A new focus of onchocerciasis in Mwanza District, Malawi.

Anecdotal information suggested that a new focus of onchocerciasis had recently developed in Mwanza in Malawi, a district not contiguous with Thyolo, the only district in Malawi with recognized autochthonous transmission. We carried out a survey of the northern half of Mwanza district, randomly selecting 62 villages for assessment. Two iliac crest skin snips were taken from 2215 residents over the age of 15 years; one-quarter had Onchocerca volvulus microfilariae. Prevalence increased with age and was higher overall among men than women. The age-adjusted prevalence among migrants from Thyolo was 31%. Among residents who had never been outside Mwanza, the gender-specific prevalences and microfilarial loads were similar. The intensity of infection was low. Autochthonous transmission of O. volvulus has thus been demonstrated in Mwanza and illustrates the possibility of the spread of the disease to new foci in Malawi and the surrounding countries, due to significant population shifts.