Assessment of potential factors for plague transmission in the plague endemic area of Karatu District, Northern Tanzania
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Date
2024-05
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Sokoine University of Agriculture
Abstract
Background: Plague, caused by Yersinia pestis, is a severe
zoonotic disease with rodents and fleas as primary hosts and
vectors. Despite being quiescent in several foci in Tanzania, plague
remains a significant public health concern due to its recurring
nature. The Karatu district experienced plague outbreaks in 1996
and 1997. Subsequent studies have reported evidence of Yersinia
pestis DNA in rodents in this endemic area in 2006 and 2013.
Consistent surveillance of the plague pathogen in rodent
communities and monitoring rodents and flea abundance is crucial
to assess the risk of human plague outbreak and the health of the
rodent populations in the vicinity of human habitats. This study
aimed to assess potential attributes for plague transmission in
Karatu district during the periods of disease quiescence. This study
was conducted in Karatu district in January and February, 2022 (wet
season) and repeated in July 2022 (dry season) The study aimed to
enhance surveillance of plague in plague foci and adjacent non-
plague foci - villages to determine the prevalence of plague bacilli in
the rodents, and identify the need for enhancing control measures.
This work is at the nexus of the One Health approach, as it involves
the surveillance of the plague zoonotic pathogen in the small
mammals for the purpose of enhancing public health. The results of
this study will be useful in informing public health, vector control, and
environmental institutions about the potential factors that may
underlie the transmission of the plague agent, Y. pestis, in the
plague-endemic area of Karatu district, northern Tanzania.
Methods: A repeated cross-sectional study was conducted in four
selected villages of Karatu district, involving two plague foci villages
and two non-plague foci villages, during the wet and dry seasons. A
total of 120 Sherman traps were used to capture rodents in farm
land habitats, peridomestic areas, bush lands, and forest buffer
zones, with an approximate distance of 300 to 500m between
habitats. The captured rodents were anaesthetised and identified using a dichotomous key after taking various morphological
measurements. Blood samples were collected from the
anaesthetised rodents through the supraorbital vein using glass
micro capillary tubes. Ectoparasites (fleas) and internal organs
(lungs and spleen) were collected and preserved in 70% ethanol and
absolute ethanol, respectively. Fleas were processed in the
laboratory and identified to genus and species levels using the
available dichotomous key. Blood smears were prepared on
microscope glass slides and preserved in slide boxes at room
temperature. In the laboratory, the blood smears were stained with
Wright-Giemsa stain and observed for the presence of Y. pestis
coccobacilli under a compound light microscope at ×1000
magnification. The presence of bipolar-stained coccobacilli bacteria
triggered suspicion for Y. pestis. The microscopy results were then
confirmed using qPCR to detect the presence of the pla gene of Y.
pestis DNA, extracted from the rodent lungs and spleens.
Results: A total of 291 rodents belonging to nine species were
captured in peridomestic areas, farmlands, bushes, and forest buffer
zones. Mastomys natalensis was the most abundant species
(41.92%), followed by Arvicanthis niloticus (24.05%), Lemniscomys
striatus (14.78%), Rattus rattus (8.59%), Lophuromys flavopunctatus
(4.47%), Grammomys spp (4.12%), Otomys spp (1.03%),
Graphiurus murinus (0.69%), and Praomys delectorum (0.34%).
Rodent abundance varied significantly among species and across
seasons however, was not statistically significant different between
plague and non-plague foci villages or across habitats. The rodent
diversity, expressed as Shannon-Wiener diversity index, was slightly
higher in plague foci villages compared to non-plague foci villages
(0.7 and 0.6, respectively). Peridomestic areas had the highest
diversity index (0.92), followed by farmlands (0.65), bush lands
(0.64), and forest buffer zones (0.00). Out of the 291 rodent blood
smears examined, 34 were suspected positive for Y. pestis
coccobacilli, with one sample from M. natalensis confirmed positive
for the plasminogen activator (pla) gene of Y. pestis DNA by qPCR. The findings further indicated that 190 fleas belonging to four
different species were collected from 73 rodents, belonging to six
species. Among the collected fleas, Dinopsyllus lypusus was the
most abundant (46.32%), followed by Ctenophthalmus spp
(26.84%), Xenopsylla brasiliensis (16.32%), and Xenopsylla cheopis
(10.53%). A total of 38.42% fleas were collected from M. natalensis,
22.63% from L. striatus, and 18.42% from R. rattus. The highest flea
infestation prevalence was found on R. rattus with preponderance of
X. cheopis and X. brasiliensis. The specific flea index of X. cheopis
on R. rattus was (1.0 SFI) in plague foci and (<0.5 SFI) in non-
plague foci villages. The result of the GLM final model indicated that
the flea abundance on rodents was significantly dependent on
rodent species, season, habitats, rodent weight, sex, and location.
There was a weak positive correlation between rodent weight and
flea abundance.
Conclusion: Overall, the study suggests a possible enzootic
circulation of Y. pestis in sylvatic rodents in the plague foci villages
of Karatu district. The high abundance of rodents and fleas,
particularly in farmlands and peridomestic areas, increases the
potential risk for the transmission of the plague pathogen to the
surrounding population and thus poses a threat to public health,
especially considering that these habitats serve as crucial sites for
human economic activities, interactions, and livelihoods. Therefore,
it is of paramount importance to maintain vigilant rodent control to
mitigate any risks of repeated plague outbreaks.
Description
MSc Dissertation
Keywords
Plague, Rodent abundance, Yersinia pestis DNA, Trap success, Flea index, Rodent blood smear.