Evidence of the absence of Human African Trypanosomiasis in northern Uganda: analyses of cattle, pigs and tsetse flies for the presence of Trypanosoma brucei gambiense
Authors: Cunningham Lucas J. , Lingley Jessica K. , Tirados Iñaki , Esterhuizen Johan , Opiyo Mercy A. , Mangwiro Clement T. N. , Lehane Mike J. , Torr Stephen J. 
Published: August 2019

Background

Large-scale control of sleeping sickness has led to a decline in the number of cases of Gambian human African trypanosomiasis (g-HAT) to <2000/year. However, achieving complete and lasting interruption of transmission may be difficult because animals may act as reservoir hosts forT. b. gambiense. Our study aims to update our understanding ofT. b. gambiensein local vectors and domestic animals of N.W. Uganda.

Methods

We collected blood from 2896 cattle and 400 pigs and In addition, 6664 tsetse underwent microscopical examination for the presence of trypanosomes.Trypanosomaspecies were identified in tsetse from a subsample of 2184 using PCR. Primers specific forT. bruceis.l. and forT. bruceisub-species were used to screen cattle, pig and tsetse samples.

Results

In total, 39/2,088 (1.9%; 95% CI=1.9-2.5) cattle, 25/400 (6.3%; 95% CI=4.1-9.1) pigs and 40/2,184 (1.8%; 95% CI=1.3-2.5) tsetse, were positive forT. bruceis.l.. Of these samples 24 cattle (61.5%), 15 pig (60%) and 25 tsetse (62.5%) samples had sufficient DNA to be screened using theT. bruceisub-species PCR. Further analysis found no cattle or pigs positive forT. b. gambiense, however, 17/40 of the tsetse samples produced a band suggestive ofT. b. gambiense. When three of these 17 PCR products were sequenced the sequences were markedly different toT. b. gambiense, indicating that these flies were not infected withT. b. gambiense.

Conclusion

The absence ofT. b. gambiensein cattle, pigs and tsetse accords with the low prevalence of g-HAT in the human population. We found no evidence that livestock are acting as reservoir hosts. However, this study highlights the limitations of current methods of detecting and identifyingT. b. gambiensewhich relies on a single copy-gene to discriminate between the different sub-species ofT. bruceis.l.

Author Summary

The decline of annual cases of West-African sleeping sickness in Uganda raises the prospect that elimination of the disease is achievable for the country. However, with the decrease in incidence and the likely subsequent change in priorities there is a need to confirm that the disease is truly eliminated. One unanswered question is the role that domestic animals play in maintaining transmission of the disease. The potential of cryptic-animal reservoirs is a serious threat to successful and sustained elimination of the disease. It is with the intent of resolving this question that we have carried out this study whereby we examined 2088 cattle, 400 pigs and 2184 tsetse forTrypanosoma brucei gambiense, the parasite responsible for the disease. Our study foundT. bruceis.l. in local cattle, pigs and tsetse flies, with their respective prevalences as follows, 1.9%, 6.3% and 1.8%. Further analysis to establish identity of these positives to the sub-species level found that no cattle, pigs or tsetse were carrying the pathogen responsible for Gambian sleeping sickness. Our work highlights the difficulty of establishing the absence of a disease, especially in an extremely low endemic setting, and the limitations of some of the most commonly used methods.