Susceptibility of Mammalian Hosts to Experimental Infection with Heterobilharzia americana

Abstract

Observations were made on the relative susceptibility of the raccoon, a natural host, and nine species of laboratory animals to infection with Heterobilharzia americana. Cercariae, collected from experimentally infected Pseudosuccinea columella, were observed to penetrate the skin of all hosts tested. There was great variation in the degree of development in the various hosts. The mouse, hamster, and especially the dog, were found to be suitable laboratory hosts; the infections in these hosts being comparable in most respects with those observed in the raccoon. In the cotton rat relatively few worms reached maturity, but viable eggs were produced. The rabbit, guinea pig, and cat exhibited a degree of host resistance which did not bar maturation of the worms but did suppress the production of viable eggs. The albino rat and rhesus monkey were very resistant, only a small number of stunted worms being recovered early in the infections. Heterobilharzia americana Price 1929 has been found in five naturally infected host species-the raccoon (Procyon lotor), bobcat (Lynx rufus), nutria (Myocastor coypus), rabbit (Sylvilagus aquaticus), and domestic dog. Although the worms obtained from the naturally infected animals were sexually mature, viable eggs in the feces were not in all instances observed or recorded. It is apparent, however, that the principal natural mammalian host of H. americana, in the areas studied, is the raccoon (Malek et al., 1961). In experimental studies on the life cycle of H. americana, it has been shown that normal development occurs in the white mouse (Lee, 1962). The purpose of the present study was to determine the relative susceptibility of various laboratory animals to infection with H. americana, and thus to determine the relative potential usefulness of these animals in experimental studies on schistosomiasis in mammals. MATERIALS AND METHODS The criteria used as a basis for comparing one animal with another were: (1) the percentage Received for publication 9 July 1962. *A portion from a thesis submitted to the Graduate School of Tulane University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The study was supported in part by research grants 2E-2 and E-2898 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Public Health Service. t Present address: Department of Parasitology, Faculty of Medicine, University of Singapore, Singapore. of cercariae recovered as worms following exposure to known numbers of the cercaria; (2) the capacity of the parasite to develop to anatomical and physiological maturity; (3) the proficiency of the worms in the production of eggs; (4) the viability of eggs recovered from the feces, intestine, and liver; (5) the length of time required for the worms to reach maturity; and (6) the pathological manifestations of infection. The animals tested included the albino mouse and rat, hamster (Mesocricetus auratus), cotton rat (Sigmodon hispidus), common laboratory rabbit and guinea pig, domestic dog and cat, and rhesus monkey (Macaca mulatta). The only raccoon used in this study was captured while still a very young animal from Marsh Island Wildlife Refuge, Louisiana, where raccoons and nutrias were found to be negative for H. americana. This raccoon was maintained in the laboratory for several months and was known to be free from Heterobilharzia infection. For each exposure cercariae were obtained from a number of experimentally infected snails (Pseudosuccinea columella) to assure the presence of both sexes. Different numbers of cercariae were used for different species of animals and different animals of the same species. Two methods were utilized to infect mammalian hosts, depending upon the size of the animals. Smaller animals such as mice and hamsters were infected by placing them in 250-ml and 600-ml beakers respectively for about 1 hr in shallow water containing cercariae. For the larger animals the following method was employed: After the animal was anesthetized with Nembutal, it was secured in the horizontal position to an animal board. On the shaved abdominal area a plastic ring, 2 cm in height, was held in place by dropping melted vaseline along the rim. The diameter of the ring varied with the volume of cercarial suspension. The desired number of cercariae was placed with a small amount of water in the center of the ring. After 1 hr,