Predicting temperature-dependent transmission suitability of bluetongue virus in livestock

Linking vector favourable environmental conditions with serological evidence of widespread bluetongue virus exposure in livestock in Ecuador

Despite existing knowledge of bluetongue disease (BT) in Latin America, little information is available on its actual spread and overall burden. As a vector-borne disease, high-risk areas for BT coincide with environmental conditions favourable for the prevailing vector. In Ecuador, information on the presence of BT is limited to singled out virological findings. In this study, we obtained serological evidence for BT virus exposure from the passive surveillance system of the National Veterinary Service, which monitors reproductive-vesicular diseases, including FMD and BT, as part of differential diagnosis. Bioclimatic factors relevant to Culicoides development as the main vector and host abundance at the parish level were considered as risk factors and analysed using a logistic regression model. The results reveal widespread evidence of bluetongue virus exposure, geographically aligning with favourable vector ecosystems within a temperature range of 12-32 °C. Key variables for predicting high-risk BT areas include cattle population, maximum temperature of the warmest month, minimum temperature of the coldest month, temperature seasonality, and precipitation of the driest month. This analysis, the first of its kind for an Andean country with diverse ecosystems, provides a foundation for initial strategic approaches for targeted surveillance and control measures, considering a One Health approach.

Parameterisation of a bluetongue virus mathematical model using a systematic literature review

Bluetongue virus (BT) is a vector-borne virus that causes a disease, called bluetongue, which results in significant economic loss and morbidity in sheep, cattle, goats and wild ungulates across all continents of the world except Antarctica. Despite the geographical breadth of its impact, most BT epidemiological models are informed by parameters derived from the 2006-2009 BTV-8 European outbreak. The aim of this study was to develop a highly adaptable model for BT which could be used elsewhere in the world, as well as to identify the parameters which most influence outbreak dynamics, so that policy makers can be properly informed with the most current information to aid in disease planning. To provide a framework for future outbreak modelling and an updated parameterisation that reflects natural variation in infections, a newly developed and parameterised two-host, two-vector species ordinary differential equation model was formulated and analysed. The model was designed to be adaptable to be implemented in any region of the world and able to model both epidemic and endemic scenarios. It was parameterised using a systematic literature review of host-to-vector and vector-to-host transmission rates, host latent periods, host infectious periods, and vaccine protection factors. The model was demonstrated using the updated parameters, with South Africa as a setting based on the Western Cape's known cattle and sheep populations, local environmental parameters, and Culicoides spp. presence data. The sensitivity analysis identified that the duration of the infectious period for sheep and cows had the greatest impact on the outbreak length and number of animals infected at the peak of the outbreak. Transmission rates from cows and sheep to C. imicola midges greatly influenced the day on which the peak of the outbreak occurred, along with the duration of incubation period, and infectious period for cows. Finally, the protection factor of the vaccine had the greatest influence on the total number of animals infected. This knowledge could aid in the development of control measures. Due to gradual climate and anthropological change resulting in alterations in vector habitat suitability, BT outbreaks are likely to continue to increase in range and frequency. Therefore, this research provides an updated BT modelling framework for future outbreaks around the world to explore transmission, outbreak dynamics and control measures.

Immunological and Pathogenic Differences of Two Experimental Bluetongue Virus Serotype Infections Evaluated in Two Disparate Host Species

Bluetongue virus (BTV) is a prevalent midge-borne pathogen that infects ruminant species worldwide. BTV infections range from asymptomatic to lethal, with mechanisms that determine the severity of infection remaining largely undefined. Although it is relatively poorly understood, the immune response to BTV infection is thought to be critical for both the propagation of disease as well as the resolution of infection. To bridge this gap in knowledge, we infected cohorts of sheep and muntjac deer with two serotypes of BTV (BTV10 and BTV17) for longitudinal analysis (30 days). Interestingly, species-specific differences were observed. Circulating virus was detected early and remained detectable for the duration of the sheep study, while infections in muntjac showed faltering detection of BTV10 at 3 weeks post infection. The magnitude of the immune response was subdued in the muntjac when compared to the sheep cohorts, though similar responses were observed. We also assessed midge viral uptake and the ability to replicate BTV. Midges successfully fed on both species, yet those that fed on sheep resulted in more efficient BTV transmission. Our findings demonstrate that differences in BTV infections, immune responses, and vector competence across host species and serotypes will impact global BTV emergence and strategies for mitigation.

