Epidemiology of a Hybrid Swarm: Evidence of 11 Feline Infectious Agents Circulating in a Population of Sympatric European Wildcat Hybrids and Free-Living Domestic Cats, in Scotland.

Hybridisation between wild and domestic species poses a serious challenge to conservation management and can, potentially, lead to extinction. Alongside it, disease transmission will inevitably occur. However, the link between these two phenomena has historically been neglected. In Scotland, the European wildcat is particularly threatened by hybridisation with the domestic cat, a process promoted by long-term habitat loss, human encroachment, and persecution. Between 2015 and 2019, free-living cats ( = 120) were captured in six conservation priority areas of northern Scotland. Samples were collected for infectious disease screening (feline immunodeficiency virus, feline leukaemia virus, feline calicivirus, feline herpesvirus, , , , , Candidatus , Candidatus , and ) and genetic analysis. Polymerase chain reaction and reverse transcriptionPCR were used to detect infectious DNA or RNA, respectively. The hybrid score (Q) for each individual cat was determined using a 35-SNP-marker test. Statistical analysis investigated the association between Q and probability of infection, accounting for spatial clustering. The results confirmed the presence of 11 infectious agents circulating in the free-living cat population of northern Scotland, which consists of a hybrid swarm between and . For eight of them (feline leukaemia virus, feline herpesvirus , , , , Ca. , and ), there was no significant association between infection probability and Q, supporting our hypothesis that the hybrid swarm may be functioning as a single epidemiological unit. Considering the impact of infectious diseases on health, welfare, and population dynamics of domestic cats, their presence in the extremely fragile and hybridised population of in Scotland could be population limiting or, potentially, contribute to local extinction. Comprehensive disease surveillance, risk analysis, and domestic cat management will be essential for the European wildcat conservation, particularly where hybridisation is increasing and anthropogenic factors are prevalent.