Search

The major vectors of the blinding form of human onchocerciasis in West Africa are two blackfly species, Simulium sirbanum and Simulium damnosum s.s. (Diptera: Simuliidae), identified at the adult stage as the 'savanna group' of the Simulium damnosum complex. In 1988, in the central part of Sierra Leone, the average daily biting rate (females/man/day) by savanna blackflies (mostly S. sirbanum) during the peak of the dry season (April-May) was 59.9, making up 69.1% of total captures on average. There was evidence of a strong long-range immigration of adult females of S. sirbanum through eastern Guinea in the dry season, with a reverse movement towards Guinea in the rainy season. Therefore, in 1989, the World Health Organization's Onchocerciasis Control Programme (OCP) extended its vector control operations from central West Africa to rivers of central and northern Sierra Leone, and to rivers of eastern Guinea. Four years of efficient larviciding drastically reduced adult populations of S. sirbanum in Sierra Leone. In the peak of the dry seasons of 1993 and 1994, the average biting rate by savanna blackflies in central Sierra Leone had dropped to 1.0, making up only 4.3% of total captures on average. Yearly biting rates by S. sirbanum in central Sierra Leone were therefore reduced to 2% of their pre-intervention levels. Based on larval samples, the S. sirbanum has been replaced by two forest species, S. leonense in the south and S. squamosum in the north. Since 1992, it has been possible to calculate accurate transmission rates for blinding onchocerciasis, based on DNA-probe identifications. From 1993, the risk of transmission has not only been reduced by vector control but also by mass distribution of ivermectin to rural communities. In terms of control strategy, the authors conclude that larviciding operations could be alleviated in central Sierra Leone without increasing the risk of blinding onchocerciasis transmission, as long as the migration of S. sirbanum through eastern Guinea and northern Sierra Leone is prevented.

Background Mass treatment with ivermectin controls onchocerciasis as a public health problem, but it was not known if it could also interrupt transmission and eliminate the parasite in endemic foci in Africa where vectors are highly efficient. A longitudinal study was undertaken in three hyperendemic foci in Mali and Senegal with 15 to 17 years of annual or six-monthly ivermectin treatment in order to assess residual levels of infection and transmission, and test whether treatment could be safely stopped. This article reports the results of the final evaluations up to 5 years after the last treatment. Methodology/Principal Findings Skin snip surveys were undertaken in 131 villages where 29,753 people were examined and 492,600 blackflies were analyzed for the presence of Onchocerca volvulus larva using a specific DNA probe. There was a declining trend in infection and transmission levels after the last treatment. In two sites the prevalence of microfilaria and vector infectivity rate were zero 3 to 4 years after the last treatment. In the third site, where infection levels were comparatively high before stopping treatment, there was also a consistent decline in infection and transmission to very low levels 3 to 5 years after stopping treatment. All infection and transmission indicators were below postulated thresholds for elimination. Conclusion/Significance The study has established the proof of principle that onchocerciasis elimination with ivermectin treatment is feasible in at least some endemic foci in Africa. The study results have been instrumental for the current evolution from onchocerciasis control to elimination in Africa., Author Summary The control of onchocerciasis, or river blindness, is based on annual or six-monthly treatment with ivermectin of populations at risk. This has been effective in controlling the disease as a public health problem but it was not known whether it could also eliminate infection and transmission to the extent that treatment could be safely stopped. Many doubted that this was feasible in Africa. A study was undertaken in three hyperendemic onchocerciasis foci with seasonal transmission in Mali and Senegal where treatment has been given for 15 to 17 years. As a result of this treatment, infection and transmission levels had fallen everywhere below postulated thresholds for elimination. Treatment was therefore stopped in each focus. Follow-up evaluations up to 5 years after the last treatment showed no evidence of recrudescence after stopping treatment but instead a consistent decline in infection and transmission levels, reaching zero in two sites. The study has established the proof-of-principle that onchocerciasis elimination with ivermectin treatment is feasible in at least some endemic foci in Africa. The results of the study have greatly contributed to the current evolution from onchocerciasis control to elimination in Africa.

Background Mass treatment with ivermectin is a proven strategy for controlling onchocerciasis as a public health problem, but it is not known if it can also interrupt transmission and eliminate the parasite in endemic foci in Africa where vectors are highly efficient. A longitudinal study was undertaken in three hyperendemic foci in Mali and Senegal with 15 to 17 years of annual or six-monthly ivermectin treatment in order to assess residual levels of infection and transmission and test whether ivermectin treatment could be safely stopped in the study areas. Methodology/Principal Findings Skin snip surveys were undertaken in 126 villages, and 17,801 people were examined. The prevalence of microfilaridermia was, Author Summary The control of onchocerciasis, or river blindness, is based on annual or six-monthly ivermectin treatment of populations at risk. This has been effective in controlling the disease as a public health problem, but it is not known whether it can also eliminate infection and transmission to the extent that treatment can be safely stopped. Many doubt that this is feasible in Africa. A study was undertaken in three hyperendemic onchocerciasis foci in Mali and Senegal where treatment has been given for 15 to 17 years. The results showed that only few infections remained in the human population and that transmission levels were everywhere below postulated thresholds for elimination. Treatment was subsequently stopped in test areas in each focus, and follow-up evaluations did not detect any recrudescence of infection or transmission. Hence, the study has provided the first evidence that onchocerciasis elimination is feasible with ivermectin treatment in some endemic foci in Africa. Although further studies are needed to determine to what extent these findings can be extrapolated to other areas in Africa, the principle of onchocerciasis elimination with ivermectin treatment has been established.