High Mortality of Wild European Rabbits during a Natural Outbreak of Rabbit Haemorrhagic Disease GI.2 Revealed by a Capture-Mark-Recapture Study.

Rabbit haemorrhagic disease virus (RHDV) GI.2 has caused significant declines in the abundance of wild European rabbits (), contributing to the species being recently classified as "endangered" in its native range. The epidemiology of this virus is still poorly understood despite its relevance for domestic and wild rabbits. During a longitudinal capture-mark-recapture (CMR) study of wild Iberian rabbits, , in a semiextensive breeding enclosure, an outbreak of RHDV GI.2 took place in January-February 2022, allowing us to estimate key epidemiological parameters of a natural outbreak. From April 2021 to July 2022, 340 rabbits were captured 466 times and individually identified, and some were vaccinated against myxoma virus (MYXV) and/or RHDV GI.2. Sera were collected and tested for IgG specific for MYXV and RHDV GI.2, and data were analyzed using multievent CMR models. During six weeks in January-February 2022, an estimated 81.0% (CI 77.1-84.3%) of the population died. Intensive aboveground searches could recover 189 carcasses (50.5% of the estimated mortality, CI 41.8-63.4%), with RHDV GI.2 detected in 6/7 tested. Apparent RHDV GI.2 seroprevalence rose from 15.4% (CI 8.0-27.5%) in January 2022 to 87.9% (CI 72.7-95.2%) in February 2022. The apparent mortality of RHDV GI.2-seropositive rabbits during the outbreak was estimated as null, while for seronegative rabbits, it was 76.0% (CI 53.8-90.3%). Among the seronegative rabbits, mortality was higher in unvaccinated (100%) than in recently vaccinated (60.0 ± 16.6%) and in females (100%) than in males (52.0 ± 17.1%). Infected carcasses in the burrows might explain the medium-term disease persistence in the population following the outbreak. Rabbits with antibodies at the cutoff for seropositivity were fully protected from fatal infection. Females had a higher fatality rate than males, underscoring the impact of RHDV GI.2 on the population dynamics of this endangered species.