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 10,922 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 1,398 poultry flocks in 50 states. Of those, 633 flocks have been commercial and 765 have been backyard flocks. A total of 133.91 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. 

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, 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, red fox, ringed seals, sea lions,, skunks, tigers, and Virginia opossum.

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 924 dairy cow herds in 16 states (California, Colorado, Idaho, Iowa, Kansas, Michigan, Minnesota, Nevada, New Mexico, North Carolina, Ohio, Oklahoma, South Dakota, Texas, Utah, and Wyoming) 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. 

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 April 1, 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 recorded 74 human cases (67 confirmed and 7 probable) since 2022, with all but one of those cases occurring since the spring of 2024. The cases have occurred 10 states (California [37], Colorado [10], Iowa [1], Louisiana [1], Michigan [2], Missouri [1], Oregon [1],Texas [1], and Washington [11], and Wisconsin [1]. Forty four of the cases occurred after exposure to infected cattle and 23 after exposure to infected poultry. One case was thought to be due to exposure to wild birds, backyard flocks, or mammals. The source of exposure was unknown in 2 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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