Following a single exposure of Macaca mulatta to 1000 cercariae of Schistosoma mansoni, nearmaximum egg recoveries persisted for only 60 to 90 days; they were at a low level by 200 days and negligible within 300 days. When challenged about 2 years after initial exposure, no increase or renewal of egg production occurred. When a series of 26 monthly exposures (25–50 cercariae each) was given, egg-counts increased for about 200 days, then remained at a moderately constant level for about 300 days, declined sharply thereafter, and became negligible before the series of exposures was completed. Another group of animals received varied combinations of moderate and light exposures at varied intervals. Some of these animals acquired only light infections and failed to resist additional exposures 250 or 500 days later. Among the animals used in the various experiments, there was evidence of considerable individual variation in the natural and acquired resistance.
The number of worms reaching the hepatic portal system of resistant animals after a second challenge was determined by examining the animals 6 to 65 days after exposure to cercariae. No worms were found in the liver 6 days after the challenge. At 15 days a few schistosomules were present. In animals examined at 22 to 42 days about 10% of the cercariae used in the challenge exposure had reached the liver. These worms were either undeveloped or stunted and apparently would not have reached maturity. At 55 and 65 days, stunted worms were present in two animals but in two others the worms had essentially disappeared. In one animals examined 97 days after the first challenge, no worms were found.
Present address: Office of the Surgeon, Hq. Fourth Army, Ft. Sam Houston, Texas.
Present address: Division of Immunology, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington 12, D. C.
The authors wish to acknowledge the professional and technical assistance of P. M. Bauman, Awilda Salas, Wilda Knight and L. A. Berrios-Duran. The authors also are grateful to Dr. E. Harold Hinman, formerly Dean of the School of Medicine, School of Tropical Medicine, University of Puerto Rico, for his co-operation and interest in the project, as well as for supplying the monkeys and facilities for their care; and to Dr. Seymour Garson, Dr. Franz van Lichtenberg, and Major Robert I. Anderson, for their interest and suggestions.