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Avian Influenza H5N1

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

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 China in 1996. A big poultry outbreak in Hong Kong in 1997 led to the first documented human deaths. 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 emerged in October 2020 in the Netherlands that was better adapted to infect all birds and spread faster and farther than any predecessor.  The European Food Safety Authority reported that more than 58 million birds had died or been culled in 37 European countries since October 2021.

Avian influenza H5N1 arrived in North America by December 2021. Since then, more than 58.3 million wild and domestic birds have been infected across 47 states in the United States. This is the first avian influenza outbreak in the U.S. since 2016. The previous avian flu pandemic lasted 6 months. The current outbreak has already lasted twice that time.

The World Organization for Animal Health (WOAH) raised concerns about the rapid spread of highly pathogenic H5N1 avian influenza 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 from breathing in the droplets or aerosols containing viral particles. The disease is not transmitted by eating poultry or eggs.

An outbreak of avian influenza H5N1 in October 2022 on a mink farm in Spain provided the strongest evidence so far that the H5N1 strain of flu can spread from one mammal to another. Genetic sequencing showed that the mink had been infected with a new variant of H5N1, which included genetic material from an avian influenza strain found in gulls, as well as a genetic change known to increase the ability of some animal influenza viruses to reproduce in mammals. Scientists believe that respiratory transmission spread the mink outbreak. The weekly mortality rate reached 4.3% per week. Workers were forced to cull all 51,986 mink on the farm. Eleven farm workers had been in contact with the infected mink, but all tested negative for H5N1.

In the wild, mink are solitary animals or live in small families. They are unlikely to spread the virus far or infect humans. At mink farms, thousands of such solitary carnivores are forced to live together, creating ideal conditions for the avian virus to adapt to other mammals including humans. Mink farming needs to end.

On February 6, Peru's National Agrarian Health Service (SENASA) announced its surveillance had detected H5N1 virus in dead sea lions and one dolphin. SENASA said at least 585 sea lions and 55,000 wild birds had been found dead in seven of the country's coastal nature preserves, likely due to avian flu. Peru’s health ministry also announced that a zoo lion had died from an H5N1 infection.

Over the past year, H5N1 has shown an increasing ability to jump from birds to mammals. The H5N1 clade circulating in birds and poultry has gained a mutation that makes the virus more recognizable by mammalian airway cells. In addition to minks, sea lions, and dolphins the list of mammals with confirmed infections in Europe and the Americas now includes badger, black bear, grizzly bear, bobcat, coyote, dolphin, ferret, fisher cat, fox, leopard, lynx, opossum, otter, pig, polecat, seals, raccoon, raccoon dog, and skunk.

Some infected mammals have exhibited neurological symptoms. Some grizzly bears became disoriented and partially blind. Some infected seals were unable to orient themselves and swim properly.

At this time, the risk of avian influenza to humans remains low, most likely because H5N1’s receptor binding site has not mutated to bind more avidly to cells in human’s respiratory tract. So far there have been seven confirmed human infections during the current global wave, including one death, among people who had direct contact with infected poultry. There has been no evidence of human-to-human transmission. However, the outbreak must be monitored closely because H5N1 has the potential to mutate into a variant that is more adept at infecting humans.


Jetelina K and Leining L. Why are there no eggs? Avian flu and keeping human risk low, Your Local Epidemiologist,, January 24, 2023

Sidik SM. Bird flu outbreak in mink sparks concern about spread in people. Nature News, January 24, 2023

Agüero, M. et al. Highly pathogenic avian influenza A(H5N1) virus infection in farmed minks, Spain, October 2022, Eurosurveillance 28, issue 3, Jan 19, 2023.

Kupferschmidt, K. Bird flu spread between mink is a ‘warning bell’. Science, January 27, 2023.

Schnirring L, Peru confirms H5N1 avian flu in marine mammals, part of southward spread, CIDRAP, February 7, 2023.

Abbasi J. Bird Flu has begun to spread in mammals – Here’s what’s important to know. JAMA Published online February 8, 2023. doi:10.1001/jama.2023.1317

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