U. S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Laboratory of Parasitic Diseases, Bethesda, Maryland 20014
The decline in infectivity of Schistosoma mansoni cercariae was studied under controlled laboratory conditions considered favorable for cercarial survival. Cercariae of known age were tested by mouse exposure challenge at intervals up to 30 hours from the time they were shed from snails. Infectivity decreased with time and the decrease was rapid. About eight hours after the cercariae were shed, one-half the infectivity had been lost. Since there was no detected loss of infectivity in the first three hours, the time for infectivity to fall to one-half of the maximum level was less than five hours. Infectivity was about one-tenth the initial level after 14–15 hours.
Since conditions for cercarial survival in the experiments were relatively favorable, it can be expected that under field conditions, which are almost always more rigorous, the cercarial half-life is probably only a few hours even in the absence of predators.
With the technical assistance of Rodney Duvall and Judith McBride.
Present address: Pan American Health Organization, Washington, D. C.