Onchocerciasis (“oncho”) is similar in some ways to Lymphatic Filariasis in that it is a vector-borne nematode parasitic disease that causes severe disability. Oncho affects approximately 33 million people, mostly in 30 countries in sub-Saharan Africa (with small foci in Latin America and Yemen).
Onchocerciasis causes blindness and severe skin disease, and it is spread by black flies. O. volvulus adult worms live in subcutaneous nodules and are less sensitive to available drug treatments than those adult worms of the species that cause LF. They also have a longer lifespan (approximately 14 years rather than the estimated 7 years for LF parasites).
Several programs and developments have greatly improved the Oncho situation since the 1970’s when the Onchocerciasis Control Programme (OCP) in West Africa was initiated. OCP relied exclusively on vector control in its early years. However, following the appearance of Ivermectin (Mectizan) on the scene in the late 1980’s, OCP transitioned to become a drug distribution program using annual Ivermectin for mass drug administration (MDA) in 11 countries. OCP ended in 2002 and was replaced by the African Programme for Onchocherciasis Control (APOC) which coordinates community directed distribution of Ivermectin MDA in 28 African countries (including the former OCP countries).
OCP and APOC have done a good job of reducing parasite infection intensities and Oncho disease rates in many endemic countries. APOC’s commission will end at after 2015. Efforts to eliminate onchocerciasis will be taken up by a new organization named Expanded Special Project for Elimination of Neglected Tropical Diseases or ESPEN. There is growing evidence that it may be possible to eliminate oncho in some areas by MDA with Ivermectin (alone, or combined with vector control), but elimination programs in most African countries will require active maintenance for many years to come. While Ivermectin has good activity against the parasite larvae that cause disease in the skin and eye (microfilariae or Mf), it does not kill O. volvulus adult worms, and they resume production of Mf that can lead to transmission of new oncho cases by black flies after a few months. Current MDA activities are focused on areas with moderate or high infection rates (where disease risks are highest). Low endemicity areas in Africa (where fewer than 20% of adult men have oncho nodules detectable by palpation) are not receiving interventions for oncho and the geographic distribution of infections has not been fully mapped. These areas are not disease free (oncho dermatitis can be severe in hypoendemic areas, A.K.A. “sowda”), and they also may serve as a source for reintroduction of the parasite into previously controlled areas after interventions stop. While local elimination of Oncho in isolated areas may be achieved in the near future, computer modeling studies predict that it may take several decades to eliminate Oncho in some countries in Africa (WHO).
Therefore more effective drugs or dosing schedules for MDA against Oncho are needed that could shorten the number of years required to interrupt oncho transmission. Improved treatments should also make it more feasible to extend MDA into areas that are currently not being helped. These changes have the potential to be a game changer for global elimination of onchocerciasis.