Assessment of potential factors for plague transmission in the plague endemic area of Karatu District, Northern Tanzania

MSc Dissertation

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.

Africa Centre of Excellence for Innovative Rodent Pest Management and Biosensor Technology and Development (ACE IRPM&BTD)