Human onchocerciasis is the second most important cause of infectious blindness after trachoma, with 99% of the cases occurring in sub-Saharan Africa.
Onchocerciasis is endemic in 27 sub-Saharan African countries, the Yemen, and was imported through the slave trade to six Latin American countries (Colombia has now been certified to be free of the disease). The extent of the disease has been estimated by REMO (rapid epidemiological mapping of onchocerciasis). Presently, it is estimated that 37 million people carry Onchocerca volvulus, with 90 million at risk in Africa.
Causative organism: Human onchocerciasis is caused by the filarial parasitic nematode Onchocerca volvulus, a close relative of the parasite responsible for lymphatic filariasis.
Adult worms (macrofilariae) live in subcutaneous nodules and deeper worm bundles, were fertilized females can produce, during an average of 10 years, millions of microfilariae, responsible for the morbidity associated with the infection.
Human onchocerciasis is also known as ‘river blindness’ because the vectors that transmit the infection breed in fast flowing rivers, and because the microfilariae can enter the eye and cause irreversible visual impairment and blindness. Immunological responses to filarial products, either of parasite origin or particularly of their endosymbiotic Wolbachia bacteria, lead to long-standing, non-resolving inflammation associated with chronic onchocerciasis pathology.
Skin pathology ranges from troublesome itching to (disfiguring) skin changes, including early-stage reactive lesions, and late-stage depigmentation (leopard skin) and atrophy.
The burden of onchocercal disease has been estimated by the Global Burden of Disease 2010 Study.
Ingested during a bloodmeal by Simulium (black fly) vectors, the microfilariae develop within the fly into infective (L3) stages that are then transmissible to other people. Many Simulium species have been incriminated to a greater or lesser degree in the transmission of O. volvulus, their relative vectorial roles contributing to shape diverse transmission patterns across endemic areas.
There are three major control programmes: Onchocerciasis Control Programme in West Africa (OCP, 1974 – 2002), African Programme for Onchocerciasis Control (APOC, 1995 – Present) and Onchocerciasis Elimination Program for the Americas (OEPA, 1993 – Present).
The OCP started as a vector control programme aiming at reducing vector density (by weekly insecticide application to vector breeding sites along fast-flowing rivers) for prolonged periods (given that adult parasites live, on average, for a decade). APOC and OEPA have adopted the strategy of mass drug administration (MDA) with ivermectin, the former through the modality of annual community-directed treatment with ivermectin, CDTI, and the latter through biannual (twice yearly) treatment by mobile teams. Implementing treatment strategies which reduce the prevalence and intensity of microfilariae have the double benefit of reducing morbidity and decreasing the incidence of infection.
The costs and benefits of different treatment strategies (annual vs. biannual) and the cost-effectiveness of control are currently under investigation. Modelling papers which consider these issues will appear in the list below shortly (papers by Coffeng et al., Turner et al.).
–What is the optimal ivermectin treatment frequency and duration according to epidemiological setting to reach operational thresholds for elimination?
–What other interventions could be deployed? (e.g. in areas co-endemic with loiasis where ivermectin may be contra-indicated in some individuals)
–How long should surveillance be in place after cessation of treatment to detect any recrudescence of infection?
–How can modelling help in the detection and management of anthelmintic resistance should this occur and spread?