Pathogenesis of Bagaza virus infection in experimentally infected red-legged partridges

Bagaza virus (BAGV) is a mosquito-borne flavivirus of the Ntaya serocomplex that emerged in 2010 in Spain in an outbreak in gamebirds, primarily red-legged partridges. It reappeared in 2019 and again in 2021 when it emerged also in Portugal. An experimental infection confirmed the high susceptibility of red-legged partridges to the virus and direct transmission to uninfected contacts. Here we study pathogenesis and viral antigen distribution in experimentally infected partridges in comparison to what was found in field cases. Sixteen five-month-old red-legged partridges were inoculated with BAGV (strain Spain RLP-Hcc1/2010). Four additional partridges were kept as contacts in the same cage, and ten sham-inoculated partridges as controls. Two partridges were euthanized at 4, 7 and 10 days post-infection (dpi) and another three birds died on days 6, 7 and 10 dpi, respectively. Tissues (brain, lung, pancreas, liver, kidney, spleen, heart, bursa of Fabricius, large and small intestines) were fixed in 10% neutral buffered formalin and processed for histopathology and immunohistochemistry. Slides were scored for lesion severity and number and type of viral antigen positive cells per 0.2mm2. Partridges euthanized at 4 dpi only had congestion and inflammatory infiltrates in spleen and heart, while from 7dpi on, the main findings were congestion and endothelial hypertrophy in addition to mononuclear inflammatory infiltrates in the spleen, kidney, liver and heart. Perivascular cuffs of inflammatory cells, endothelial cell swelling, and neuronal necrosis and gliosis were observed in the brain, especially of birds euthanized/dead on 10 dpi. Hemosiderosis, one of the hallmarks of BAGV infection in free-living red-legged partridges, was only observed sporadically. BAGV antigen was found on 4 dpi in the caeca and spleen. On 7 and 10 dpi, BAGV antigen were mostly identified in the spleen, bursa, heart, and kidney. No BAGV antigen was detected in the brain except for two Purkinje ce