A Modification of the Formol-Ether Concentration Technique for Increased Sensitivity in Detecting Schistosoma Mansoni Eggs

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  • San Juan Laboratories, Bureau of Laboratories, Center for Disease Control, Public Health Service, U. S. Department of Health, Education, and Welfare, Puerto Rico Nuclear Center, San Juan, Puerto Rico 00936

The low Schistosoma mansoni egg counts generally found in Puerto Rico require a sensitive technique for epidemiologic studies. The Ritchie formol-ether concentration technique has been modified to make it more useful for this purpose. The modified technique was compared with the older technique by analyzing fecal specimens from ten individuals with varying levels of infection. It was also compared with the Kato thick-smear technique with specimens from 25 other patients. In both series, three replicates of each technique were done on each stool. Results indicated that the modified technique was more sensitive than the older technique in terms of number of eggs counted, and that the sediment was smaller and clearer and required about 15% less time to examine. In comparison with the Kato technique, it was more successful in detecting light infections, although at higher levels of intensity, when expressed on an eggs-per-gram basis, the thick smear detected relatively more eggs. For the series as a whole, coefficients of variation for the three replicates done on each stool were smaller for the modified concentration technique than for the Kato technique. This is interpreted to reflect satisfactory reproducibility of the concentration technique when compared to the Kato technique. The concentration technique has the additional advantages over the thick smear of detecting other intestinal parasites and allowing for transportation and storage after feces are preserved in formalin.

Author Notes

San Juan Laboratories, CDC, PHS, HEW, GPO Box 4532, San Juan, Puerto Rico 00936. Send reprint requests to this address.

Director, Puerto Rico Nuclear Center, San Juan, Puerto Rico 00935.