Trypanosoma brucei gambiense sleeping sickness follows a long asymptomatic phase and persists in ancient foci from which epidemic clinical disease arises. A putative focus of T. b. gambiense infections has been identified, initially in mothers and young children, on the Lake Albert shoreline of Western Uganda leading to mass screening of 6207 individuals in September 2008. T. b. gambiense infections were identified by Card Agglutination Test for Trypanosomiasis (CATT) and sub-species-specific PCR although parasitological methods failed to confirm any patent trypanosome infections. In April 2009, CATT positives were re-visited; diagnosis of individuals by CATT and PCR was unstable over the two time points and parasites remained undetected, even using mini Anion Exchange Centrifugation Technique (mAECT). These observations suggest the possibility of a silent focus of disease, where all infected individuals are in a latent stage, and highlight our limited understanding of the local natural history and disease progression of T. b. gambiense in children and adults.

The epidemiology of T. b. gambiense is reviewed in the light of recent findings, and potential animal reservoir hosts are listed. The discovery of distinct zymodemes in pigs with isozyme characteristics identical with stocks previously isolated only from man, and the human plasma resistant character (BIIT positive) of these stocks, is indicative of a zoonotic source of infection of man in some areas of West Africa. The likely epidemiological differences between savanna and forest foci are emphasised.

The focality of West African sleeping sickness is discussed in relation to the various hypotheses which have been propounded to explain this phenomenon. Recent observations which suggest Glossina can move longer distances on wind fields than hitherto supposed, and its possible implications as an explanation of some residual foci, is discussed. The possible impact of onchocerciasis on the distribution of foci is mentioned.