The Molluscan Intermediate Host and Schistosomiasis Japonica

II. Observations on the Production and Rate of Emergence of Cercariae of Schistosoma japonicum from the Molluscan Intermediate Host, Oncomelania quadrasi


This investigation was conducted in order to obtain greater numbers of naturally shed cercariae for use in the laboratory of the Commission on Schistosomiasis in the Philippine Islands. The observations and experiments performed provided the following observations:

  1. 1.Alkaline water (optimum pH 7.6) was a critical factor in the natural release of cercariae of Schistosoma japonicum from the molluscan host, Oncomelania quadrasi on Leyte.
  2. 2.Temperature variations of the water, between 19° and 30°C., did not influence the release of cercariae from the naturally infected host, Oncomelania quadrasi.
  3. 3.Artificial illumination and darkness were similarly unimportant factors; however the natural release of cercariae is of a definite nocturnal periodic nature. Cercariae were released in significant quantities throughout the early evening and continued emerging after midnight, with the peak of production occurring from 9:00 to 11:00 p.m., steadly decreasing as daylight approaches.
  4. 4.Naturally infected Oncomelania quadrasi will liberate cercariae on two to three successive nights, and then an equally long period will pass before any considerable number of cercariae will emerge. Not a single known positive O. quadrasi under observation for 30 days discharged cercariae consistently for more than three consecutive days, thus indicating that the natural emergency of S. japonicum cercariae from this molluscan host is of a cyclic nature.

Author Notes

Army Medical Department Research and Graduate School, AMC, Washington, D. C.

United States Public Health Service, Lexington, Ky.

New York University