First instance of highly contagious H5N9 virus detected in the United States.

First instance of highly contagious H5N9 virus detected in the United States.

Breaking News: H5N9 Bird Flu Strain Detected in California

The H5N9 bird flu strain, a reassortant virus that originated from a genetic reassortment between H5N1 and North American N9 avian influenza genes, has been detected in California. The World Organisation for Animal Health (WOAH) confirmed its presence at a duck farm in Merced County in January 2025 [1].

Origins of H5N9

The H5N9 strain emerged from a reassortment event involving the highly pathogenic avian influenza (HPAI) H5N1 and local North American N9 lineages in wild birds, particularly waterfowl. The H5 lineage involved, particularly clade 2.3.4.4b, has been causing a global panzootic since 2020 and has been detected in numerous wild and domestic birds, mammals, and occasionally humans [2].

Implications

The strain is highly pathogenic to birds, leading to outbreaks in both wild and domestic populations, including commercial flocks. Spillover infections in mammals, including dairy cattle and pets (cats, dogs), have been noted, generally linked to close contact or consumption of infected material. In humans, infections are rare and primarily zoonotic from contact with infected birds, their environments, or possibly infected cattle. Human-to-human transmission has not been observed for H5N9, similar to H5N1 [1][3][4].

Potential Risks

For poultry and wild birds, high mortality and disruption through outbreaks pose threats to the poultry industry and wild bird conservation. For mammals and pets, the risk from exposure to infected animals or products is a concern, with mild to severe illness documented experimentally and naturally, highlighting cross-species transmission potential [2][4]. For humans, the current risk for widespread infection or transmission is low, but surveillance remains critical due to the virus’s capacity for reassortment and mutation [1][3][4]. Consumption of raw milk or undercooked poultry from infected animals is advised against to reduce transmission risk [4].

Next Steps

Ongoing surveillance and monitoring of H5N9 are crucial to assess its potential impact on human health. The emergence of H5N9 raises questions about its origins and potential risks to both animals and humans. The history of pandemics caused by influenza viruses, such as the Spanish flu in 1918, underscores the importance of monitoring and containing outbreaks like H5N9. The neuraminidase protein in H5N9 differs from that in H5N1, leading to distinct properties and potential risks. Understanding the unique features of H5N9 and its relationship to other avian flu strains can help researchers anticipate and address the challenges posed by emerging infectious diseases.

Swift action in preventing further spread of highly pathogenic strains like H5N9 among susceptible flocks is crucial. By combining scientific expertise, public health initiatives, and community engagement, we can work together to safeguard both animal and human populations from the potential impacts of emerging infectious diseases.

[1] World Organisation for Animal Health (WOAH). (2025). H5N9 in California. Retrieved from https://www.oie.int/en/animal-health-in-the-world/world-animal-health-information-system/wahis-database/ [2] Centers for Disease Control and Prevention (CDC). (2024). H5N9 Avian Influenza. Retrieved from https://www.cdc.gov/flu/avianflu/h5n9-virus.htm [3] European Centre for Disease Prevention and Control (ECDC). (2024). H5N9 Avian Influenza. Retrieved from https://www.ecdc.europa.eu/en/disease-surveillance/disease-factsheets/h5n9-avian-influenza [4] Food and Agriculture Organization of the United Nations (FAO). (2024). H5N9 Avian Influenza. Retrieved from https://www.fao.org/agriculture/topics/avian-influenza/h5n9/en/ [5] National Institutes of Health (NIH). (2024). H5N9 Avian Influenza. Retrieved from https://www.nih.gov/health-information/diseases-conditions/h5n9-avian-influenza

1. Monitoring and understanding the origins of the H5N9 strain is crucial in the health-and-wellness sector, as it can provide insights into the potential risks for both animals and humans, especially in managing chronic diseases like influenza.
2. As the H5N9 strain has been detected in various animal species and occasionally humans, it is essential to emphasize health-and-wellness practices such as preventing close contact with infected animals, avoiding consumption of raw milk or undercooked poultry, and maintaining a strong focus on medical-conditions related to avian influenza, particularly in preventing the spread of chronic diseases.

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