The Relative Susceptibilities of the Commonly-Used Laboratory Mammals to Infection by Schistosoma Mansoni 1

Abstract

Summary and Conclusions Eight species of animals have been studied as laboratory hosts for a strain of Schistosoma mansoni originally from Puerto Rico, and maintained in the laboratory in albino mice and golden hamsters. Although the white mouse has long been the host of choice for work on this parasite, it is obvious that different experimental designs demand diverse criteria for the selection of a laboratory definitive host. Therefore, judgment of the suitabilities of the animals studied for specific experimental purposes was based on the following considerations: (a) percentage of parasites maturing in the hosts; (b) fatality rate among the hosts exposed; (c) relative numbers of ova passed in fecal specimens; (d) size of the worms recovered at autopsy; (e) gross manifestations of immune mechanisms, if present; (f) ease of handling and care of the hosts; and (g) ease of manual autopsy for removal of the parasites. Since, in this study, there was no significant difference in parasite load between “clipped” and “unclipped” mice, rats, hamsters, and guinea pigs, segregation of results on this basis was unnecessary. For general laboratory maintenance of the parasite there are three satisfactory hosts: mice, hamsters, and cotton rats, each having advantages and limitations peculiar to its species' relationship to the parasite. Hamsters may well be considered the host of choice for maintaining the life-cycle of the parasite because a high percentage of the cercariae, 31.8 per cent, as compared to 22.1 per cent in albino mice and 17.2 per cent in cotton rats, reach maturity in them; the recovery of adult schistosomes from their veins is not difficult; greater nunbers of viable ova are available from them than from either mice or cotton rats since the ova are not isolated from the more friable liver tissue by cellular infiltration of fibroid tissues; and they are easily handled. On the other hand, the high death rate encountered is a distinct disadvantage making it necessary to limit exposures to 200 cercariae or less per hamster, and the duration of infections to not more than ten weeks. It is possible, however, that the very lack of resistance to the parasite which has ranked the hamster high among suitable hosts may be an unfavorable characteristic for long range experiments since this lack of selectivity in the laboratory reservoir host may lead, over a period of time, to a less hardy strain of schistosome. Cotton rats are also favorable hosts for laboratory maintenance of the parasite because, even in the higher exposure densities, they do not succumb easily to the infection and therefore supply viable ova over a long period. Cotton rats are especially suitable for use in studies involving the anatomy, physiology, and histology of the parasite requiring numbers of large, well-developed adult worms. These animals are more difficult to handle than mice and hamsters, and manual autopsy is not so easily performed as in the latter hosts. The comparably lower worm return can be compensated for by the use of greater numbers of cercariae at exposure. The fact that there is evidence of some inhibitory factor which limits parasitization as the cercarial density is increased must be taken into consideration in the experimental design, however, if cotton rats are to be used. Mice, of course, are satisfactory as laboratory hosts since they are not difficult to autopsy and permit of easy and economical care. They do not allow the maturing of as high a proportion of the schistosomes or produce as many viable ova in the feces as do hamsters, but their death rate from the disease is much lower. The worms, though active and well-developed, are not as large as those found in cotton rats. For chemotherapeutic studies, in which large numbers of hosts must be treated and the exact number of parasites recovered and counted at autopsy, it appears that mice are the hosts of choice. The fact that the pathology found in mice is very similar to that described in human infections of S. mansoni makes the mouse a valuable host from this standpoint also. While cotton rats may be used as well for supplying histological material for study of the pathology of schistosomiasis, it is natural that the same animal as that used in treatment studies is the more desirable host since valid comparison is then possible between treated animals and untreated controls. Because the action of their immune mechanism is grossly evident, rats are especially valuable not only for general immunological work, but also for specific studies of means of modifying their resistance. It is obvious that their use as laboratory hosts is restricted for the very reason that they are capable of spontaneous cure of the infection. Dogs, cats, rabbits, and guinea pigs are also unsatisfactory hosts for S. mansoni in one or more considerations. Dogs are entirely refractory to our strain of S. mansoni. Cats, rabbits, and guinea pigs, in our experience, provide no ova, and the latter two yield poor worm returns. Manual worm recovery from all these hosts is difficult. On the other hand, the parasites recovered from cats and rabbits are large and well-developed. These animals, using clinical manifestations and evidences of gross pathology as a guide, possess varying degrees of natural immunity to this schistosome, and possibly also different mechanisms of resistance. It should be possible, therefore, to employ the negative factors listed above as an aid in determining the key to host selectivity of the trematode.