A survey of livestock to identify the presence of human infective trypanosomes in the Trypanosoma brucei gambiense endemic districts of north-west Uganda
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Background
Gambian human African trypanosomiasis (gHAT) is a neglected tropical disease caused by Trypanosoma brucei gambiense transmitted by tsetse flies ( Glossina ). Humans are the major host and mass screening and treatment of the human population has contributed to the elimination of gHAT as a public health problem in many countries, including Uganda. There is some evidence that animals may act as hosts for T. b. gambiense which may undermine efforts to eliminate transmission. We undertook a study to determine whether cattle and pigs were harbouring T. b. gambiense in a region of north-western Uganda where efforts to eliminate transmission of gHAT were ongoing.
Methodology
Blood was collected from 2,775 cattle and 571 pigs and examined for the presence of trypanosomes by the haematocrit centrifugation technique (HCT). In addition, polymerase chain reaction (PCR) was used to amplify the trypanosomal internal transcribed spacer (ITS1). Those whose ITS-1 results indicated the Trypanozoon group ( T. brucei ) were further tested by sub-species specific PCRs for Trypanosoma brucei gambiense and T. b. rhodesiense . The results were analysed by species of infecting parasite, geographic location and presence or absence of vector control activities in the area.
Principal Findings
No samples were found positive for potentially human infective trypanosomes. Among the cattle samples, 2.2% were positive by HCT (95% CI = 1.7% - 2.8%), while 5% were positive by PCR (95% CI = 4.3% - 5.9%), but this varied depending on location with up to 26.5% positive in one village. Among pigs, 3 samples (0.6%; 95% CI 0.2% - 1.6%) were positive by HCT and none were positive by PCR. There was no significant association between trypanosome species and vector control activities (p > 0.05). Based on these results it was concluded with 95% confidence that human infective trypanosomes are not circulating in livestock at a prevalence greater than 1/1000, and when this is augmented with results of previous surveys the parasite is not circulating at a prevalence greater than 1/2734. At the level of the trypanosome species, there is no significant difference between the frequencies of species in vector controlled versus non-vector controlled areas (p > 0.05).
Conclusions
These results support the evidence base that livestock are not an important reservoir and therefore not a threat to the elimination of Gambian human African trypanosomiasis in Uganda.
Author summary
Two subspecies of the same parasite cause human African trypanosomiasis (HAT) known as sleeping sickness. The east African form has a reservoir in livestock and wildlife with spillover to human hosts, whilst the west African form circulates in humans. However there have been some reports of the west African parasite in animals. Uganda is nearing elimination of the west African form, however to ensure elimination it is essential to verify the absence of the parasite in livestock in the endemic area. In this study a comprehensive livestock survey with testing of blood samples did not find the parasite in livestock. Whilst it is practically impossible to verify the total absence of the parasite, this study combined with studies show that the prevalence of the parasite in cattle or pigs is less than 1/2734 making a very unlikely reservoir.
