Loa loa, the causative agent of loiasis is thought to infect 13 million people worldwide (Nutman et al., 1988). Loiasis is endemic to the rainforests and forest edges of central and western Africa (see map). Infection by Loa loa is dependent upon presence of Chrysops (mango fly) species that serve as both intermediate host and vector. While Loa loa larvae are capable of developing in numerous species of Chrysops, Chrysops silacea and Chrysops dimidiata are thought to be responsible for most parasite transmission. While the parasite larvae can effectively develop in the North American deerfly species Chrysops atlanticus (Orihel et al., 1975), an endogenous case of loiasis has never been reported in the United States. Chrysops breed under the rainforest canopy and in mud at the side of streams so these flies are more common in the rainy season.
Loiasis is endemic areas of Nigeria, Cameroon, Zaire, Angola, Gabon, Chad, Sudan, and the Central African Republic. Heaviest microfilaremic loads (over 150,000 microfilariae per ml of blood) are found in Cameroon, Gabon, Congo, Zaire and the Central African Republic (Burnham, 1997). Rubber plantations in these areas bring humans into the dense high canopy forest, increasing risk for exposure to Chrysops. Because of different exposure patterns, loiasis is more prevalent among adults than among children.
Loa loa transmission is dependent upon presence of the Chrysops vector and upon infected humans who serve as parasite reservoirs. Therefore, effective loiasis prevention measures include vector control, vector bite prevention, and reduction of microfilaremia in the human reservoir population.