United States: A California-based duck farming operation has entered the spotlight this week following the release of a report from the World Organization of Animal Health. According to findings submitted by US authorities, an avian influenza strain labeled H5N9 was identified among ill birds within the farm’s population.
While H5N9 has previously been detected in avian populations across the country, the US Department of Agriculture (USDA) confirmed this as the inaugural instance of the strain manifesting as “highly pathogenic avian influenza” within domestic poultry—a classification denoting its capacity to inflict severe illness, according to CBS News.
“Observed symptoms included heightened mortality rates. State authorities have since imposed quarantine measures on the implicated premises,” the USDA stated in its report.
Differentiating H5N9 from H5N1
Avian influenza viruses are categorized based on two critical proteins present within their structure: hemagglutinin (e.g., H5 or H3) and neuraminidase (e.g., N1 or N9). The interplay of these proteins, alongside other genetic variations, influences the severity of illness and transmission dynamics among humans and animals.
H5N1 has been the predominant strain driving recent outbreaks across the US, impacting wild birds, poultry flocks, dairy herds, and domestic pets, and even infecting 67 confirmed human cases. Notably, the USDA identified both H5N9 and H5N1 in ducks at the California farm. Experts suggest that H5N9 may have emerged via “reassortment,” wherein H5N1’s genetic material blends with other influenza viruses inside an infected host.
“H5N1 frequently reassorts with other avian influenza strains. The H5 hemagglutinin protein currently affecting dairy cattle and commercial poultry farms in the US has demonstrated such behavior,” remarked Andy Pekosz, a microbiology and immunology professor at Johns Hopkins University, in an email exchange, as per the reports by CBS News.
S. Mark Tompkins, who heads the Center for Influenza Disease and Emergence Research at the University of Georgia, noted that it would not be “particularly startling” for such reassortment events to give rise to strains infecting poultry.
“The pervasive circulation of H5N1 across North America creates ample opportunity for reassortment within species susceptible to influenza viruses,” Tompkins explained.
Historical Context of H5N9 in the US
Although H5N9 has surfaced in American birds before, its effects have often been milder, a condition veterinary experts refer to as “low pathogenic avian influenza.”
“Instances of H5N9 have been documented in North American wild bird populations as recently as 2023 and 2024,” said Tompkins, noting earlier outbreaks, as reported by CBS News.
One of the earliest recorded instances of H5N9 dates to the 1960s when turkeys in Wisconsin exhibited mild respiratory issues and significant declines in egg production. The strain detected in the California outbreak, however, is markedly distinct, being part of the 2.3.4.4b lineage of avian influenza that has devastated poultry populations in recent years.
“The existence of H5N9 isn’t novel, and other variants like H5N5 also exist. Detection of this strain doesn’t inherently signify a crisis,” commented Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai. However, Krammer expressed greater concern about the H5N5 strain, which has recently spread from Europe and demonstrated the ability to infect mammals across Canada.
Potential Human Health Implications
As with H5N1, the H5N9 strain would require specific mutations to enable efficient human-to-human transmission. Current data from the Centers for Disease Control and Prevention (CDC) indicates no evidence of such transmission within the US, attributing most human cases to direct contact with infected animals. However, a few isolated cases remain unexplained due to the absence of an identifiable source.
As per CBS News, it remains unclear whether authorities investigated potential human exposure among workers at the Merced County duck farm. Local officials have yet to comment on the matter.
“The human-adapted H1N1 influenza strain possesses a neuraminidase protein somewhat related to H5N1’s but diverges from H5N9. Consequently, cross-recognition of H5N9 within the human population may be more limited,” Pekosz explained.
The Merced County Outbreak
The USDA reported that the H5N9 virus was discovered on a commercial duck farm in Merced County, California. The outbreak began on November 23, 2024, leading to the culling of 118,954 birds by December 2 as part of containment efforts, as reported by CBS News.
Merced County has borne the brunt of avian influenza outbreaks in California, with over three million birds culled across various farms within the past year alone. The state has endured unprecedented losses due to widespread outbreaks, particularly within the egg industry.
Authorities have yet to identify the specific source of infection at the duck farm. A USDA spokesperson has not provided further comment on the situation.
