Pathological and serological insights into Lagovirus diseases dynamics in the European brown hare (Lepus europaeus): A nine-year longitudinal study.

The European brown hare syndrome virus (EBHSV; GII.1) and rabbit haemorrhagic disease virus 2 (RHDV2; GI.2) are pathogenic lagoviruses affecting the European brown hare (Lepus europaeus). EBHSV/GII.1 causes periodic epidemics, while RHDV2/GI.2 infections emerge from spillover events in areas where hares are sympatric with European rabbits (Oryctolagus cuniculus). In the northeast of the Iberian Peninsula, the overlap of these species provides a unique opportunity to investigate how the epidemiology of these viruses correlates with disease course. We analysed the presence of lagoviruses in 113 European brown hare carcasses recovered in Catalonia (NE Spain) between 2015 and 2024. Animals were necropsied, and tissue and serum samples were collected for histopathology, virological investigation, and serology. Sera from hunted hares apparently healthy (n = 89, 2015-2023) were also included in the study. PCR on liver samples (n = 58) and virological ELISA on positive sera (n = 52) confirmed 28 EBHSV/GII.1 and 24 RHDV2/GI.2 cases. After the first EBHSV/GII.1 detection in 2016, antibody titres decreased progressively until 2020-2021, coinciding with an outbreak. No conclusive seropositivity for RHDV2/GI.2 was observed during the study. Pathology revealed more acute lesions in RHDV2/GI.2-infected hares compared to EBHSV/GII.1. These lesions, resulting in sudden death due to a deficient immune response, may explain this distinct epidemiological scenario. Despite a decade of circulation, RHDV2/GI.2 has not fully adapted to hares. However, ongoing monitoring is essential, as mutations or recombination events could increase its epizootic potential. The co-circulation of both lagoviruses, combined with other co-factors, might jeopardise the viability of European brown hare populations at the southern limit of their range.