BackgroundMass 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 FindingsSkin snip surveys were undertaken in 126 villages, and 17,801 people were examined. The prevalence of microfilaridermia was <1% in all three foci. A total of 157,500 blackflies were collected and analyzed for the presence of Onchocerca volvulus larvae using a specific DNA probe, and vector infectivity rates were all below 0.5 infective flies per 1,000 flies. Except for a subsection of one focus, all infection and transmission indicators were below postulated thresholds for elimination. Treatment was therefore stopped in test areas of 5 to 8 villages in each focus. Evaluations 16 to 22 months after the last treatment in the test areas involved examination of 2,283 people using the skin snip method and a DEC patch test, and analysis of 123,000 black flies. No infected persons and no infected blackflies were detected in the test areas, and vector infectivity rates in other catching points were <0.2 infective flies per 1,000.Conclusion/SignificanceThis study has provided the first empirical evidence that elimination of onchocerciasis with ivermectin treatment is feasible in some endemic foci in Africa. Although further studies are needed to determine to what extent these findings can be extrapolated to other endemic areas in Africa, the principle of elimination has been established. The African Programme for Onchocerciasis Control has adopted an additional objective to assess progress towards elimination endpoints in all onchocerciasis control projects and to guide countries on cessation of treatment where feasible.

BackgroundMass 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 FindingsSkin 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/SignificanceThe 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.

An important variable in the epidemiology of arthropodborne diseases is the intensity of transmission, which is a function of host-vector contact and the prevalence of infection in the vector population. This latter value is often difficult to estimate. It is possible to envision the application of polymerase chain reaction (PCR) assays to this problem. To accomplish this, the assay must detect a single infected vector in a pool containing a large number of uninfected individuals. It must also be possible to calculate the prevalence of infection from the number of positive pools. A PCR assay for detecting Onchocerca volvulus in pools of vector black flies is described, and an algorithm is presented to calculate the prevalence of infection in the vector population, based upon the proportion of PCR-positive pools. This algorithm should be applicable to any disease for which a PCR assay is available.

The internal transcribed spacer (ITS) of the rRNA gene cluster has been used as a model for the study of the action of concerted evolution and molecular drive on repeated sequence families. In contrast to this general finding, preliminary DNA sequence analysis of cloned representatives of the ITS from the West African black fly species complex Simulium damnosum s.1. demonstrated extensive intra-individual and intra-specific polymorphisms. Variability in the ITS was primarily confined to the ITS1 domain. The degree and type of intra-individual and intra-specific variability within the ITS was further characterized using gel electrophoresis, DNA hybridization, and heteroduplex analysis of the PCR products generated from the ITS1 domain. ITS1 copies from individual S. damnosum s.1. differed in length and sequence composition. These results, when taken together, demonstrate that a large degree of intra-individual and intra-specific heterogeneity exists in the ITS of S. damnosum s.1. The intra-individual heterogeneity was greater in the savanna-dwelling than forest-dwelling sibling species of S. damnosum s.1. This heterogeneity may be due in part to inter-breeding among sympatric sibling species, coupled with disturbance of S. damnosum s.1. populations resulting from intensive vector control efforts.

BackgroundHuman landing collections are currently the standard method for collecting onchocerciasis vectors in Africa and Latin America. As part of the efforts to develop a trap to replace human landing collections for the monitoring and surveillance of onchocerciasis transmission, comprehensive evaluations of several trap types were conducted to assess their ability to collect Simulium ochraceum sensu lato, one of the principal vectors of Onchocerca volvulus in Latin America.Methodology/Principal FindingsDiverse trap designs with numerous modifications and bait variations were evaluated for their abilities to collect S. Ochraceum s.l. females. These traps targeted mostly host seeking flies. A novel trap dubbed the “Esperanza window trap” showed particular promise over other designs. When baited with CO2 and BG-lure (a synthetic blend of human odor components) a pair of Esperanza window traps collected numbers of S. Ochraceum s.l. females similar to those collected by a team of vector collectors.Conclusions/SignificanceThe Esperanza window trap, when baited with chemical lures and CO2 can be used to collect epidemiologically significant numbers of Simulium ochraceum s.l., potentially serving as a replacement for human landing collections for evaluation of the transmission of O. volvulus.