A novel Orthonairovirus detected in grey seal (Halichoerus grypus)

The <i>Orthonairovirus</i> genus is currently markedly expanding, with several recently detected species causing febrile disease in humans. Orthonairoviruses have tripartite, negative-sense RNA genomes, with the segments denoted according to size: S, M and L. They infect birds and mammals and are primarily transmitted by ectoparasite vectors such as ticks. In April 2020, a dead grey seal pup was found in the Östhammar municipality on the Swedish coast of the Baltic Sea. Histological examination showed pathological disruption of the normal hepatic architecture caused by fatty degeneration, necrosis and inflammation suggestive of viral hepatitis. Deep sequencing of liver, spleen, lung and brain tissue revealed the presence of a novel orthonairovirus. The complete genome was determined and showed the closest similarity to the Yezo virus, originally detected in human patients with febrile disease on Hokkaido, Japan. Phylogenetic analysis shows that this virus, for the M and L segments, belongs to a clade where many members cause febrile disease in humans. Sequence similarity for the L-segment (90%) is below the ICTV species demarcation criterion, and the species name 'Östhammar virus' is therefore proposed. Screening with qPCR of archived liver and spleen samples from stranded seals and, as a potential ectoparasitic vector, archived ethanol-fixed seal lice, which had been collected along the same coastline, was all negative. Future work should screen the tick population in the vicinity of where the seal pup was found and investigate whether there are unexplained febrile disease cases among the human population in the area.