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Leptospirosis in the Seychelles : geographic, molecular and epidemiological investigations of a zoonotic disease in a tropical insular environment

Abstract : Leptospirosis is an emerging neglected disease representing a heavy burden in the tropics, especially in tropical islands such as Seychelles, which record among the highest human incidence worldwide. This thesis aims at exploring the eco-epidemiology of leptospirosis in Seychelles by (i) using rats as markers of environmental exposure to Leptospira infection, (ii) describing the molecular epidemiology of the disease in humans and animals in a One Health framework, and (iii) identifying occupational and behavioural risk factors while comparing the current situation to that described 25 years ago. The combination of fine spatial distribution, molecular and clinical epidemiology complement each other in providing a comprehensive picture of the continuum involving reservoirs and human hosts within a shared environment.Habitat fragmentation and proximity to nutritional sources are found good predictors of Leptospira-laden Rattus spp. Geospatial analyses determined a selection of other important variable factors that are strongly correlated with Leptospira infection in Rattus spp., including altitude or distance to surface water (negative correlation), urbanization and heavy rainfall (positive correlation). Results of these analyses can guide policy makers and especially urban planners to best implement landscape structures for conservation or pest control goals leading to reduced exposure of humans to rat-borne diseases.Rattus norvegicus is found significantly more infected than Rattus rattus. Therefore, increased infection in urbanized/fragmented habitats may result at least in part from Rattus spp distribution, as R. norvegicus is mostly found in urban areas. Most importantly, genotyping of Leptospira in human acute cases and rats suggests that these rodents are involved in only a third of human acute infections, while most human cases originate from yet to be identified reservoir(s).An annual incidence of 54.6 (95% CI 40.7-71.8) per 100,000 confirms the major medical and public health importance of the disease in the country. The disease affects mainly men (96%) and displays a case fatality rate of 11.2%, mostly associated with severe forms (acute renal failure, hepatic failure and pulmonary haemorrhage). Farming and gardening related activities, proximity to cattle and cats, thrombocytopaenia, leukocytosis, elevated bilirubin and high values for renal function tests are predictors of leptospirosis. The geographical distribution of human cases poorly overlaps districts of high prevalence in rats in keeping with a restricted role of rats in human disease.The comparison of figures reported herein and in previous studies published 25 years ago reveals changes in behaviour and exposure, and shows that the development of health care has lowered the case fatality despite still high disease incidence in the country. A low level of knowledge on leptospirosis is reported, urging the need for implementing health education campaigns. Altogether, the data presented in this thesis strongly supports the implementation of a research program aiming at discovering alternative reservoir(s) to provide a full understanding of the epidemiological situation, which will allow fine tuning preventive measures for an efficient control of a disease that is still recognised as the infectious disease causing the highest mortality in the country.
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Leon Biscornet. Leptospirosis in the Seychelles : geographic, molecular and epidemiological investigations of a zoonotic disease in a tropical insular environment. Human health and pathology. Université de la Réunion, 2020. English. ⟨NNT : 2020LARE0013⟩. ⟨tel-02952395⟩

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