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Laboratory and Field Evaluation of a Direct Filtration Technique for Recovery of Schistosome Cercariae^*

H. Kloos, C. H. Gardiner, A. Selim AND G. I. Higashi

The use of cercariometry in epidemiological studies has been limited by turbidity, difficulty of recovering cercariae at low concentrations in natural waters, and by complex apparatus that requires a power source. The technique of differential filtration developed by Theron has been modified and tested in the laboratory and in the field in Upper Egypt for detection of Schistosoma haematobium cercariae. A recovery filter with a pore size of 30 µm was found to give the best results. The recovery rate in the laboratory was 51% on filtration of 5-liter samples of formalin-treated water, and 19–30% with 10- and 20-liter samples. This moderate efficiency is offset by simplicity of the technique, rapid filtration, and ease of reading the recovery filter, permitting sampling of large volumes of water in the field by persons with little training. Average densities of 0.024 cercariae per liter of water were found at the Nile and 0.029 cercariae per liter in the irrigation canals. Most cercariae were recovered between 0700 and 0900 hours.

Accepted for publication July 1, 1981.

^* The authors want to express their appreciation to Mr. Hugh E. Darby of Fresno for his generous assistance with designing and constructing the apparatus, to Drs. Andre Theron and Michael A. Prentice for their advice and encouragement, and to Captain Raymond Watten, NAMRU-3, and Dr. Donald Heyneman, University of California, San Francisco, for providing facilities for testing the technique in the laboratory. This study was supported by a grant from the Edna Mc-Connell Clark Foundation and by the United States Medical Research and Development Command, Work Unit No. MR041.09.01-0153. The opinions and assertions contained herein are the private ones of the authors and are not to be construed as official or as reflecting the views of the Department of the Navy or the Naval Service at large.

Address reprint requests to: Dr. H. Kloos, 2307 N. Backer, Fresno, California 93703.

Present address: 2307 N. Backer, Fresno, California 93703.

Present address: Department of Veterinary Disease Pathology, Armed Forces Institute of Pathology, Washington, D.C. 20306.

Present address: Department of Epidemiology, School of Public Health, The University of Michigan, Ann Arbor, Michigan 48109.