Volume 42, Issue 2
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



The impact of mass treatment with ivermectin on the intensity of transmission by the black fly () was evaluated on the Liberian Agricultural Company rubber plantation in Liberia, West Africa. The adult pre-treatment prevalence of onchocerciasis was > 80%, and the average intensity of infection was 5.35 mf/mg of skin. The drug was administered at 2 annual intervals, reaching 58–60% of the ∼ 14,000 people living in 73 camps. Landing/biting catches of black flies made in central and peripheral plantation zones indicated similar fly activity before and after ivermectin treatment (man hr index of 2.1 and 2.4 within the plantation, and 10 and 10.9 outside the plantation, respectively). The number of infected flies with developing larvae (L, L, L stages) of in treated areas was reduced by 93.4–95%; the number of infective flies with L larvae was reduced by 81.7–89.3%. Parasite loads of infected (L, L) and infective flies (L stages only) outside the plantation also decreased by 86.8% and 80%, respectively. Monthly transmission potential (MTP) showed a similar decrease: from 22.9 to 5.8 (74.6% reduction) in the treated area, and from 210 to 158.8 (24.4% reduction) in untreated areas. Mass treatment with ivermectin efficiently controlled, and at least temporarily interrupted, transmission of by black fly vectors.


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