STOP-FiLAR

Onchocerciasis and loiasis are vector-borne parasitic diseases caused by Onchocerca volvulus (Ov) and Loa loa (Loa), respectively. Onchocerciasis is transmitted by the bites of black flies (Simulium spp.), and loiasis by the bites of tabanids (Chrysops spp.).

Onchocerciasis is endemic in 31 African countries, as well as in a large focus spanning the Venezuela/Brazil border and in Yemen, while loiasis is present only in Central Africa. An estimated 37 million people are infected with Ov and 15 million people are infected with Loa.

From the early 1990s, African endemic countries implemented onchocerciasis control programmes based on annual mass drug administration (MDA) of ivermectin (IVM). In hyper- and mesoendemic areas, where the disease constitutes a public health problem, all individuals aged ≥5 years are offered the drug without prior diagnosis. IVM has a strong microfilaricidal effect, i.e. it kills most parasite’s larval stage (microfilariae, mf) present in the skin. However, it has little effect on the adult worms (macrofilariae) viability, which live up to 15 years. Consequently, MDA have to be repeated annually for at least 15 years to interrupt onchocerciasis transmission in a defined focus. In 2010, the African Programme for Onchocerciasis Control (WHO-APOC) shifted its objective from control of the disease to the elimination of infection, meaning that interventions have to be expanded to hypoendemic areas.

Loiasis is not currently targeted by a dedicated control programme, but it impedes onchocerciasis elimination activities in areas where both diseases coexist because IVM can induce an encephalopathy with coma in individuals with Loa microfilarial densities (MFD) exceeding 30,000 mf/mL of blood. In co-endemic areas where onchocerciasis is hypoendemic, the risk of post-IVM SAE outweighs the individual benefits of treatment. To date, there is no short-term perspective of a treatment that can be used widely and that is both effective against onchocerciasis and safe in subjects with high Loa MFD. Besides this programmatic issue, individual treatment of subjects with very high Loa MFD remains challenging. Treatment strategies for these subjects include the use of apheresis or a 3-week daily course of albendazole (ALB) followed by IVM and/or diethylcarbamazine. These protocols are difficult to apply both in Central Africa and in Europe for the management of imported cases, because of SAEs that may occur.

Solutions have been proposed to treat areas where onchocerciasis is hypoendemic and loiasis is co-endemic. These so-called “alternative treatment strategies” (ATS) include the pre-treatment of the whole population with a drug reducing progressively (thus safely) the Loa MFD under the threshold above which a post-IVM SAE can occur, and apply IVM MDA subsequently. Elimination of onchocerciasis could be accelerated by treating all subjects (either infected or not) living in endemic areas with a “chemo-prophylactic” drug preventing the parasite development from the first larval stages in humans to fecund adult worms. This strategy would reduce new active infections and lead to a progressive attrition of the parasite reservoir in the exposed population. It could also be applied at individual level to uninfected subjects visiting endemic areas.