Loa loa is one of the eight parasitic nematode (roundworm) species that account for most cases of filariasis in humans (this form of filariasis is sometimes known as loiasis), although it is responsible for less morbidity than are many of the other seven species (notably, Wuchereria bancrofti and Brugia malayi, which cause lymphatic filariasis, and Onchocerca volvulus, which causes onchocerciasis (river blindness)). Loa loa is endemic to Africa. (Centers for Disease Control Parasites and Health website) In particular, loiasis is really endemic to just a few Central and West African countries, where it afflicts around 20 million people (Hoerauf 2009). This nematode is often known as the African eye worm because the adult worm can sometimes be seen moving through the sclera (white portion) of the eye. Migration causes severe eye pain, inflammation, and sometimes blindness and may take a few minutes or up to several hours. Most cases of loiasis are asymptomatic. The first clinical signs may appear within a few months of infection, but often do not appear for more than a decade. (Padgett and Jacobsen 2008 and references therein)
The primary vectors for Loa loa filariasis are two day-biting Chrysops deerflies, C. silacea and C. dimidiata. During a blood meal, an infected fly introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound. The larvae develop into adults that commonly reside in subcutaneous tissue, where they can live for several years. The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, whereas the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. Adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs. The fly ingests microfilariae during a blood meal. After ingestion, the microfilariae lose their sheaths and migrate from the fly's midgut through the hemocoel to the thoracic muscles of the arthropod. There the microfilariae develop into first-stage larvae and subsequently into third-stage infective filariform larvae. The third-stage infective larvae migrate to the fly's proboscis and, just a few weeks after the microfilariae were ingested by the fly, can infect another human when the fly takes a blood meal. (Centers for Disease Control Parasites and Health website)
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