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.