Abstract
During the summer of 2020, West Nile virus (WNV) caused an important outbreak in south-western Spain, with the highest impact on humans in the country up to that time, resulting in 77 clinical cases (including eight fatalities). Concurrently, equine WNV foci were reported within the same region. Meanwhile, WNV circulation was also detected in horses and birds in north-eastern Spain (Catalonia), although no human cases were notified. This striking difference in human case incidence between these two affected areas may be due to characteristics of the strains circulating in each site. One of these intrinsic viral strain factors that may account for these differences is the competence of avian reservoir hosts. A higher host competence leads to a higher viral spread in the enzootic cycle, consequently, increasing the risk of spillover to humans and horses. To assess differences in host competence, WNV strains circulating in both areas during the summer of 2020 were studied through in vivo inoculation of a transmission-competent avian species susceptible to WNV infection, the red-legged partridge, autochthonous to the Iberian Peninsula. The south-western strain SPA20-02, belonging to lineage 1, and the north-eastern strain AC924, belonging to lineage 2, were inoculated in parallel in red-legged partridges. The SPA20-02 strain exhibited higher and longer viraemias than the AC924 strain, resulting in a higher competence index for this avian species. The lower competence index of red-legged partridges for transmission of AC924 suggests that this strain exhibits a lower transmission capacity and, consequently, lower spread risk. These findings indicate that the lower severity of the 2020 outbreak in north-eastern Spain could, at least partially, be explained by the reduced transmission potential of the AC924 strain.