Bird flu is on the move — here are the different strains around the world

Avian influenza, or bird flu, has public health officials on alert after an unprecedented spread in dairy cows in the United States this year. Four dairy workers have also tested positive in the country.

A particularly severe variant of the H5N1 strain has been spreading around the world in animals since 2020, causing lethal outbreaks in commercial poultry and sporadic infections in other species from alpacas to house cats. Until this year, it had never infected cows.

Different bird flu strains have been found in Australia and Mexico in humans, while different H5 subtypes are also present around the world in both animals and humans, in countries including China and Cambodia.

Most of the human cases reported exposure to poultry, live poultry markets, or dairy cattle prior to infection, but scientists are worried the virus could mutate in ways that make it more easily spread from person-to-person, which could spark a pandemic. The World Health Organization says the risk to people is low at this point.

Below are occurrences of varying types of the bird flu virus that have been found in humans this year.

The first known cases of infected dairy cattle occurred in Texas in March, and is now in dairy herds in 12 states. The U.S. Agriculture Department said tests so far indicate that the virus detected in cows is the same H5N1 virus affecting wild birds and commercial poultry flocks. The four dairy workers who have tested positive for the virus this year had mild symptoms such as conjunctivitis, or pink eye.

The H5N1 virus in the United States belongs to the clade 2.3.4.4b, genotype B3.13, a genotype detected only in North America so far, the European Food Safety Agency said in a scientific report.

A resident of Mexico died with the first known cases of H5N2 avian influenza in humans, the WHO said on June 5. Mexico’s government said chronic illness, rather than bird flu, was the cause of death. The person had no known exposure to animals.

The WHO on June 7 said a child with H5N1 bird flu reported by Australia had traveled to Kolkata, India. Genetic sequencing showed the virus was a subtype of H5N1 and part of a strain that circulates in Southeast Asia and has been detected in previous human infections and in poultry.

Australia is separately dealing with three outbreaks of different strains of the virus on poultry farms – H7N3, H7N8 and H7N9 – that authorities say likely arrived on farms via wild birds.

The WHO on June 11 reported a case of human infection with bird flu caused by the H9N2 subtype in a four-year-old child in the eastern Indian state of West Bengal. It was the second human infection of H9N2 bird flu from India following a case in 2019, the agency said. While the H9N2 virus typically tends to cause mild illness, the United Nations agency said further sporadic human cases could occur as this is one of the most prevalent avian influenza viruses circulating in poultry in different regions.

Vietnam reported a 21-year-old student had died from the H5N1 bird flu in March. He had no underlying medical conditions, but had been exposed to wild birds from hunting a couple of weeks prior to onset of symptoms. No contact with dead or sick poultry was reported at the time.

Vietnam also reported an outbreak of H9N2 in a 37-year-old man, EFSA said.

The Southeast Asia nation and Vietnam neighbor has reported five human cases of H5N1 as of June 20.

China this year detected human cases caused by the H5N6, H9N2 and H10N3 strains, with two fatal H5N6 cases in the Fujian province. Both of those cases had exposure to backyard poultry before the onset of symptoms, EFSA said.

The case of H10N3 avian influenza was the third one ever reported globally.

Germany reported a rare outbreak of highly pathogenic H7N5 bird flu on a farm in the western part of the country, near the border with the Netherlands, the World Organisation for Animal Health said on July 4. It was the first outbreak anywhere of H7N5 on WOAH’s public records.