Walter Reed Army Institute of Research, Walter Reed Army Medical Center, School of Medicine, University of Puerto Rico, U. S. Army Tropical Research Medical Laboratory, Washington, D. C. 20012, Puerto Rico
Nineteen experimental monkeys were subjected to five basic regimens of cercarial exposure. Fundamentally different patterns of egg excretion were noted for the heavy and light infections. Although 12 of the animals were passing eggs at the time of the final challenge, previous challenges had indicated that all the animals were highly resistant to superinfection. At necropsy, 755 to 3,082 days after the initial exposure to cercariae, adult worms were found in only nine animals. The most interesting observations from challenges in monkeys with highly developed, acquired resistance to S. mansoni included the following: 1) there was considerable individual variation in host response; 2) a great many cercariae penetrated the skin; 3) the migration of the schistosomules was greatly retarded but 10% of them reached the liver; 4) the development of these worms was also delayed, and they did not reach maturity; 5) the number of young worms began to decline between 55 and 65 days after challenge, at the time the control animals died; and 6) the worms from the last exposure to cercariae had disappeared in all animals examined 97 or more days after challenge.