Immunization with a zoophilic strain of Schistosoma japonicum: A re-evaluation of the Formosan strain of S. japonicum in rhesus monkeys

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  • 1 Divisions of Experimental Parasitology and Experimental Immunology, Clinical Medical Sciences Department, Naval Medical Research Institute, National Naval Medical Center, Bethesda, Maryland 20014, and Department of Microbiology, The University of Chicago, Chicago, Illinois 60637

The zoophilic Formosan strain of Schistosoma japonicum has been reported to be very effective in immunizing rhesus monkeys against the pathogenic Japanese strain. Before its potential as a human vaccine could be evaluated, more information was needed on its protective properties against other pathogenic strains, on its ability to decrease clinical disease and on the amount of pathology resulting from its use. Rhesus monkeys given five immunizing exposures to the Formosan strain had significant reductions in quantitative egg excretions and in worm fecundity after challenge with the Philippine strain. Three of 4 such monkeys had greatly reduced worm burdens. The prepatent period was extended almost a week in immunized monkeys. Evidence is offered that fecal egg counts should not be used as a definitive criterion of immune status. Unimmunized monkeys challenged with the Philippine strain experienced a leucocytosis and hyperglobulinemia (IgG) at the onset of egg excretion, neither of which occurred in immunized monkeys. Quantitative egg counts in tissue digests demonstrated that immunization did not significantly reduce the number of eggs trapped in the liver. Granulomas in the liver and intestinal tissues were comparable in immunized monkeys and their controls. Liver and spleen to body weight ratios and serum enzyme levels were not altered either by immunization or by challenge. Immunization alone induced dermal immediate hypersensitivity reactions in all monkeys and circulating specific reaginic antibody detectable by P-K testing in 6 of 8 monkeys. Challenge with the Philippine strain resulted in reaginic antibody in all instances 60 days later. Circumoval precipitins were detected not only in monkeys harboring adult worms, but also in those exposed to only the immunizing infection. A marked eosinophilia resulted from multiple exposures to the Formosan strain. All monkeys exposed only to the Formosan strain had worms at autopsy, including mature egg-laying females. These latter animals displayed liver granulomas. These results indicate the Formosan zoophilic strain of S. japonicum can immunize partially but not completely against challenge with a pathogenic strain.