Daily Periodicity of Schistosoma Mansoni Cercariae in Puerto Rican Waters

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The density of Schistosoma mansoni cercariae in Puerto Rican waters was found to undergo a marked daily fluctuation. In the rivers and stream studied, the peak of cercarial abundance was found to occur between 11 a.m. and 1 p.m. Few cercariae were present in the water before 9 a.m. or after 4 p.m. In a heavily infested pond, cercariae appeared at 10 a.m. and their numbers accumulated to a peak at 4 p.m. After 4 p.m., the density declined rapidly at first and then more slowly to a low at 8:30 a.m. the following morning. On the basis of these experiments, the hazard of exposure to cercariae in Puerto Rican waters is considered to be from 20 to 100-fold less between the hours of 5 a.m. and 9 a.m. than at most other times of the day.

The recovery apparatus and technique used in these experiments are suggested as an accurate timesaving device for stream survey work, and as a useful aid to the development of more economical means of schistosomiasis control.

Author Notes

Puerto Rico Field Station, Communicable Disease Center, Public Health Service, P. O. Box 52, San Juan, Puerto Rico.