A comparison of individual dissection and mass separation for recovery of Onchocerca larvae from vector black flies

The effectiveness of a mass separation technique, previously used for the extraction of larvae of lymphatic-dwelling filarial worms from batches of vector mosquitoes, was tested as a means of recovering infective-stage larvae of Onchocerca volvulus from Simulium ochraceum in Guatemala. Blood-engorged flies, collected from 10 infected human attractants, were maintained for 9 days to allow ingested microfilariae to develop to the infective stage. The numbers of Onchocerca larvae recovered after groups of these flies were crushed and washed into tissue culture fluid in Baermann funnels was compared with the numbers obtained by individual dissections of flies fed on the same subjects. The mass separation procedure gave a mean recovery rate of 0.03 larva/fly and detected larvae only in flies which had fed on those subjects with the highest microfilarial skin densities. Dissections yielded 0.50 larva/fly (a 16.7-fold increase) and detected larvae in flies collected from all test subjects. The explanation for the ineffectiveness of the mass separation technique may lie in the observed sluggishness of infective-stage Onchocerca larvae and a consequent inability to free themselves from the fly fragments in the Baermann funnel.