Images of Memorable Cases: Case 67 Peripheral blood film from a native of West Africa. 67. Loiasis The image shows a sheathed microfilaria of Loa loa, a threadlike worm distributed irregularly in West and Central Africa. Flies of the genus Chrysops are the intermediate hosts and vectors. Adult worms inhabit the subcutaneous connective tissues of the body, causing localized edema called “Calabar swellings.” Occasionally, the adult worm can be seen crossing the eye beneath the conjunctiva. The immature forms (microfilariae) are diurnal and appear in greatest concentrations in daytime blood. Finding the microfilariae in routine blood films establishes the diagnosis.