Investigating trypanosome infection prevalence in three districts of Northern Uganda: a study of trypanosome populations in cattle and humans
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Date
22/05/2023Item status
Restricted AccessEmbargo end date
22/05/2026Author
Watson, Ross
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Abstract
The African Trypanosomiases are a suite of diseases affecting humans and animals
across sub-Saharan Africa, caused by protozoan trypanosome parasites and
transmitted by tsetse flies (Glossina spp.). A number of different trypanosome
species cause Animal African Trypanosomiasis (or Nagana) in wildlife and livestock,
and have a major impact on animal health, agriculture and the economy. Two subspecies
of African trypanosomes infect humans: Trypanosoma brucei gambiense
causes a chronic disease in Central and Western Africa and Trypanosoma brucei
rhodesiense causes an acute disease in Eastern and Southern Africa. Human African
Trypanosomiasis (HAT), or sleeping sickness, is fatal if left untreated. Uganda is the
only country to have active but geographically discrete foci of both T. b. gambiense
HAT (gHAT) and T. b. rhodesiense HAT (rHAT). Recently, Northwards migration of
rHAT has been observed in Uganda, associated with movements of cattle that serve
as reservoirs for the zoonotic T. b. rhodesiense parasite. Convergence of the two
forms of HAT in districts of Northern Uganda would impact on correct diagnosis and
treatment since rHAT and gHAT require different diagnostics and treatment.
The work described in this thesis explores the impact of recent cattle-based
interventions for the control of zoonotic rHAT, specifically targeted at preventing
the Northwards migration of T. b. rhodesiense in cattle in Uganda.
Previously, a cross-sectional survey of cattle in 150 villages across 32 rHAT-affected
districts of Uganda was undertaken in 2014 using the Polymerase Chain Reaction
(PCR); screening determined the prevalence of human-infective T. b. rhodesiense
and non-human-infective Trypanosoma brucei brucei parasites. In 2018, cattle from
20 villages in three districts of Northern Uganda were screened to determine
change in trypanosome prevalence across these 20 sites.
Between 2014 and 2018, overall trypanosome prevalence was observed to have
significantly reduced (p < 0.0001) from 59.05% (95% CI 56.02% - 62.02%; 607/1,028)
to 15.17% (95% CI 13.07% - 17.54%; 150/989), a trend observed for all individual
trypanosome species. The prevalence of Trypanosoma brucei sensu lato decreased
from 35.51% (95% CI 32.64% - 38.48%; 365/1,028) to 6.88% (95% CI 5.46% - 8.63%;
68/989), (p < 0.0001), and minimum prevalence of human-infective T. b.
rhodesiense reduced from 4.67% (95% CI 3.54% - 6.14%; 48/1,028) to 2.12% (95% CI
1.39% - 3.22%; 21/989), (p = 0.0020).
Between 2014 and 2018, the prevalence of Trypanosoma vivax decreased from
15.86% (95% CI 13.75% - 18.22%; 163/1,028) to 4.75% (95% CI 3.59% - 6.26%;
47/989), (p < 0.0001), and the prevalence of Trypanosoma congolense decreased
from 31.42% (95% CI 28.66% - 34.32%; 323/1,028) to 5.46% (95% CI 4.21% - 7.06%;
54/989), (p < 0.0001). The scale (and/or direction) of change in prevalence was
observed to vary at both district and village level.
A human survey for HAT was also undertaken in the same 20 villages in 2018; 5,383
individuals were screened for the presence of Trypanosoma brucei sensu lato DNA
using PCR. In total, 10 samples were considered positive for T. brucei sl (0.19%; 95%
CI 0.10% - 0.35%) and were examined to determine the sub-species. Association
between villages with PCR-positive human samples and the presence of humaninfective
T. b. rhodesiense parasites was observed.
Human-infective T. b. rhodesiense parasites and non-human-infective T. b. brucei
parasites co-exist in the cattle population in Uganda. This thesis explores the
dynamics of distribution of human- and non-human-infective T. brucei in cattle
before and after treatment; in particular, the impact of interventions on the
proportion of T. b. rhodesiense within the T. brucei sl population. On very few
occasions does the relative prevalence of T. b. rhodesiense appear to exceed that of
T. b. brucei in cattle populations, despite epidemiological theory predicting
otherwise. Explanations for these observations are discussed, and progress towards
elimination of Human African Trypanosomiasis as a public health problem is
considered, taking into account the findings of this thesis.
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