Impact of repeated large scale ivermectin treatments on the transmission of Loa loa

The impact of large-scale treatment with ivermectin on the transmission of Loa loa was studied in a forest village in south Cameroon where loiasis was highly endemic (prevalence of 30%). After one year of parasitological and entomological surveillance without treatment, all consenting residents (788 at the beginning of the study) aged >5 years received ivermectin (200 µg/kg) every 3 months for 2 years (April 1993-95). Parasitological and entomological surveillance continued for 18 months after the end of treatment. The prevalence of loiasis was reduced to <10% and the mean microfilaraemia decreased by 90% over 2 years. The prevalence and average intensity of infection remained stable during the 18 months after treatment ended. Two vector species were identified, Chrysops dimidiata (representing approximately 90% of the fly population) and C. silacea. Compared to the pre-treatment values, the infection rate (all stages of L. loa) in Chrysops decreased by 75% and the infective rate (percentage of Chrysops harbouring 3rd-stage larvae of L. loa in the head) decreased by 85% in C. dimidiata and became zero in C. silacea following treatment. After the end of treatment, the infection and infective rates increased gradually. Although large-scale treatment seems an efficient method for the control of L. loa, the high risk of adverse effects when using the current microfilaricidal drugs renders such a strategy unacceptable.