caught tsetse. Parasite population structure was investigated using a panel of microsatellite markers 3 together with a reference strain of T.brucei equiperdum (OVI) and a T.b.brucei positive control.Ten cases (5 horses, 5 donkeys) with naturally occurring CNS trypanosomosis were included. Horses presented with rapidly progressive spinal ataxia while donkeys showed slowly deteriorating cerebral dysfunction and cranial nerve abnormalities. CNS trypanosomosis was confirmed post-mortem using

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... histochemistry and PCR. Histopathological evaluation revealed diffuse lymphocytic-plasmacytic meningoencephalomyelitis. Microsatellite fragment analysis showed a heterogenous parasite population with a large range of alleles present, inconsistent with a clonal population. Parasite populations from donkey versus horse, and from blood versus CNS tissue were not found to be significantly different, suggesting that host factors are important in progression of neurological disease. Of 405 tsetse trapped locally and dissected, 11 contained microscopically visible midgut trypanosome infections. DNA extracted from the positive tsetse midguts was positive for equid DNA in 3/11 cases, confirming vector involvement. 5/11 flies were positive for T.brucei but with different microsatellite patterns to that found in infected CNS tissue. Further work is required to develop an optimal panel for use in both tsetse and equine-derived samples. Continued efforts are required to improve understanding of the transmission of this disease to enable the development of effective preventative measures.