A new strain of bird flu has been detected on a duck farm in California — the first confirmed outbreak of the H5N9 strain of the virus in poultry in the US — the US government has revealed.

The virus was found alongside the more common H5N1 strain on backyard ducks at a facility in the western state. In a report to the World Organization for Animal Health (WOAH), the US Department for Agriculture (USDA) said it was the first confirmed case of the strain in poultry in the country, and it was working with animal health and wildlife officials to investigate the case and enhance surveillance.

A spokesperson for the USDA tells Food Ingredients First that although the strain is less common than H5N1, the finding was “not unexpected.”

“The case of H5N9 confirmed in backyard ducks at a facility in California was a reassortment of the H5N1 D1.1 virus that has been circulating in US birds. The finding was not unexpected, as it is not uncommon to see reassortments and ducks are reservoir hosts for influenza A viruses,” they say. 

“APHIS [Animal and Plant Health Inspection Service] reported the detection to the WOAH, as is done for all cases of HPAI in domestic birds in the US. APHIS will continue to conduct comprehensive epidemiological investigations and enhanced surveillance in response to detections of HPAI and will continue reporting cases to WOAH.”

Numerous strains of both highly pathogenic avian influenza and low pathogenic avian influenza are circulating in the US. While the reassortments have most commonly included an H5 and an N1 protein, that is not the case with the outbreak in California. According to the US National Library of Medicine, it is unclear whether the “novel [H5N9] virus” will cause human infections and become a pandemic subtype.

Avian flu is a contagious disease primarily found in birds. However, the first outbreak in dairy cows was detected in March 2024 in the US, leading to a new National Milk Testing Strategy (NMTS) rollout to stop it from spreading.

Other bird flu concerns 

The latest news follows the Food and Drug Administration (FDA) recently advising pet food companies in the US to “reanalyze food safety plans” to protect against avian flu. A number of cats have gotten sick or died after consuming raw pet food or milk contaminated with the more common H5N1 virus. Meanwhile, the UK government has also confirmed a human case in the West Midlands region. 

In recent guidance, the FDA called on manufacturers of cat and dog foods to take extra precautions. Cats are especially vulnerable to the H5N1 virus if exposed to it through raw pet food or contaminated milk. 

“The reanalysis is necessary to respond to the recent domestic cat illnesses and deaths described above and to scientific data indicating that cats and dogs have become ill from consuming H5N1 virus,” says the FDA. 

“Manufacturers that implement a preventive control for the H5N1 hazard as a result of their reanalysis will be taking an important step toward protecting cat and dog health and helping to prevent the spread of H5N1. Addressing H5N1 will require a concerted effort across sectors, including by government, businesses, and consumers.” 

Taking action against avian flu

The FDA is encouraging pet food manufacturers to seek ingredients from healthy flocks or herds and to take steps such as heat treatment to help “inactivate” viruses. The government agency also recommends implementing a supply-chain-applied control to provide assurance that ingredients used in animal food do not come from H5N1-infected animals.  

In the UK, the person infected with bird flu caught the infection on a farm, after close and prolonged contact with a large number of infected birds. The birds were infected with the DI.2 genotype, one of the viruses known to be circulating in birds in the UK this season. This strain is different from strains circulating among mammals and birds in the US. 

The government confirmed that the infected individual was admitted to an infectious disease unit.

UK chief veterinary officer Christine Middlemiss says: “While avian influenza is highly contagious in birds, this is a very rare event and is very specific to the circumstances on this premises.”

“We are seeing a growing number of avian flu cases in birds on both commercial farms and in backyard flocks across the country. Implementing scrupulous biosecurity measures will help protect the health and welfare of your birds from the threat of avian influenza and other diseases.”

The government says the risk to the wider public is “very low.”