Equine central nervous system trypanosomosis in The Gambia is caused by genetically diverse populations of trypanosoma brucei parasites
In many countries diseases affecting working equid performance and productivity are detrimental both to equid welfare and to local economic development. Central nervous system (CNS) trypanosomosis, caused by Trypanosoma brucei spp 1, is a severe manifestation of trypanosomosis, which is usually fatal. The causative agent of this condition in the Gambia was investigated further in this study, to determine genotypic variation, mode of transmission and future management interventions. The presence of trypanosomes in local tsetse flies (Glossina spp.) was also investigated due to suspected vector involvement in disease transmission. Working equids exhibiting signs of CNS trypanosomosis were clinically evaluated. Blood was stored in EDTA and on FTA® cards prior to DNA extraction. In advanced neurological disease where prognosis was hopeless euthanasia was performed. CSF and CNS tissue samples were collected post-mortem. CSF was stored on FTA® cards and tissue samples were collected in formalin and RNAlater®. To confirm CNS T.brucei spp infection, immunohistochemistry and T.brucei-specific PCR 2 was performed on tissue samples. DNA was also extracted from blood collected from patients with evidence of generalised T.brucei infection with normal neurological function, and from the midguts of locally 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 immunohistochemistry 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.
This work is funded by The Donkey Sanctuary.