Heterologous immunity against Schistosoma mansoni in mice by administration of Heterobilharzia americana.

Three groups of Swiss albino mice were exposed to cercariae of Heterobilharzia americana a mammalian schistosome in Southern United States. they were challenged at different intervals with cercariae of a Puerto Rican strain of Schistosoma mansoni, and a fourth group (control for the first two groups) was exposed only to S. mansoni. With a patent infection (two months) of H. americana there was a noticeable reduction of the worm recovery rates of S. mansoni and its eggs deposited in the tissues. In the two other groups exposed simultaneously or at a 3-week interval, there was no significant reduction in the recovery rates of adult S. mansoni and the number of eggs exceeded in some cases that noted for the control group. Thus a patent infection with H. americana is necessary to confer immunity against a challenge infection with S. mansoni.