The impact of the bluetongue serotype 3 outbreak on sheep and goat mortality in the Netherlands in 2023

In September 2023, bluetongue virus serotype 3 (BTV-3) emerged in the Netherlands, infecting over five thousand livestock farms. In sheep, high morbidity and mortality rates were reported that were unlike previously described bluetongue outbreaks. This study aimed to quantify the impact of BTV-3 in the small ruminant population in the Netherlands in 2023. Sheep and goat movement census data and BTV-3 notification data were available from 2020 until the end of 2023. Data were aggregated to farm and week level and mortality indicators were calculated for lambs (<1 year) and adult animals (≥1 year). Population averaged GEE models with a Negative-binomial distribution and a log-link function correcting for repeated measures per farm in time were used to quantify the association between BTV-3 and mortality. In 2023, 2994 sheep farmers and 89 goat farmers notified clinical signs of BTV-3 to the NVWA. During this BTV-3 outbreak period, an additional 55,000 sheep died compared to the same period in 2020-2022. At flock level a high variety in mortality was observed, with a clear increase in mortality in both flocks that were not notified but that were located in infected areas and in flocks of which the farmer notified clinical signs. During the BTV-3 outbreak period, mortality in infected areas increased 4.2 (95 % CI: 4.0-4.3) times in sheep lambs (<1 year) and 4.6 (95 % CI: 4.4-4.8) times in sheep (≥1 year) compared to BTV-3 free areas. Flocks with a confirmed BTV-3 infection that were notified in September showed a 12.8 (95 % CI: 11.4-14.3) times higher mortality in lambs and a 15.1 (95 % CI: 13.7-16.6) times higher mortality in sheep compared to flocks in BTV-3 areas. In flocks of which the farmer notified clinical signs after September, mortality was 4.6 (95 % CI: 4.2-5.0) and 5.6 (95 % CI: 5.1-6.0) times higher in lambs and sheep compared BTV-3 areas respectively. In goats, around 4000 additional deaths were recorded during the BTV-3 outbreak period. In farms that were notified, mortality of goats (≥1 year) was 1.8 (95 % CI: 1.2-2.8) times higher compared to BTV-3 free areas. Since May 2024, multiple BTV-3 vaccines are available in the Netherlands. In June 2024, the first new infections of BTV-3 were confirmed in Dutch sheep flocks. Hopes are that with the possibility to vaccinate, the spread and impact of BTV-3 in the Netherlands will rapidly decline and that losses as observed in 2023 will no longer be seen.

Epidemiological Analyses of the First Incursion of the Epizootic Hemorrhagic Disease Virus Serotype 8 in Tunisia, 2021-2022

Epizootic hemorrhagic disease (EHD) is a non-contagious arthropod-transmitted viral disease and a World Organization for Animal Health (WOAH)-listed disease of domestic and wild ruminants since 2008. EHDV is transmitted among susceptible animals by a few species of midges of genus Culicoides. During the fall of 2021, a large outbreak caused by the epizootic hemorrhagic disease virus (EHDV), identified as serotype 8, was reported in Tunisian dairy and beef farms with Bluetongue virus (BTV)-like clinical signs. The disease was detected later in the south of Italy, in Spain, in Portugal and, more recently, in France, where it caused severe infections in cattle. This was the first evidence of EHDV-8 circulation outside Australia since 1982. In this study, we analyzed the epidemiological situation of the 2021-2022 EHDV outbreaks reported in Tunisia, providing a detailed description of the spatiotemporal evolution of the disease. We attempted to identify the eco-climatic factors associated with infected areas using generalized linear models (GLMs). Our results demonstrated that environmental factors mostly associated with the presence of C. imicola, such as digital elevation model (DEM), slope, normalized difference vegetation index (NDVI), and night-time land surface temperature (NLST)) were by far the most explanatory variables for EHD repartition cases in Tunisia that may have consequences in neighboring countries, both in Africa and Europe through the spread of infected vectors. The risk maps elaborated could be useful for disease control and prevention strategies.