The highly pathogenic H5N9 avian influenza has been identified in ducks at a commercial farm in California, marking the first time this strain has been found in U.S. poultry. This discovery adds another layer of concern to the ongoing battle against avian influenza, alongside the prevalent H5N1 strain.
The U.S. Department of Agriculture (USDA) reported the finding to the World Organization for Animal Health (WOAH) over the weekend, although the outbreak initially occurred in November 2024 at a Merced County farm. Alarmingly, both H5N9 and H5N1 were present on the farm.
H5N1 has garnered significant attention in recent years due to outbreaks in wild birds and poultry, and its emergence in dairy cows in early 2024. Over 60 human H5N1 cases have been documented in the U.S. since 2024, primarily linked to contact with infected animals. While most human cases have been mild and the general public risk remains low, a recent H5N1-related death in Louisiana highlights the potential severity.
This recent outbreak underscores that H5N1 isn’t the only highly pathogenic avian influenza variant circulating. The term “highly pathogenic” refers specifically to the virus’s ability to cause severe disease in birds. According to the WOAH report, the outbreak was detected in late November after increased duck deaths at the farm. Initial tests by the USDA’s National Veterinary Services Laboratory identified an unknown H5 influenza type. Subsequent genetic testing, completed by mid-January, confirmed the presence of both H5N9 and H5N1, both belonging to the 2.3.4.4b clade of highly pathogenic avian influenza viruses.
H5N9 Outbreak Response and Investigation
The WOAH report confirms this as the first confirmed case of HPAI H5N9 in U.S. poultry. Following the detection, state officials quarantined the farm, and approximately 120,000 birds were culled in early December. The USDA’s Animal and Plant Health Inspection Service (APHIS), in collaboration with state animal health and wildlife officials, is actively investigating the situation.
The Risk of Reassortment
The emergence of H5N9 raises concerns about reassortment, a process where different influenza variants infect the same host and exchange genetic material, potentially creating new strains with unpredictable characteristics. This could include changes facilitating human transmission. APHIS confirms that reassortment led to the H5N9 strain in this outbreak, specifically a reassortment of the circulating H5N1 D1.1 virus. While this instance involved only avian influenza strains, a more concerning scenario would involve H5N1 reassorting with human-adapted flu strains.
Ducks as a Reservoir for Avian Influenza
Ducks are a particular concern as they often show no signs of illness, potentially making them effective vectors for spreading the virus to other birds and animals. Currently, this H5N9 outbreak appears isolated, with no further cases detected.
Ongoing Surveillance and Communication
While the USDA continues to communicate with the WOAH, there has been limited public information released about the outbreak. However, APHIS emphasizes its commitment to continued surveillance.
Conclusion: Vigilance and Continued Monitoring are Crucial
The detection of H5N9 in U.S. poultry underscores the importance of ongoing surveillance and rapid response to avian influenza outbreaks. While the immediate public health risk remains low, the potential for reassortment and the role of ducks as a reservoir highlight the need for continued vigilance and research. APHIS remains committed to investigating and monitoring the situation, working to mitigate the spread of both H5N9 and H5N1.
