Researchers in Sweden have made a concerning discovery regarding the H5N1 avian influenza virus.
In a study published in the Emerging Infectious Diseases journal by the U.S. Centers for Disease Control and Prevention, it was reported that the virus was found in a harbor porpoise that had washed up on shore.
This suggests a potential crossover of the highly pathogenic virus from avian species to cetaceans. The H5N1 strain has already caused many bird deaths in Europe and North America, and it is now believed to have infected over 112 species, including marine birds.
Since December 2021, there have been increasing reports of H5N1 infections in wild bird species in Europe, and even mammals such as red foxes and mustelids are not immune to viral spillover.
According to recent findings, harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus) in North America are susceptible to the highly pathogenic H5N1 avian influenza virus. Although seals have been known to be vulnerable to influenza A viral infections, there are very few reports of influenza infections in cetaceans. Hence, the discovery of an H5N1 infection in a Phocoena phocoena (harbor porpoise) individual is alarming.
In this study, researchers reported a case of a stranded male P. phocoena individual on the west coast of Sweden. The porpoise appeared to be unwell, and it was swimming in circles, unable to right itself. Unfortunately, the porpoise died shortly after getting stranded, and therefore the carcass was subjected to necropsy.
Typically, when porpoises are found stranded, they undergo screening to detect the presence of cetacean morbilliviruses and influenza A virus, both of which can affect the nervous system in cetaceans as well as other species.
To test for cetacean morbillivirus, samples of the spleen, lung, and brain are analyzed using a technique called real-time reverse-transcriptase polymerase chain reaction (rRT-PCR). Additionally, samples taken from the bronchial and lung swabs are examined to detect the presence of influenza A viral genome.
After identifying the presence of the influenza A virus in the swab samples, further analysis was conducted on other organs to measure the viral loads. Moreover, a technique called immunohistochemical analysis was carried out using a primary monoclonal antibody specific to the influenza A nucleoprotein, which is commercially available, to determine the distribution of the viral antigen and its relationship with pathological lesions.
The necropsy results indicated no major macroscopic findings except for pulmonary edema, which resulted from drowning. However, microscopic analysis of the brain revealed evidence of lymphoplasmacytic meningoencephalitis, characterized by gliosis, necrosis, vasculitis, and perivascular cuffing. Additionally, the lung tissue showed a higher number of alveolar macrophages and evidence of mononuclear septal thickening.
Although cetacean morbilliviral ribonucleic acid (RNA) was not detected by rRT-PCR, swab samples revealed the presence of the H5N1 highly pathogenic avian influenza virus. High levels of the virus were found in the brain, followed by the lungs, kidney, liver, and spleen. No traces of the virus were found in the blubber, muscles, or intestines.
During the immunohistochemical analysis, the researchers observed moderate levels of immunolabelling in the cytoplasm and nuclei of neurons, choroid plexus epithelial cells, and glial cells. Lower levels of antigens were found in the cytoplasm and nuclei of scattered alveolar cells, including sloughed epithelial cells and macrophages. No viral antigens were detected in any other tissue.
The researchers hypothesized that the meningoencephalitis could have been responsible for the unusual behavior observed in the harbor seal, ultimately leading to its death by drowning. The presence of high levels of the virus in the brain was consistent with infections of H5N1 clade 2.3.4.4b in mammalian species. Since the viral strain closely resembled the H5N1 strain circulating among wild birds, it is possible that the virus was transmitted through contaminated water or direct contact with an infected bird.
The findings suggest that the H5N1 virus has an expanding host range, and its sporadic infection in other mammalian species increases the risk of its growing transmissibility and virulence.
In summary, this study identified the highly pathogenic H5N1 avian influenza virus in a stranded harbor seal in Sweden. The necropsy showed microscopic evidence of lymphoplasmacytic meningoencephalitis, which was believed to be the probable cause of the abnormal behavior and subsequent drowning.
Genomic and immunohistochemical analyses revealed high levels of the H5N1 virus in the brain and lungs. These findings underscore the growing risk of H5N1 spillover to mammalian species and highlight the urgent need for enhanced animal health surveillance and intervention strategies.
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