Loiasis, commonly called the African eye worm, is caused by the parasitic filarial worm Loa loa and transmitted by deerflies. Loiasis is found in Central and West Africa with an estimated 29 million people at risk of infection. Ten countries within this region report high rates of infection with over 40% of people stating they have had the infection in the past.
Loa loa is transmitted by flies of the genus Chrysops. When an infected Chrysops bits a person the infective L3 stage larvae are introduced onto the skin of the human where they are able to penetrate the bite wound. The larvae then develop into adults and are found in the subcutaneous tissues of the body where the females produce large numbers of microfilariae.
Similar to other species of filarial worms the microfilaria migrate throughout the body based on the time of day, during the day they can be found in the peripheral blood where they can be ingested by a Chrysops taking a blood meal. Once ingested microfilariae develop into the third stage or L3 infective larvae within the fly’s midgut, from there they migrate to the fly’s proboscis a where they can infect another human.
Most infections are asymptomatic though in cases where symptoms develop they typically do not show for months after the infection. If symptoms develop they will commonly present as itchy, non-painful swelling that come and go, these swellings can appear anywhere but are most commonly seen near joints. Less common are symptoms of itching over the entire body, muscular and joint pain as well as fatigue. In addition the worms may crawl across the surface of the eye, which lends to the name African eye worm, though can be seen under the skin in some cases.