Laboratory comparison of four cercaria recovery techniques

The sampling of rivers and large streams for schistosome cercariae requires a method that is both highly accurate and capable of analysing several hundred litres of water regardless of turbidity. Four techniques-direct filtration by Rowan's vacuum paper filtration method, a modification of Rowan's method, continous centrifugation, and an overlay filtration method-were compared for recovery accuracy, volume of water sampled, and ease of reading under simulated field conditions. The modified Rowan technique ranked first for mean recovery accuracy (79.4%), followed by continuous centrifugation (53.8%), Rowan's technique (25.9%), and the overlay method (22.4%). With the direct filtration techniques, the filtering rate was directly related to certain turbidity factors, the maximum being 10 litres/min with the modified Rowan technique and clear water. The continuous centrifugation and overlay methods were independent of turbidity variations. Filters could be read most quickly (mean time 3.4 min/filter) with the continuous centrifugation technique because the concentrated sample made it possible to use small (47-mm) filters. Mean reading times with the other techniques (89-mm and 90-mm filters) ranged from 5.6 to 8.4 min per filter but were highly variable in relation to turbidity, particle size, and number of cercariae recovered. Direct filtration by the modified Rowan technique is considered to be the simplest and most efficient method. When flocculation is included, the resulting additional loss of cercariae (16%) is within acceptable limits.