Lawrence S. Ritchie

The Biology of Schistosoma mansoni in Laboratory Rats

The course of infection with Schistosoma mansoni was studied in two strains of rats over a period of 40 weeks. An improved procedure was used for exposure of the animals to cercariae. Worm recovery rates in Sprague-Dawley rats each exposed to 1,000 cercariae were as follows: 4 weeks, 20 to 37%; 8 weeks, 4 to 7%; 16 weeks, 2 to 5%; 21 to 32 weeks, 2%; 40 weeks, 2%. Canadian Hooded rats gave similar yields. Exposures to reduced numbers of cercariae did not alter the 4-week recovery rate. However, an increased recovery (16%) resulted at 8 weeks in rats exposed to 250 cercariae. On the basis of worm recovery rates, Sprague-Dawley rats ranging in age from a few days to 24 months were equally susceptible to infection. Observations on the growth and survival of male and female worms showed that the natural defenses of the rat were more effective against female worms. But neither sex attained the sizes characteristic of worms from mice. There was a marked tendency for worms to inhabit the liver rather than the mesenteries of the rat. Eggs were not found in the feces or intestines, and those in the liver were mostly immature, degenerate, or mere shells. A few miracidia were recovered, but these failed to produce infection in snails. The biology of S. mansoni was similar in SpragueDawley and Canadian Hooded rats. The use of the rat is discussed in relation to studies on the mechanisms of host resistance to S. mansoni. Several investigators have concluded that the

Effects of water velocities on worm burdens of animals exposed to Schistosoma mansoni cercariae released under laboratory and field conditions

Abstract 1. 1. The infectivity of cercariae was reduced in exposed mice when water velocities attained 2.7 miles per hour. 2. 2. Intermittent drying intervals between animal exposures to schistosome cercariae showed no detectable difference in animal worm burdens. 3. 3. Cercariae-free flowing waters following animal exposure to cercariae did not reduce the worm burden. 4. 4. The animal worm burden acquired from cercariae-infested waters was inversely proportional to the distance cercariae were carried downstream. 5. 5. No loss in the number of released cercariae carried as far as 2000 feet downstream was detected by Rowans cercariometer apparatus; thus, suggesting that cercariae lose their infectivity prior to loss of morphological integrity. 6. 6. Some cercariae carried up to 5000 feet downstream were capable of completing their life cycle in the vertebrate host. 7. 7. In a carefully controlled experiment with exposures between one to 120 minutes, mice exposed 30 minutes or longer not only acquired more worms per mouse but had a higher infection rate than mice exposed 15 minutes or less. 8. 8. Two important factors in limiting infections of schistosomiasis are: (1) velocity of water and (2) distance cercariae are carried downstream.

Schistosoma mansoni infections in Cercopithecus sabaeus monkeys.

Cercopithecus sabaeus (green) monkeys discharged eggs up to 3.5 years after initial exposure and for a comparable period after reexposure. The number and development of worms from challenge infections were similar to initial infections, but excretion of eggs was delayed after challenge. Monkeys exposed repeatedly to moderate or small numbers of cercariae tolerated a burden of worms that would have been lethal as a single initial infection. There was evidence that worms lived for 5 years, and without regression in size. Large accumulations of eggs still occurred in the liver and intestine after 5 years, and the relative numbers of immature, mature, and degenerate eggs were similar to early stages of first infections. The response of the green monkey to S. mansoni is in marked contrast to that of Macaca mulatta, which suppresses excretion of eggs after 6 to 9 months and quickly acquires strong resistance to challenge infections. The longer output of eggs by the green monkey is more like what occurs in man. A corresponding comparison of acquired resistance is not possible because too little is known about resistance imposed against schistosomes in the human host. The rhesus and the green monkeys may provide a valuable experimental model for investigating the immune mechanism against schistosomes. The green monkey may be useful in the evaluation of schistosomicidal drugs. The need for experimental hosts of human schistosomes that react to the infections more like man than the commonly used Macaca mulatta (rhesus) monkey was emphasized by Sadun et al. (1966a). Their studies and those of Sadun and Bruce (1964) on the biology and pathology of schistosomiasis in 10 species of primates showed that worm recovery rates were highest in three species of Macaca, but self-cure occurred within a few months. The baboon and chimpanzee discharged eggs for much longer periods, but worm recovery was relatively low. For other species, the infections were abortive from the beginning. Only the chimpanzee developed pipe-stem fibrosis of the liver. The need for information on other primate hosts was noted by the above authors. Information on acquired resistance among primate hosts is essentially limited to the rhesus monkey. For man, too little is known to recognize a suitable model. The current study is concerned with another primate, Cercopithecus sabaeus, the green Received for publication 22 May 1967. * Assigned to Walter Reed Army Institute of Research, Washington, D. C. (20012), with duty station at the Department of Preventive and Social Medicine, University of Ibadan, Ibadan, Nigeria. t Present address: Tropical Disease Section, Ecological Investigations Program, Communicable Disease Center, U. S. Public Health Service, San Juan, Puerto Rico. monkey, and includes observations on the length of the prepatent period, duration and pattern of egg discharge, life-span of the infection, distribution and condition of eggs in tissues, and acquired resistance. These objectives were not all completed as originally planned, due to closing of this laboratory. A number of investigators have exposed the green monkey to human schistosomes for limited objectives, and only a few animals were involved, or the infections were of short duration (Archibald, 1923; Black, 1932; Vogel and Minning, 1953; Kuntz, 1955; Vogel, 1962; Hsii and Hsii, 1962; Meisenhelder and Thompson, 1963; and Ritchie et al., 1964). The green monkey was found to discharge eggs for relatively long periods. In contrast to M. mulatta, challenge infections developed to maturity in animals that had been infected

