Animal model studies on the antelope schistosomes, Schistosoma margrebowiei and S. leiperi, with particular reference to their proposed role in limiting the distribution of human intestinal schistosomiasis.

Abstract

It has been postulated that the absence of human and cattle schistosomiasis in parts of southern Africa where lechwe antelope (Kobus leche) occur is a consequence of an immunologically-mediated protection induced by repeated exposure to the cercariae of Schistosoma margrebowiei and S. Leiperi, which are common parasites of these animals. The aim of the studies described was the development of animal models in which to investigate this hypothesis. The infection characteristics of the antelope schistosomes in BALB/c mice and Mastomys Coucha were assessed. Both schistosome species reached full patency in these hosts, although S. Margrebowiei infections deteriorated rapidly in M.Coucha. While they differed markedly in terms of egg production rates and preferred sites of tissue egg deposition, both species caused severe hepatosplenomegaly and portal hypertension in the mouse model. Modulation of the granulomatous responses to ova in the tissues was demonstrated. Mice harbouring mature antelope schistosome infections displayed strong partial resistance to challenge infections with both homologous parasites and the human schistosome, S. Mansoni. However, the failure of challenge parasites to become established was considered to be due largely to changes in the portal-hepatic vasculature resulting from egg-induced immunopathology. Resistance to S. Mansoni challenge did not develop in mice infected with radiation-attenuated cercariae of the antelope schistosomes. The suitability of rats and guinea pigs as alternative models was assessed. Worm recoveries from rats were low and there was no evidence of egg-deposition. Worm yields from the guinea pig were relatively high, but sexual development was poor and short-lived. Since excretion of S. Margrebowiei eggs has occasionally been reported from humans, and since the guinea pig supports full sexual maturation of S. Mansoni, this animal appeared to provide a particularly appropriate model for the present investigation. However, repeated exposure of guinea pigs to cercariae of the antelope schistosomes, over a period of 24 weeks, failed to induce significant resistance to S. Mansoni challenge infection. The need for further experimental and field studies is discussed. An area in the Okavango Delta (Ngamiland, Botswana) has been identified as a possible site for field work.