The past three decades have seen an increasing number of emerging arthropod-borne viruses in temperate regions This process is ongoing, driven by human activities such as inter-continental travel, combined with the parallel emergence of invasive arthropods and an underlying change in climate that can increase the risk of virus transmission and persistence. In addition, natural events such as bird migration can introduce viruses to new regions. Despite the apparent regularity of virus emergence, arthropod-borne viruses circulating in temperate regions face the challenge of the late autumn and winter months where the arthropod vector is inactive. Viruses therefore need mechanisms to overwinter or they will fail to establish in temperate zones. Prolonged survival of arthropod-borne viruses within the environment, outside of both vertebrate host and arthropod vector, is not thought to occur and therefore is unlikely to contribute to overwintering in temperate zones. One potential mechanism is continued infection of a vertebrate host. However, infection is generally acute, with the host either dying or producing an effective immune response that rapidly clears the virus. There are few exceptions to this, although prolonged infection associated with orbiviruses such as bluetongue virus occurs in certain mammals, and viraemic vertebrate hosts therefore can, in certain circumstances, provide a route for long-term viral persistence in the absence of active vectors. Alternatively, a virus can persist in the arthropod vector as a mechanism for overwintering. However, this is entirely dependent on the ecology of the vector itself and can be influenced by changes in the climate during the winter months. This review considers the mechanisms for virus overwintering in several key arthropod vectors in temperate areas. We also consider how this will be influenced in a warming climate.

An outbreak of disease in a Swedish beef cattle herd initiated an in-depth study to investigate the presence of bacteria and viruses in the blood of clinically healthy (n = 10) and clinically diseased cattle (n = 20) using whole-genome shotgun sequencing (WGSS). The occurrence of infectious agents was also investigated in ticks found attached to healthy cattle (n = 61) and wild deer (n = 23), and in spleen samples from wild deer (n = 30) and wild boars (n = 10). Moreover, blood samples from 84 clinically healthy young stock were analysed for antibodies against Anaplasma phagocytophilum and Babesia divergens. The WGSS revealed the presence of at least three distinct Mycoplasma variants that were most closely related to Mycoplasma wenyonii. Two of these were very similar to a divergent M. wenyonii variant previously only detected in Mexico. These variants tended to be more common in the diseased cattle than in the healthy cattle but were not detected in the ticks or wild animals. The DNA of A. phagocytophilum was detected in similar proportions in diseased (33%) and healthy (40%) cattle, while 70% of the deer, 8% of ticks collected from the cattle and 19% of the ticks collected from deer were positive. Almost all the isolates from the cattle, deer and ticks belonged to Ecotype 1. Based on sequencing of the groEL-gene, most isolates of A. phagocytophilum from cattle were similar and belonged to a different cluster than the isolates from wild deer. Antibodies against A. phagocytophilum were detected in all the analysed samples. In conclusion, uncommon variants of Mycoplasma were detected, probably associated with the disease outbreak in combination with immune suppression due to granulocytic anaplasmosis. Moreover, A. phagocytophilum was found to be circulating within this cattle population, while circulation between cattle and deer occurred infrequently.

Background: Tick-borne encephalitis virus (TBEV) is expanding its range in Europe, with increasing human cases reported. Since the first detection of TBEV in ticks in the United Kingdom in 2019, one possible, two probable and two confirmed autochthonous cases in humans have been reported. Aim: We aimed to understand the environmental and ecological factors limiting TBEV foci at their range edge and predict suitable areas for TBEV establishment across Great Britain (GB) by modelling patterns of exposure to TBEV in deer. Methods: We developed spatial risk models for TBEV by integrating data between 2018 and 2021 on antibodies against tick-borne flavivirus in fallow, muntjac, red and roe deer with data on potential risk factors, including climate, land use, forest connectivity and distributions of bank voles and yellow-necked mice. We overlayed modelled suitability for TBEV exposure across GB with estimations on number of visitors to predict areas of high human exposure risk. Results: Models for fallow, muntjac and roe deer performed well in independent validation (Boyce index > 0.92). Probable exposure to TBEV was more likely to occur in sites with a greater percentage cover of coniferous woodland, with multiple deer species, higher winter temperatures and rates of spring warming. Conclusion: The resulting TBEV suitability maps can be used by public health bodies in GB to tailor surveillance and identify probable high-risk areas for human exposure to guide awareness raising and vaccination policy. Combining animal surveillance and iterative spatial risk modelling can enhance preparedness in areas of tick-borne disease emergence.