DETECTION OF ONCHOCERCA-VOLVULUS DNA IN POOLS OF WILD-CAUGHT SIMULIUM-OCHRACEUM BY USE OF THE POLYMERASE-CHAIN-REACTION

The presence of Onchocerca volvulus DNA in experimentally infected fli es can now be detected by use of the PCR, so that, for example, one in fected Simulium damnosum can be detected in a pool of 100 uninfected f lies or one S. ochraceum can be detected in pools of 20-40. As this PC R technique is specific for O. volvulus, the results are not confounde d by the presence of other, unimportant, Onchocerca species, and the t echnique could replace time-consuming, manual dissection of flies. In 1996 and 1997, pools of 16-21 Simulium ochraceum were tested by the PC R technique. These flies had been collected biting man, between 1992 a nd 1994, from two hyperendemic coffee estates (fincas) in Guatemala, a nd stored in commercial (95%) ethanol. Collections at finca Buena Vist a (869 flies in 52 pools) were made 1-2 weeks and 46 weeks after 45% o f eligible subjects had been treated with ivermectin for the first tim e. At finca El Brote, collections (360 flies in 18 pools) were made 13 weeks before and 7 weeks after 97% of eligible subjects had received their first treatment. DNA was easily recovered from simuliids that ha d been stored in ethanol for up to 4 years. Of the nine pools of flies with visible blood collected at Buena Vista, each of 20 flies, eight tested positive for O. volvulus DNA. In flies without blood, 13 of 22 pools collected at Buena Vista just after treatment tested positive, w hereas there were 14 positives in 22 pools taken 46 weeks later (P > 0 .05). At El Brote, nine of 10 pre-treatment pools were positive, compa red with three of eight taken 7 weeks post-treatment (P = 0.04), indic ating that the treatments in this finca had reduced infection in the v ector, and possibly transmission, by about 60%. A sub-sample of Buena Vista flies was divided into 19 sets of three separate sub-pools conta ining heads, thoraces and abdomens. Three pools of heads alone were po sitive, and had corresponding pools of positive abdomens. Three positi ve pools of thoraces had negative corresponding pools of heads and abd omens. These results show that PCR can be used to determine the preval ence of O. volvulus DNA in wild-caught S. ochraceum. As the infection rates observed were higher than expected from dissections reported by other workers, PCR-determined rates may not be directly comparable wit h traditional parameters based on the dissection of flies to reveal O. volvulus larvae.