EFFECT OF IVERMECTIN OVER ONCHOCERCA-VOLV ULUS TRANSMISSION IN SOUTHERN MEXICO

We assessed the transmission potential and the mean infected and infec tive biting density of S. ochraceum in an onchocerciasis endemic commu nity in the southern Mexican state of Chiapas. In addition, the impact of first treatment of ivermectin during the peak of natural infection of host-seeking S. ochraceum populations was also evaluated. Monthly entomological sampling was carried out during the year before treatmen t to assess the seasonal biting behavior of parous host-seeking S. och raceum females, and either infected (with whatever stage of O. volvulu s) or infective females (with only third larval stage of O. volvulus). The mean of infective biting density was unimodal with the peak in Fe bruary. Higher densities of infective females were detected from Janua ry to March. Annual infective biting density estimated was 95.2 and th e annual transmission potential of 152.3. After treatment with ivermec tin, entomological collections were carried out during the December-Ap ril period. In these months, the first ivermectin mass treatment produ ced a reduction of natural infection rates of 55 per cent in the mean infected females density (p< 0.001) and of 30 per cent in the transmis sion potential. However, significative differences were found in parit y rates of biting females during the evaluation periods. Decrease of i nfection rates in vector populations was caused by immediate effect of ivermectin over the skin microfilariae. Microfilariae community load available for transmission declined significantly from 14 Dmg/mg to 5. 9 Dmf/mg. These means were statistically different (p< 0.001) and were reduced by 58 per cent from the baseline value.