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Murine typhus is a flea-borne zoonotic disease that has been recently reported on Reunion Island, an oceanic volcanic island located in the Indian Ocean. Five years of survey implemented by the regional public health services have highlighted a strong temporal and spatial structure of the disease in humans, with cases mainly reported during the humid season and restricted to the dry southern and western portions of the island. We explored the environmental component of this zoonosis in an attempt to decipher the drivers of disease transmission. To do so, we used data from a previously published study (599 small mammals and 175 Xenopsylla fleas from 29 sampling sites) in order to model the spatial distribution of rat fleas throughout the island. In addition, we carried out a longitudinal sampling of rats and their ectoparasites over a 12 months period in six study sites (564 rats and 496 Xenopsylla fleas) in order to model the temporal dynamics of flea infestation of rats. Generalized Linear Models and Support Vector Machine classifiers were developed to model the Xenopsylla Genus Flea Index (GFI) from climatic and environmental variables. Results showed that the spatial distribution and the temporal dynamics of fleas, estimated through the GFI variations, are both strongly controlled by abiotic factors: rainfall, temperature and land cover. The models allowed linking flea abundance trends with murine typhus incidence rates. Flea infestation in rats peaked at the end of the dry season, corresponding to hot and dry conditions, before dropping sharply. This peak of maximal flea abundance preceded the annual peak of human murine typhus cases by a few weeks. Altogether, presented data raise novel questions regarding the ecology of rat fleas while developed models contribute to the design of control measures adapted to each micro region of the island with the aim of lowering the incidence of flea-borne diseases., Author summary Murine typhus is a neglected zoonotic disease, as the number of human cases is likely underestimated in the absence of specific symptoms. It is caused by Rickettsia typhi, a pathogenic bacterium transmitted by rat fleas (Xenospylla spp). The distribution and dynamics of this disease result from complex interactions involving vectors, reservoirs and humans within a shared environment. In this study, we explored the environmental drivers of rat fleas’ abundance on Reunion Island, where murine typhus has recently emerged. Results showed that i) rat fleas’ abundance is highly dynamic, characterized by a peak at the end of the dry season and ii) among the factors investigated, rainfall, temperature and land cover are the main determinants of rat fleas’ abundance. We modeled a predictive map of flea distribution that strongly correlates with the spatial distribution of human cases on the island. This study highlights the importance of accounting for environmental and climatic characteristics to better understand the spatial and temporal drivers of flea-borne diseases.

Additional file 2: Table S6. Observed Culicoides abundance during the sampling campaign from 7 to 22 March 2018.

Additional file 1: Table S1. Model parameters for C. bolitinos. Table S2. Model parameters for C. enderleini. Table S3. Model parameters for C. grahamii. Table S4. Model parameters for C. imicola. Table S5. Model parameters for C. kibatiensis.

Additional file 2: Table S1. Characteristics of trapping sites. Table S2. Temporal variables considered on mixed-effect negative binomial hurdle models. Table S3. Non-dynamic variables considered on mixed-effect negative binomial hurdle models.

Bluetongue and epizootic hemorrhagic disease viruses are recognized worldwide as important vector-borne viruses with major consequences on livestock trade and breed-ing. They are both transmitted by certain small hematophagous biting midge species belonging to the Culicoides genus, to cattle, sheep and goats. If the epidemiological sit-uation in epizootic territories has been recently widely studied after massive outbreaks in Europe, endemic areas have been neglected. In this presentation, we will review three studies implemented between 2016-2018 in La Réunion Island to give better in-sights to the local situation: (i) a virological and serological survey on cattle to identify the two virus dynamics and prevalence, (ii) an entomological study on Culicoides spe-cies diversity and seasonal dynamics, and (iii) a virus screening in Culicoides species to identify and characterize vector species of both viruses, the level of infection in wild species populations and virus dynamics in Culicoides populations. Altogether, these three studies helped to have a better understanding of the epidemiological situations in the island. Five Culicoides species are described on the island, presenting an altitudi-nal gradient and 4 species are implicated in both virus transmissions. Seasonal dynam-ics is particularly marked for 3 species. Therefore, bluetongue might be considered as an endemic disease with continuous virus circulation, limited clinical cases and high serological prevalence while Epizootic hemorrhagic disease appeared as an epizootic disease with seasonal transmission patterns.

Reunion Island regularly faces outbreaks of epizootic haemorrhagic disease (EHD) and bluetongue (BT), two viral diseases transmitted by haematophagous midges of the genus Culicoides (Diptera: Ceratopogonidae) to animals of economic importance such as cattle, sheep and goats. To date, five species of Culicoides are recorded in Reunion Island: Culicoides bolitinos, C. enderleini, C. grahamii, C. imicola, and C. kibatiensis. Although epizootics and Culi-coides diversity are already well documented, abundance and seasonality of the five species are not. According to a recent viral screening of local Culicoides populations (unpublished data), at least four species are involved in the transmission of each virus. Therefore, cha-racterizing the risk period by modelling the temporal dynamics of the five Culicoides species is a key step to better understand BT and EHD epidemiology and improve their control. Between 2016 and 2018, 55 biweekly Culicoides catches using OVI traps were set up in 11 sites. A hurdle model (i.e. a presence/absence model combined with an abundance model) was developed for each species in order to determine climatic and environmental drivers of presence and abundance of Culicoides. Regarding abundance, average Culicoides catch per site ranges from 4 to 45,875 individuals. Also, diversity differ between sites with C. imicola being dominant at low altitude and C. kiba-tiensis at high altitude. A marked seasonality is observed for the 3 other species. Eleven me-teorological and environmental determinants were used to model presence and abundan-ce of each species: temperature, humidity, rain, wind, global radiation, vegetation index, eco-climatic area, land use, farm density, animal density and length of nearby watercourse. The association of these determinants to explain presence and/or abundance depends on the species, but each plays a role in at least one species. This is the first study to model Culicoides population dynamics in Reunion Island. In the absence of vaccination and vector control strategies, determining periods of high abundance of Culicoides is a crucial first step towards identifying periods at high risk of transmission for both viruses.

Objectives: Bluetongue (BT) and epizootic hemorrhagic disease (EHD) viruses are two important vector-borne orbiviruses with a worldwide distribution that have major impacts on livestock breeding and trade. They are transmitted by hematophagous Culicoides species. If epidemiological investigations in epizootic territories conducted after massive outbreaks enabled to better understand the epidemiology of these diseases, their epidemiology remain poorly studied/understood in areas of endemicity. In this context, three studies were implemented in Reunion Island located in the South Western Indian Ocean Materials and Methods: The three studies were: (i) a virological and serological survey on cattle to identify BT and EHD viruses seasonal dynamics and prevalence, (ii) an entomological study on Culicoides species diversity, seasonal dynamics and spatial distribution, (iii) a viral screening in Culicoides species to identify and better characterize the vector species involved in the viral transmission, the level of infection in field populations and virus dynamics in Culicoides populations. Results: The period with highest levels of seroconversion occurred is the hot and rainy season, when incidence of antibody acquisition reached 36.7% (IC95% = [31.7%; 41.7%]) for EHD virus and 20.7% (IC95% = [16.3% ; 25.1%]) for BT virus. Five Culicoides species are described on the island, and seasonal dynamics is particularly marked for three of them. Four species are involved in the transmission of each virus. Conclusion: Overall, BT is endemic with continuous virus circulation, limited clinical cases and high serological prevalence while EHD is epizootic with a marked seasonal transmission pattern.