Avian influenza refers to disease in birds caused by infection with avian influenza Type A viruses. These viruses occur naturally among wild aquatic birds such as ducks, geese, swans, gulls, terns, storks, plovers, and sandpipers. They can infect domestic poultry and other bird and animal species.

Avian influenza A viruses are classified into two categories: low pathogenicity avian influenza (LPAI) A viruses, and highly pathogenic avian influenza (HPAI) A viruses. Most avian influenza A viruses are low pathogenic and cause few signs of disease in infected wild birds. Highly pathogenic avian influenza viruses cause severe disease and high mortality in infected poultry. Only some avian influenza A(H5) and A(H7) viruses are classified as HPAI A viruses.

Poultry may become infected with avian influenza A viruses through direct contact with infected wild birds or through contact with surfaces that have been contaminated with the viruses. H5N1 was first detected at a goose farm in Guangdong China in 1996 and then spread to poultry farms across China and Hong Kong in 1997. During that outbreak, 860 people became infected and half died.

Around 2005, the virus spilled over into migratory birds, which have since spread it across the world in several big waves. A new variant (H5N1 clade 2.3.4.4b) emerged in October 2020 in the Netherlands that was better adapted to infect all birds and spread faster and farther than any predecessor. H5N1 began killing tens of thousands of seabirds in Europe before moving to South Africa. 

HPA1 H5N1 arrived in the United States and Canada by December 2021. This was the first avian influenza outbreak in the U.S. since 2016. The 2016 avian influenza pandemic in the US ended after 6 months. The current outbreak has already lasted much longer and killed many more animals. 

Since January 1, 2022, the Animal and Plant Health Inspection Service (APHIS) of the US Department of Agriculture (USDA) has confirmed 12,467 HPA1 H5N1 avian influenza outbreaks in captive wild birds. Outbreaks have occurred in all 50 states. 

APHIS has also confirmed HPA1 H5N1 avian influenza in 1,635 poultry flocks in 50 states and Puerto Rico. Of those, 771 flocks have been commercial and 870 have been backyard flocks. A total of 166.17 million poultry birds have been affected since the start of the outbreak on February 8, 2022. Genetic sequencing has shown the birds were infected with HPAI H5N1 viruses from clade 2.3.4.4. 

To provide context on the overall size of the U.S. poultry flock, there are more than 378.5 million egg-laying chickens in the United States. In 2023, more than 9.4 billion broiler chickens and 218 million turkeys were processed in the United States.

The escalating avian influenza outbreak has killed more than 100 million egg-laying hens, causing a shortage of eggs. This shortage has led to record high prices. The price of a dozen large eggs in the Southeast and South Central, US exceeded $7.00. 

H5N1 spread to South American in 2022. The World Organization for Animal Health (WOAH) has raised concerns about the rapid spread of HPA1 H5N1 in Central and South America. Newly affected countries included Bolivia, Chile, Colombia, Costa Rica, Ecuador, Honduras, Panama, Peru, and Venezuela. 

Infected birds shed avian influenza through their mucous, salvia, and feces. Other mammals can get sick by inhaling droplets or aerosols containing viral particles.

Over the past year, HPA1 H5N1 has shown an increasing ability to jump from birds to mammals. The H5N1 clade (2.3.4.4b) circulating in birds and poultry has gained a mutation that made the virus more recognizable by mammalian airway cells. The list of mammals with confirmed infections now includes: African servals, alpacas, American mink, Amur/Bengal tiger, badgers, Bengal cat, black bears, bobcats, bottlenose dolphins, brown bears, cheetah, coatis, , cougars, coyotes, desert cottontail, domestic cats, dolphins, Eastern grey squirrel, elephant seals, Eurasian lynx, ferrets, fisher cats, foxes, Geoffroy cat, gray seals, grizzly bears, harbor seals, kookaburra, leopards, lions, lynxes,  mice, mink, pigs, mountain lion, North American river otter, opossums, otters, panthers, pigs, polar bears, polecats, prairie vole, raccoons, raccoon dogs, rats, red fox, ringed seals, sea lions,, skunks, tigers, and Virginia opossum.

