Volume 13, Issue 4
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



Experiments conducted with 57 rhesus monkeys indicated that exposure to irradiated cercariae induced a marked resistance to a subsequent challenge with nonattenuated cercariae of . This acquired resistance was manifested by the ability of monkeys to survive otherwise lethal infections, by a greatly decreased worm burden, and by a much smaller number of eggs in the stools throughout the course of the infection. Furthermore, at necropsy the liver, spleen and intestines of animals which had been previously immunized appeared essentially normal grossly except for some occasional discoloration, hemorrhages and ascites.

Cercariae irradiated at 2,500 roentgens induced a greater protection than cercariae irradiated at 4,000 roentgens and much greater protection than cercariae irradiated at 10,000 roentgens. Five exposures to 5,000 irradiated cercariae induced greater protection than a single exposure to 25,000 cercariae. A considerably reduced number of worms was observed in the immunized monkeys as late as four months after challenge.

Detailed histopathological observations confirmed the parasitological data. Extensive quantitative serological studies showed that exposure to irradiated cercariae produced rapid antibody response. The geometric mean of titers in monkeys exposed to irradiated cercariae was similar to that in monkeys exposed to nonirradiated cercariae. No correlation was observed between the level of irradiation and antibody production, nor between the rise of antibody titers and acquisition of immunity. All titers decreased rapidly following challenge. This decrease was inversely related to the amount of acquired resistance observed.

A note of caution must be raised because of the histologic findings of apparent hypersensitivity damage to colonic blood vessels and the liver in some of the immunized monkeys. These findings hint that immunization, while clearly affording protection against the lethal effects and extensive tissue damage of acute schistosomiasis, may possibly enhance some of the late deleterious effects of the disease and point out the need for further studies prior to considering irradiated cercariae safe for immunization of humans against schistosomiasis.


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