Feasibility of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: first evidence from studies in Mali and Senegal

Abstract

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 <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/significance: This 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.

Figure 1. Location of the three study areas in Mali and Senegal.

Figure 2. Prevalence of onchocerciasis infection in the R. Gambia focus.

Figure 3. Prevalence of onchocerciasis infection in the R. Bakoye focus.

Figure 4. Prevalence of onchocerciasis infection in the R. Faleme focus.

Figure 5. Study design and study phases in each study site.

Figure 6. Vector infectivity rates in the three study areas after 15 to 17 years of ivermectin treatment.

Figure 7. Vector infectivity rates and prevalence of infection 16 to 22 months after the last treatment in the test area in the R. Gambia focus.

Figure 8. Vector infectivity rates and prevalence of infection 16 to 22 months after the last treatment in the test area in the R. Bakoye focus.

Figure 9. Vector infectivity rates and prevalence of infection 16 to 22 months after the last treatment in the test area in the R. Faleme focus.