STUDIES ON INFECTION AND IMMUNITY IN EXPERIMENTAL TYPHOID FEVER II. SUSCEPTIBILITY OF RECOVERED ANIMALS TO RE-ExPosuRE* Bx SIDNEY GAINES,* PH.D.,

While it is commonly believed that recovery from typhoid fever confers a fairly solid immunity, the older literature contains a number of reports of second attacks (1). Generally speaking, the second attack rate has been reported to range from 1 to 4 per cent, a figure compatible with (although by no means proving) the assumption that recovery leads to immunity. However, there are significantly higher second attack rates on record (15 per cent, (1)) high enough to raise doubts as to the assumption stated above. An outstanding recent example of such a report is that of Marmion, Naylor, and Stewart (2), who published an account of recurrences of typhoid fever in a British Royal Air Force unit in the Suez Canal Zone in 1950. Two large outbreaks, occurring in the same community within 5 months, produced 11 instances of second infections. The first outbreak was caused by phage-type J bacilli and the second by phage-type El organisms, indicating that the second cases were new attacks of the disease rather than relapses of the original infection. These workers concluded that an attack of typhoid fever confers only a moderate degree of specific immunity, and explained the usual low incidence of multiple infections, at least in part, on the lack of re-exposure to typhoid bacilli. Studies on experimental typhoid fever in progress at this institution for a number of years have confirmed and extended the findings of Griinbaum (3), and Metchnikoff and Besredka (4) that a disease can be produced in chimpanzees with clinical, laboratory, and histopathological findings closely resembling those occurring in human typhoid infections (5-7). During the course of these investigations the status of immunity in chimpanzees following recovery from typhoid fever was examined. The response of these animals to rechallenge is described in this report.