Development of Onchocerca volvulus from cryopreserved microfilariae in three temperate species of laboratory-reared blackflies.

Three species of British blackflies, Simulium ornatum s.l., S. erythrocephalum and S. lineatum, were infected with the cryopreserved microfilariae of Onchocerca volvulus obtained from human skin-snips in the Sudan. Doses of 5 or 10 microfilariae per fly were administered by intrathoracic injection into females, 1-2 days after eclosion from pupae. After 7 days at 27.5 degrees C and 85% relative humidity, microfilariae had completed development to third-stage larvae. Fly survival rates were highest for S. ornatum (96%) and lowest for S. lineatum (56%), and fell only marginally in each species when the larger dose of microfilariae was given. Infection rates ranged from 42% in S. lineatum up to 58% in S. ornatum following the lower dose of microfilariae, and 58% in S. lineatum up to 79% in S. ornatum following the higher dose. The proportion of microfilariae which completed development was relatively constant in each species of fly, ranging from 3.1-4.2% in S. lineatum to 10.5-16.8% in S. erythrocephalum. The greatest number of third-stage larvae recovered came from S. erythrocephalum at the higher dose of microfilariae, with a mean of 2.4 larvae per infected fly. As S. erythrocephalum has been successfully colonized through several generations in the laboratory, it is concluded that this is a promising species for reselection for increased susceptibility to O. volvulus.