One of the main priorities of the global programme to eliminate onchocerciasis (also called river blindness) is to complete a pilot elimination mapping strategy which aims to accurately identify transmission of the disease wherever it occurs. Previous mapping efforts focused on identifying areas with the highest burden of symptomatic disease.
“We are eagerly awaiting new data from this mapping, as previous efforts focused on control and not elimination of transmission,” said Dr Paul Cantey, Medical Officer in charge of onchocerciasis elimination, WHO Department of Control of Neglected Tropical Diseases. “Elimination mapping will locate any remaining areas where transmission still occurs so that treatment can begin, protecting the gains made by previous longstanding programmes. Mapping will also help to prevent the resurgence of blinding onchocerciasis at a time when areas that have successfully implemented programmes are stopping treatment.”
Different aspects of this provisional mapping strategy are being tested, mainly in three countries (Ghana, Mozambique and Nigeria), with technical input from WHO, the Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN)1 and support from various partners.
Prior to the development of the provisional strategy, some countries, such as Ethiopia and Ghana, devised their own strategies for identifying untreated areas of transmission. With the support of partners, countries are now aligning their strategies with that of the pilot elimination mapping strategy.
Elimination of transmission of onchocerciasis will require 100% geographical coverage in all areas in which transmission occurs, not just in areas where there is a lot of symptomatic disease.
Available data from these many activities will be reviewed during the forthcoming meeting of the WHO Onchocerciasis Technical Advisory Subgroup (OTS) scheduled in February 2019. Updates to the provisional strategy are expected at that time.
“One key activity will be the validation of the serological threshold for starting large-scale treatment with ivermectin2”, said Dr Cantey. “Although the current threshold is based on available data with input from models of transmission, there is a need for real world demonstration of ongoing transmission in areas that meet or exceed the provisional threshold, and this could probably be best done by evaluating blackflies for infectivity using polymerase chain reaction.”
In the meantime, the global onchocerciasis elimination programme continues to expand as new areas of low-level transmission are identified. By the end of 2017, due to the additional people it had identified as living in areas at risk for transmission, the total population considered at risk for infection had increased to 205 million people; this number is expected to increase with continued onchocerciasis elimination mapping.
More importantly, the almost half a century fight against onchocerciasis is clearly yielding results as the prevalence of infection decreases and some areas have stopped large-scale treatment.
Elimination mapping is, however, only the first step in achieving the elimination goal. Political will is needed to take up the challenge of elimination, and financial support will be required from both the country and donors to complete mapping, implement large-scale treatment, and achieve and maintain the necessary level of treatment coverage. It is only after going through these phases that a country can demonstrate the interruption of transmission. Already, a few countries are upbeat about the possibility of a new World Health Assembly resolution calling for “elimination of onchocerciasis transmission” in anticipation of 2020, which marks the end of WHO’s current NTD Roadmap.
Many consider that the adoption of such a resolution is needed to help garner support and generate the focus and political will needed to ensure elimination.
Background
The WHO Region of the Americas was the first region to focus on the elimination of onchocerciasis transmission. The Onchocerciasis Elimination Program for the Americas was launched in 1992 with the goal of elimination, not control. To date, four countries have been verified by WHO as having eliminated transmission of onchocerciasis: Colombia, Ecuador, Mexico and Guatemala.
The WHO African Region, where the disease is found in 31 sub-Saharan countries, began shifting from control to elimination in 2009 as data became available to demonstrate that elimination was possible in the Region. Consequently, in 2012, WHO’s NTD Roadmap set a goal of “elimination where feasible by 2020”, and the African Programme for Onchocerciasis Control advanced the goal to elimination in 80% of countries by 2025.
Although no country in the Region has been verified as having eliminated transmission, more than 1 million people live in areas that meet WHO criteria for having eliminated transmission. In some countries, elimination of onchocerciasis transmission is more difficult because of the presence of Loa loa (commonly known as African eye worm). Political instability, insecurity and limited access to endemic areas pose additional challenges.
ESPEN tackles four3 other common NTDs in the Region and supports countries to both expand and scale down large-scale treatment where appropriate. It is expected that many more areas in the Region will be able to scale down treatments over the coming years.
Progress is also being registered in the WHO Eastern Mediterranean Region. Sudan has met the WHO criteria for elimination of transmission in one area and has recently stopped transmission in a second.
Yemen, despite political instability and a lack of consistent donor support, has launched large-scale treatment.
The disease
Human onchocerciasis – also known as river blindness – is a parasitic disease caused by infection with the filarial worm Onchocerca volvulus. The parasite is transmitted by blackflies (Simulium spp.) that breed in fast-flowing rivers and streams, mostly in remote villages located near fertile land where people rely on agriculture.
Onchocerciasis is a disease of the eye and skin. Symptoms are provoked by the death of immature worms (microfilariae) that circulate in the subcutaneous tissue and in the eyes, inducing intense inflammatory responses. Infected people may show symptoms such as severe itching and various skin and eye lesions.
WHO recommends annual treatment with ivermectin for at least 10–15 years. Priority for semi-annual treatment is indicated where epidemiological data are not on track to achieve elimination. More than 99% of infected people live in 31 countries in sub-Saharan Africa.
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1https://afro.who.int/health-topics/expanded-special-project-elimination-neglected-tropical-disease
2 Ivermectin is effective against many types of parasites. It is used to treat river blindness and lymphatic filariasis and is considered to be effective also against scabies.
3Lymphatic filariasis, schistosomiasis, soil-transmitted helminthiases and trachoma.