In December 2024, domestic cats in Santa Barbara, Los Angeles County, and Oregon were infected with H5N1. Many of them died from severe illness or were euthanized. Some became infected by drinking raw Milk and other from eating raw pet food. 

USDA reported the first H5N1 virus infection in a dairy cow in Texas on March 25, 2024. USDA believed that infected wild birds introduced H5N1 to cattle. Ingestion of feed contaminated with feces from wild birds and poultry most likely contributed to the spread of infection on dairy farms. The outbreak then expanded as cows were shipped to other states. Current data suggested that many some infected cattle were asymptomatic, which contributed to cow to cow transmission. Other evidence that suggested that avian H5N1 has spread from cattle back to poultry.

Since March 2024, at least 985 dairy cow herds across17 states (Arizona [1], California [754], Colorado [64], Idaho [36], Iowa [13], Kansas [4], Michigan [31], Minnesota [9], Nevada [10], New Mexico [9], North Carolina [1], Ohio [1], Oklahoma [2], South Dakota [7], Texas [27], Utah [13], and Wyoming [1]) have tested positive for H5N1. Infected cattle have dehydration, decreased rumination, respiratory distress, diarrhea, decreased milk production, thickened (colostrum-like milk consistency, and spontaneous abortions. Illness in cows lasts for two to four weeks. Virus has been detected in milk, nasal swabs, blood, and tissues of infected cows. 

On August 30, 2024 - California became the 14th state to have outbreaks when the California Department of Food and Agriculture confirmed the presence of avian H5N1 virus in 3 dairy farms in the Central Valley. Since then, 754 dairy herds had become infected. The majority of the state’s infected herds have been located in the Central Valley. More recently, H5N1 infections have been confirmed in the southern part of the state. Altogether, California has 984 dairies and 1.7 million dairy cows. It is the nation’s top producer of milk, accounting for about 20% of the nation’s milk supply. Californians also consume a lot of raw milk.

The California Department of Food and Agriculture (CDFA) has been testing bulk raw milk samples for H5N1 virus weekly. On November 24, 2024, the California Department of Public Health (CDPH) announced that samples of retail raw milk produced by Raw Farm, LLC, in Fresno County had tested positive for extremely high levels of H5N1 virus. The company issued a voluntary recall on the affected batch of milk. No related human illnesses had been reported. 

On November 27, 2024, CDFA placed Raw Farm under quarantine. This action barred the distribution of the company’s raw milk, cream, kefir, butter, and cheese. On December 14, milk products were recalled from a second producer, Valley Milk Simply Bottled. 

On December 14, 2024, CDFA recalled milk products from a second producer, Valley Milk Simply Bottled, after its bulk tank tested positive for H5N1 avian influenza. The company is located in Stanislaus County, which is part of California’s Central Valley, where most of the infected dairy herds reside. A study published in Environmental Science and Technology reported that viable H5N1 virus can persist in refrigerated raw milk for up to 5 days. 

On April 1, 2024, the Texas Department of State Health Services reported that a dairy farm worker had tested positive for highly pathogenic avian influenza A(H5N1) virus after exposure to presumably infected dairy cattle.  CDC has confirmed 70 human cases of avian influenza, with all but one of those cases occurring since the spring of 2024. The cases have occurred 13 states (California [38], Colorado [10], Iowa [1], Louisiana [1], Michigan [2], Missouri [1], Nevada [1], Ohio [1], Oregon [1],Texas [1], and Washington [11], Wisconsin [1], Wyoming [3]. Most of the cases have occurred after contact with sick cattle [41] or poultry [24]. Two cases  thought to be due to exposure to wild birds, backyard flocks, or other mammals. The source of exposure source has not been identified in 3 cases. So far, no human-to-human transmission has been identified.

The United States Department of Agriculture (USDA) has been very slow in responding to this outbreak. On October 24, John’s Hopkins Center for Health Security claimed that USDA had yielded to industry pressure and had weakened an emergency order meant to prevent infected cows from moving across state lines. Not until December 6, did the USDA finally implement a mandatory milk testing program. 

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