Mass mortality event in South American sea lions (Otaria flavescens) correlated to highly pathogenic avian influenza (HPAI) H5N1 outbreak in Chile

Abstract

In Chile, since January 2023, a sudden and pronounced increase in strandings and mortality has been observed among South American (SA) sea lions (Otaria flavescens), prompting significant concern. Simultaneously, an outbreak of highly pathogenic avian influenza H5N1 (HPAIV H5N1) in avian species has emerged since December 2022. To investigate the cause of this unexpected mortality, we conducted a comprehensive epidemiological and pathologic study. One hundred sixty-nine SA sea lions were sampled to ascertain their HPAIV H5N1 status, and long-term stranding trends from 2009 to 2023 were analyzed. In addition, two animals were necropsied. Remarkably, a significant surge in SA sea lion strandings was observed initiating in January 2023 and peaking in June 2023, with a count of 4,545 stranded and deceased animals. Notably, this surge in mortality correlates 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 H5N1 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 in necropsied animals 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. Further studies are needed to better understand the pathogenesis and transmission.

Keywords: Chile; Mortality; South American sea lions; highly pathogenic avian influenza H5N1; outbreak; strandings.

Figure 1.

Statistical process control chart of Otaria flavescens strandings by month was collected from January 2009 to June 2023 (Illustrated from January 2020). the red arrow depicts January 2023 where the stranded surpasses the upper control limits.

Figure 2.

Column graph showcasing the frequency of stranded Otaria flavescens (South American sea lion) throughout the year 2023 by month.

Figure 3.

Spatial distribution and the high-risk cluster of influenza virus (IV) positive and negative samples from O. flavescens in Chile. A National situation of HPAIV, showing positive and negative samples (red and blue dots respectively) and high-risk spatial clusters for O. flavescens (light purple circles), and high-risk spatial clusters from wild bird outbreak in Chile (light brown circle). B overlaying high-risk significant HPAIV from wild birds and O. flavescens located in northern Chile. C proximity between statistically significant high-risk HPAIV spatial clusters for wild birds and positive cases of HPAIV in O. flavescens in coastal areas of central Chile.

Figure 4.

A. Brain. Male South American (SA) sea lion (245513). severe multifocal to locally extensive areas of non-suppurative encephalitis and hemorrhages. H&E, 10×. B. Brain. Male South American (SA) sea lion (245513). neuronal and glial necrosis with associated neuronophagia and gliosis. H&E, 40×. C. Brain. Male SA sea lion (245513). Intralesional positive immunostaining. IHC against Influenza A nucleoprotein in neurons and glial cells, IHC against Influenza A nucleoprotein, 40×. D. Arbor vitae of the cerebellum. Female sea lion (245719). Intralesional positive immunostaining. IHC against Influenza A nucleoprotein, 10×.

Figure 5.

The Otaria flavescens (South American sea lion) adult female necropsied. The animal tested positive for the HPAI H5N1 virus. The SA sea lion displayed a prominent wasting syndrome, characterized by inadequate body condition and muscular weakness, with notably pronounced ribs reflecting its poor physical state.

Figure 6.

Lung. Male sea lion (245513). intranuclear and intracytoplasmic staining for Influenza A nucleoprotein in peribronchial inflammatory cells and cell debris in peri-bronchial glandular area. IHC against Influenza A nucleoprotein, 20×. Inset: magnification of the positive immunostaining. 60×.