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

Abstract

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.

Transmission of Onchocerca volvulus by Simulium neavei in Mount Elgon Focus of Eastern Uganda Has Been Interrupted

“…22 The guidelines developed were based on a detailed review of the 2001 World Health Organization (WHO) criteria for certification of elimination of onchocerciasis, tropical disease research (TDR) studies, and guidelines used by the Onchocerciasis Elimination Program for the Americas. [23][24][25] The Uganda criteria include three indicators.…”

“…Microfilaria (mf) prevalence in skin snips must be less than 5% in all sampled communities and less than 1% in 90% of sampled communities. 24 Serological assessments. Infection rates in children must be 0.1%.…”

Seventeen Years of Annual Distribution of Ivermectin Has Not Interrupted Onchocerciasis Transmission in North Region, Cameroon

“…[4][5][6][7][8][9] An important study in hyperendemic foci in Mali and Senegal showed that 15-17 years of annual ivermectin treatment had eliminated onchocerciasis transmission in the north and center of the Rio Faleme focus and that mass treatment could be safely stopped. 10 The two criteria used by Diawara and others 10 to make this determination were entomological (< 0.5 infected flies/1,000 flies) and epidemiological (< 5% mf prevalence in all communities examined and < 1% in 90% examined). It is important to note that, in the south of the Rio Falema focus, there were seven villages with mf prevalence between 1% and 13% after 15-17 years of annual treatment.…”

Repositioning of an existing drug for the neglected tropical disease Onchocerciasis

“…Currently, the only drug available for mass treatment is ivermectin (Mectizan®, Merck), which was originally marketed as a veterinary anthelmintic (7). Although this drug has proven successful in reducing morbidity, the risk of severe skin or ocular disease and decrease of microfilarial loads (>99%) after 14 d of treatment, it only has modest effect on adult worms and must be continuously administered over decades (8,9); and it now appears that ivermectin-resistance is emerging (10). Thus, there is a crucial need to identify new drug targets and agents that can effectively treat onchocerciasis.…”