningitis virus, that some West African rodent may be susceptible to Lassa virus infection and may infect humans through contaminated urine.
The epidemiologic investigation identified 63 cases in the two years ending October 1, 1972. Fourteen (4.4 per 1000 population) were in Panguma and 18 (1.6 per 1000) were in Tongo; both towns are in the center of the diamond-mining area in the eastern province. The rest were scattered through other villages and towns in that province. Twenty-four patients had died (38 percent). Only 6 of the 63 were known to have had contact with a patient with Lassa fever in the month before they became ill -- 5 members of the Panguma Hospital staff and 1 household contact of a case.
A survey of the households of cases and a random sample of the households of controls in Panguma and Tongo showed that Lassa virus infection clustered in certain households. Excluding the known cases, antibody to Lassa virus was found in 13 percent of the people in case households (27 of 206) but only 6 percent of those in control households (16 of 255). Trapping and testing of a wide variety of animals yielded one -- Mastomys natalensis, known as the multimammate rat -- that was infected with Lassa virus (Ref. 7). That virus was recovered from 14 of 82 mastomys (16 percent) and none of 371 representatives of other species. Of the mastomys trapped in and around the household compounds of two persons with fresh cases of Lassa fever, more than 80 percent were positive for the virus. Mastomys is a feral rat that competes with Rattus rattus for location in human residences in much of sub-Saharan Africa. Social upheaval in the diamond-mining area may have tipped conditions in some way to favor mastomys, which -- once present in a typical West African village household -- would have ample opportunity to contaminate food supplies and surfaces with urine containing virus. Research in Sierra Leone since 1972 has focused on seeking drugs to treat Lassa fever, instituting barrier nursing to prevent person-to-person spread in hospitals, and identifying ecologic factors that might discourage mastomys from occupying houses (Ref. 8).
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