Highly pathogenic avian influenza (HPAI A H5N1) associated to mass mortality event in South American sea lion (Otaria flavescens) (H 34)

Abstract

During 2023, a pronounced increase in strandings and mortality was observed among South American (SA) sea lions (Otaria flavescens) from the coast of Chile. Simultaneously, an outbreak of highly pathogenic avian influenza H5N1 (HPAIV H5N1) in avian species emerged from late 2022 to mid 2023. To investigate the cause of this unexpected mortality, we conducted a comprehensive epidemiological and pathologic study. One hundred and sixty-nine SA sea lions were sampled to ascertain their HPAIV H5N1 status, including two animals that underwent a necropsy procedure in search of histopathological evidence of the virus. A total count of 20,123 specimens were stranded and deceased during 2023 possibly associated to HPAI H5N1 virus. Notably, this surge in mortality correlated geographically with HPAIV outbreaks affecting wild birds. Among 168 sampled SA sea lions, 34 (20%) tested positive for Influenza A virus, and 21 confirmed for HPAIV H5N 1 2.3.4.4b clade in tracheal/rectal swab pools. Clinical and pathological evaluations of the two necropsied stranded sea lions revealed prevalent neurological and respiratory signs, including disorientation, tremors, ataxia, and paralysis, as well as acute dyspnea, tachypnea, profuse nasal secretion, and abdominal breathing. The lesions identified aligned with observed clinical signs. Detection of the virus via immunohistochemistry (IHC) and real-time PCR in the brain and lungs affirmed the findings. The findings provide evidence between the mass mortality occurrences in SA sea lions and HPAIV, strongly indicating a causal relationship.