ASSESSMENT OF VECTOR MICROFILARIAL UPTAKE AS A COMPARATIVELY NONINVASIVE TECHNIQUE FOR MONITORING ONCHOCERCIASIS TREATMENT CAMPAIGNS IN THEAMERICA

Since 1992, efforts have been made to combat onchocerciasis in Guatema la through mass distribution of ivermectin. The impact of the campaign is assessed by taking skin-snips from sentinel groups within selected communities. This method gives an estimate of the prevalence and inte nsity of infection, and thus the efficacy of the treatment. In some co mmunities people are becoming reluctant to volunteer for skin-snipping , and so there is a need for an alternative technique that will give q uantitative results. In most hyperendemic communities in Guatemala, bi ting blackflies are so ubiquitous that few people object to allowing I o to 20 flies to engorge upon them. We examined data on the quantitati ve uptake of microfilariae by Simulium ochraceum before and after iver mectin distribution to see whether results similar to skin-snip data c ould be obtained. Counts of microfilariae ingested by S. ochraceum are compared to the numbers found in skin-snips from the same volunteers. In a group of 31 untreated infected persons, a skin-snip survey detec ted 64.5% positive, while feeding flies (vector microfilarial uptake, VmfU) detected 96.8%. Post-treatment, in a sample of 58 of whom 52 (89 .7%) had a history of infection, both skin-snips and VmfU detected 54. 2% Vector blood meals contained more microfilariae than a mg of skin b efore treatment, but both recorded about equal numbers after treatment . When the data set was subdivided to compare samples taken at 2-3, 6- 8 and 14-17 months post-treatment, the effect of ivermectin was still apparent at 6-8 months, but had virtually disappeared by Iq months pos t-treatment. A surprising observation was that the flies ingested fewe r microfilariae from treated persons than was expected from the skin d ensities as estimated by skin-snip. This effect lasted for over 8 mont hs, and could indicate that ivermectin has a greater effect on transmi ssion than previously suspected. We conclude that VmfU could be used a s an alternative to skin-snipping, and discuss the ethical implication s.