A field-evolved differential filtration method for recovery of schistosome cercariae

Modifications to Theron 's (1979) differential filtration technique are described which improve both sensitivity and reliability. Following pre-filtration to remove larger debris and some plankton, the sample is formalized; constant stirring prevents losses due to the cercarial stickiness normally caused by the fixation process. Once fixed, Schistosoma mansoni cercariae are reliably retained and well displayed on nylon monofilament cloth of 50 micrometers pore size. Such a coarse recovery filter permits very large samples to be processed and results in filters which are easy to read. Field comparison showed the method improved sensitivity by a factor of more than 20 times and laboratory evaluation indicated a recovery rate of over 80%.