Feasibility of Onchocerciasis Elimination with Ivermectin Treatment in Endemic Foci in Africa: First Evidence from Studies in Mali and Senegal

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

Proof-of-principle of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: final results of a study in Mali and Senegal.

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

Summary of main results.

<p>NA: not available.</p

Trend in prevalence of mf in the R. Bakoye focus.

*<p>Source: Diawara et al <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001825#pntd.0001825-Diawara1" target="_blank">[17]</a>.</p

Trend in prevalence of mf in the R. Gambia focus.

*<p>Source: Diawara et al <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001825#pntd.0001825-Diawara1" target="_blank">[17]</a>.</p

Study design.

<p>Study design.</p

Modified study design for the R. Faleme focus.

<p>Modified study design for the R. Faleme focus.</p

Location of the three study foci in Mali and Senegal.

<p>Location of the three study foci in Mali and Senegal.</p

Trend in vector infectivity rate in the R. Gambia focus.

*<p>Source: Diawara et al <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0001825#pntd.0001825-Diawara1" target="_blank">[17]</a>.</p

Prevalence of onchocerciasis infection in the R. Faleme focus before the start of treatment and 1.6 to 5 years after the last treatment round.

<p>Prevalence of onchocerciasis infection in the R. Faleme focus before the start of treatment and 1.6 to 5 years after the last treatment round.</p