There is an outbreak of bird flu. Here's what you need to know about it:
What is bird flu?
Bird flu is a so-called highly pathogenic bird flu virus. The “highly pathogenic” part concerns birds, which the virus is quite adept at killing. In virological terms, the virus is of the Influenza A type and is called H5N1. The “H” stands for the protein Hemagglutinin (HA), of which there are 16 subtypes (H1-H16). The “N” is an abbreviation for Neuraminidase (NA), of which there are 9 subtypes (N1-N9). There are many possible combinations of HA and NA proteins. The two known human influenza viruses type A are H1N1 and H3N2. (Two additional subtypes, H17N10 and H18N11, have been identified in bats).
When did this bird flu first appear?
The current H5N1 strain circulating around the world emerged in 1996 in farmed geese in China's Guangdong province. It quickly spread to other poultry and migratory birds. By the early 2000s, the disease had spread across South Asia. In 2005 it was observed in the Middle East, Africa and Europe. In 2014, the disease emerged in North America, but seemed to disappear here while the disease was still raging in Europe, the Middle East and Africa. In 2021, the disease appeared in wild birds migrating off Canada's Atlantic coast. It has since spread across North and South America.
What types of animals does bird flu affect?
Birds are the main carriers and victims of the virus. Hundreds of millions of wild and domestic birds have died around the world. Since 2021, hundreds of U.S. poultry farms have had to “depopulate” millions of birds after becoming infected, presumably from sick, migrating wild birds. The virus is highly contagious among birds and has a mortality rate of almost 100%. Mammals have also been infected and died. In most cases, these are scavenging or predatory animals that ate sick birds – and the virus died in these animals and did not become contagious between them. So far, 48 species of mammals have been infected. However, there have been some cases where the virus appears to have spread between mammals, including on European fur farms, on some South American elephant seal roosting beaches, and now among dairy cattle in the United States.
Can people get bird flu?
Since 2003, when the virus first began spreading through South Asia, 868 cases of human infection with H5N1 have been reported, 457 of which were fatal – a mortality rate of 53%. There have been only two cases in the US. In 2022, a poultry worker in Colorado became infected and suffered only mild symptoms, including fatigue. In 2024, a dairy worker in Texas became infected and complained only of conjunctivitis, or pink eye.
Why does everyone pay attention to dairy cows?
On March 25, 2024, officials announced that dairy cows in Texas had been infected with bird flu. Since then, the virus has been found in 36 herds in nine states. There are no known cases in California. A single introduction of the virus is believed to have occurred through exposure to wild birds (either passive exposure or eating contaminated feed), which likely occurred in Texas in December. The virus has now been found in milk. A study conducted by federal researchers found that 1 in 5 milk samples collected at stores contained the virus. It is believed that the virus can pass between cows and that there may be cows that show no symptoms. For the most part, it appears that dairy cows suffer only mild illness when infected, and milk production slows. They clear the virus after a few weeks.
Is it safe to drink milk?
Yes, if it is pasteurized milk. Federal officials say the virus they discovered in pasteurized milk samples is inactive and will not cause illness. In the case of raw milk, they urge people to avoid it. That's because they found high viral loads in raw milk samples. Additionally, studies on barn cats that consumed raw milk have reported serious consequences. In one cluster of 24 barn cats, half of them died after consuming raw milk, while others suffered blindness, neurological problems and profuse nasal discharge. The virus has not been found in sour cream or cottage cheese.
What about wastewater?
As health officials and researchers try to understand how widespread bird flu is among livestock and in the environment, they are analyzing municipal wastewater. A team from Emory University and Stanford University surveyed 190 wastewater treatment sites in 41 states. They have found a wave of influenza A virus at 59 locations in recent weeks. This does not necessarily mean that there is bird flu in these locations. However, in places where the team has investigated – including three in Texas where they knew there was H5N1 in dairy cattle – they found bird flu. Influenza A is generally seasonal in humans, peaking from late fall to early spring. The increase the researchers noticed – including in several locations in California – started after the flu season ended. Researchers in Texas also detected H5N1 in the wastewater of nine of the 10 cities they tested, all located in Texas. The CDC also monitors for influenza A at approximately 600 locations nationwide.
