Spatial distribution of African Animal Trypanosomiasis in Suba and Teso districts in Western Kenya
BMC Research Notes
Spatial distribution of African Animal Trypanosomiasis in Suba and Teso districts in Western Kenya
Samuel M Thumbi 0
Joseph O Jung'a
Reuben O Mosi
Francis A McOdimba
0 Centre for Infectious Diseases, School of Biological Sciences, University of Edinburgh , Kings Buildings, West Mains Road, Edinburgh, EH9 3JT , UK
Background: Studies on the epidemiology of African Animal Trypanosomiasis (AAT) rarely consider the spatial dimension of disease prevalence. This problem is confounded by use of parasitological diagnostic methods of low sensitivity in field surveys. Here we report a study combining highly sensitive and species specific molecular diagnostic methods, and Geographical information system (GIS) for spatial analysis of trypanosome infection patterns, to better understand its epidemiology. Blood samples from 44 and 59 animals randomly selected from Teso and Suba districts respectively were screened for trypanosomes using PCR diagnostic assays. Spatial distribution of the positive cases was mapped and average nearest neighbour analysis used to determine the spatial pattern of trypanosome cases detected. Findings: Trypanosome prevalence of 41% and 29% in Suba and Teso districts respectively was observed. T. vivax infections were most prevalent in both areas. Higher proportions of T. brucei infections (12%) were observed in Suba, a known sleeping sickness foci compared with 2% in Teso. Average nearest neighbour analysis showed the pattern of trypanosome infections as random. An overlay with tsetse maps showed cases lying outside the tsetse infested areas, mostly being cases of T. vivax which is known to be transmitted both biologically by tsetse and mechanically by biting flies. Conclusion: These findings suggest a need to design control strategies that target not just the biological vector tsetse, but also the parasite in cattle in order to clear the possibly mechanically transmitted T. vivax infections. There is need to also review the accuracy of available tsetse maps.
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Findings
Trypanosomiasis, a disease of humans and animals
caused by several species of trypanosomes and spread
by tsetse flies is a major constraint to livestock
production in 37 countries within the Sub-Saharan region. An
estimated 45-50 million cattle are at risk of infection in
the region, with an estimated economic loss of up to US
$ 1.3 billion in cattle production [1]. Its public health
importance has led to attempts to control the disease
nationally and regionally with initiatives as Pan Africa
tsetse and trypanosomosis eradication program
(PATTEC) [2]. These attempts rely on repeated large-scale
epidemiological studies and environmental surveys,
guiding the design and implementation of intervention
strategies. The accuracy of these surveys is limited by
use of parasitological diagnostic techniques as
microscopy due to low sensitivity [3], and the difficulty in
incorporating climatic and environmental data known to
influence tsetse distribution, and as a result disease
spread [4,5].
The high costs required to produce tsetse distribution
maps through ground-based vector surveys have
resulted in few studies looking at the spatial dimension
of disease prevalence [6]. However, the use Geographical
Information System (GIS) software now makes it
cheaper and easier to produce maps which can serve as
useful tools for policy discussion, and allow for analysis
of factors that would influence the disease patterns [7].
Polymerase Chain Reaction (PCR) diagnostic assays
overcome the low sensitivity limitations of
parasitological techniques. They are powerful tools for
identification and diagnosis of trypanosomes in their
hosts and vectors although their high cost and need for
elaborate expertise has delayed their adoption [8].
However, PCR assays able to detect all pathogenic
trypanosome species in a single reaction have been developed
[9,10]. These reduce the costs of screening a sample
from an endemic area by up to five times, and have
been suggested as suitable for large-scale
epidemiological studies [11].
Results obtained from molecular studies, associated
with geo-referenced information concerning vector,
cattle distribution and relevant environment parameters,
combined in a GIS have the potential of providing more
informative study results. This new approach of studying
complex pathogenic system is argued to lead to a better
evaluation of the risk of infection, allows for effective
risk communication, and gives scientific outputs in ways
that are understandable to non-scientists [12]. Here we
report a study combining use of highly sensitive and
species specific PCR assays, with GIS for spatial analysis
of infection patterns. The aim is to provide information
about the prevalence and distribution AAT in two areas
in Western Kenya, and suggest improved control
strategies based on the findings.
Results
Cross-sectional trypanosomiasis survey
Overall, 29% of the 44 animals and 41% of the 59
animals sampled in Teso an (...truncated)