Enhancement of acquired resistance against Schistosoma mansoni in albino mice by intraperitoneal immunizing exposures.

Immunization of albino mice against Schistosoma mansoni by multiple, low-grade cercarial exposures resulted in earlier and higher levels of resistance when exposures were administered intraperitoneally (ip) than when the percutaneous (pc) route was used. Mice which received a course of ten weekly ip injections of ten cercariae each had significantly fewer worms than their controls. This did not occur when the exposures were made percutaneously. Both courses elicited strong resistance against challenge, although the level of this resistance was higher following ip immunization. With pc immunization resistance apparently was directly related to the maturation of immunizing infections and accumulation of eggs. This was not the case with ip immunization since significant resistance against challenge was evident only 7 days after the last of five weekly ip exposures. In ip immunization the basis for resistance may be the prolonged immunologic stimulation of lungs and liver resulting from the delayed escape of young worms from the peritoneal cavity, and the tendency of worms that remain in the peritoneal cavity to persist as schistosomular stages for extended periods. Lurie and DeMeillon (1957) have reported that a series of intraperitoneal injections with small numbers of cercariae elicited strong, rapidly developing resistance against Schistosoma mansoni in mice. Essentially complete resistance was achieved with all of several schedules, suggesting that even those worms which must have developed before resistance became effective had been eliminated relatively soon thereafter. Other investigators working with the more conventional plan of immunization followed by challenge usually have not observed such remarkable levels of resistance (see reviews by Kagan, 1958; Stirewalt, 1963). The percutaneous route of exposure has been used almost invariably, and to the limited extent that single and multiple immunizing exposures have been compared, it has not been shown conclusively that either procedure is superior to the other. The study reported herein was undertaken to compare the intraperitoneal and percuReceived for publication 10 July 1964. * Presented in part at Annual Meeting, American Society of Parasitologists, November 1963. taneous routes of immunization against S. mansoni in mice, using a series of low-grade cercarial exposures. The occurrence of important differences between mean worm burdens and response to challenge infection following exposures by one route as compared with the other, indicate that the route by which an immunizing infection was acquired may be an important factor in the development of resistance. MATERIALS AND METHODS Bagg strain albino female mice with an initial weight of 18 to 22 g were used exclusively. Cercariae of a stock laboratory strain of Puerto Rican S. mansoni were used for immunization and challenge. The several cercarial pools necessary for each experiment were always obtained from groups of 30 to 50 laboratory-reared and infected Australorbis glabratus. Immunization consisted of a series of ten (or in one case only five) percutaneous or intraperitoneal exposures performed by conventional methods. In Experiments 1 and 3, on the occasion of each immunizing exposure, a new group of five mice were exposed in parallel in order to assay cercarial infectivity. The cumulative mean from these groups provided a basis for determining the resistance that mice acquired during the course of multiple exposures. In Experiment 2, however, five to ten

Effect of the duration of Schistosoma mansoni infections on the degree of protection against subsequent exposures

Abstract Prolongation of initial infection with S. mansoni in rodents results in decreased levels of acquired resistance despite continued presence of adult worms. In the albino rat a significant decrease in resistance occurs between 32 and 64 weeks after primary exposure, although resistance is still at a significant level at the latter time. In hamsters acquired resistance is lost sometime after 12 weeks and before 32 weeks. The mechanism of acquired resistance in the rat develops independently from the mechanism responsible for innate resistance in this host; is presumably unrelated to the latter; and has characteristics similar to the same mechanism in the highly susceptible mouse and hamster.