Comparative Observations on Experimental Schistosoma mansoni Infections in African Green and Rhesus Monkeys

Abstract

The pattern of egg excretion and the number of schistosomes are compared in five African green (Cercopithecus aethiops pygerythrus and C. aethiops sabaeus) and five rhesus monkeys. Egg excretion in the green monkey was lower during the acute phase and declined more slowly thereafter. The green monkeys maintained a significantly higher number of worms over a period of many months. The number of eggs passed by green monkeys during chronic infections was much less than would be expected from the number of live worms recovered at autopsy. Although primate hosts are valuable in schistosomiasis research, rhesus monkeys, which have been used mostly, rapidly develop strong immunity to schistosomes which reduces the worm burden and egg excretion (Meleney and Moore, 1954; Vogel, 1958; Naimark et al., 1960; Meisenhelder, Olszewski, and Thompson, 1960; Sadun et al., 1961; Thompson, Meisenhelder, and Najarian, 1962). Although improvisations permit the evaluation of drugs against Schistosoma mansoni infection in the rhesus monkey (Thompson et al., 1962), acquired immunity limits its value for this and many other purposes. The importance of searching for a primate host better than the rhesus monkey has been emphasized in a report to the World Health Organization (1959). Vogel (1958) stated, without elaboration, that the survival of S. mansoni and the duration of its egg excretion is much longer in African green monkeys than in the rhesus. These considerations stimulated the present observations dealing with the course of S. mansoni infection in African green monkeys relative to rhesus monkeys. MATERIALS AND METHODS The animals infected during this work included three Cercopithecus aethiops pygerythrus (the East African green monkey) and two C. aethiops sabaeus (the West African green monkey). Four of these were studied in parallel as pairs with a monkey of each subspecies in each pair, i.e., numbers 25 and 138, 38 and 39. Received for publication 9 January 1963. As detailed accounts appear elsewhere (Najarian and Thompson, 1958; Thompson et al., 1962), the materials and techniques will be described only briefly. The monkeys were anesthetized intravenously with sodium thiamylal (Surital ?, Parke, Davis and Co.) and exposed percutaneously for 45 min to S. mansoni cercariae (Puerto Rican strain) collected from 30 or more Australorbis glabratus. The course of infection was followed by semiquantitative stool examinations for eggs; acidether preparations of 1 ml of feces were examined twice weekly from the 5th week after exposure until autopsy. Egg counts were not adjusted for the water content of the feces. Infections were assessed terminally by the number of live worms found at autopsy; following perfusion, the mesenteric and portal veins, the ground liver, and the perfusate were examined for schistosomes.