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