Bird flu in humans, explained
After a dairy worker in Texas was infected with a highly pathogenic strain of bird flu, disease trackers worry the recent outbreak among cattle could develop into a larger threat.
The risk to the general public remains low, but experts are concerned about the possibility of the H5N1 virus evolving and more easily spreading from birds to other mammals. Here’s what to know about bird flu, its symptoms and treatment, and its risk to humans and pets.
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What is bird flu?
Avian influenza is a disease caused by influenza A viruses that spread widely among wild birds, particularly aquatic birds, birds of prey and waterfowl, but also domestic birds such as poultry. In the United States, highly pathogenic avian influenza has been detected in more than 9,000 wild birds and affected more than 82 million commercial poultry and backyard flocks since early 2022, according to data from the Centers for Disease Control and Prevention.
According to the World Organization for Animal Health, avian influenza viruses can survive for long periods in cold temperatures on surfaces such as farm equipment, which allows them to spread from farm to farm.
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Can humans catch bird flu?
It is rare, but humans can become infected with bird flu if they come into close contact with infected birds - whether the birds are dead or alive - or with surfaces that may have been contaminated by an infected bird’s saliva or feces.
Although the virus has been detected in wild mammals such as red foxes, raccoons, opossums and skunks - probably from eating infected wild birds - experts said the virus poses a low risk to humans. Two human cases have been reported in the United States, including the case in Texas that the CDC announced Monday. The first case occurred in 2022 when a person in Colorado with direct exposure to poultry tested positive for the same strain.
Infections could range from mild cases such as conjunctivitis - an eye infection that could occur after handling contaminated material and then touching the eyes - to more serious but rare respiratory infections, experts said.
The virus typically doesn’t infect the human respiratory tract, because humans don’t have the receptors in their throats, noses or upper respiratory tracts that are susceptible to the current bird flu strain.
A person would need to breathe in a large amount of the virus - by sweeping up and inhaling infected fecal matter deep into the lungs, for example - to develop a respiratory infection from the virus, said William Schaffner, a professor of infectious diseases and preventive medicine at Vanderbilt University.
“In those circumstances, the virus can initiate an infection in an occasional human and quickly develop into influenza pneumonia,” he said, and then “the fatality rate is very high.”
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What are the symptoms of bird flu in humans?
Some people who are infected may not experience symptoms, according to the CDC. Others may have mild symptoms such as conjunctivitis or flu-like symptoms - fever, cough, sore throat, body aches, headaches, fatigue and, in more serious cases involving pneumonia, trouble breathing.
The symptoms of bird flu depend on the strain of the virus with which each person becomes infected. The strains that have caused most of the human infections in the past 25 years are H5N1 and H7N9, the CDC said.
Bird flu infection is diagnosed in a lab, usually by swabbing someone’s nose and throat.
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How is it treated?
People who contract bird flu are typically treated with supportive care and, in serious cases, with ventilators to help them breathe. There are also antiviral medications that are effective at treating the current strains, Schaffner said.
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Is there a bird flu vaccine for humans?
Yes, there are vaccines for the bird flu. But they would need to be tested to see if they’re a match for this specific strain. Vaccine manufacturing would then need to be scaled up and mass produced.
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Can bird flu kill humans?
Yes, it can be fatal, mainly when the virus gets into the lungs and causes influenza pneumonia, but that is rare.
According to the World Health Organization, between January 2003 and February 2024, there were 887 laboratory-confirmed cases of human infection with H5N1 reported globally from 23 countries. Of those who were infected, there were 462 deaths.
While these numbers might seem scary, experts caution that the risk to the general public is low and that there is no evidence of sustained human-to-human transmission.
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Could bird flu become the next pandemic?
Each time there is highly pathogenic avian influenza outbreak, it triggers concerns that the virus could mutate to infect humans more readily and start spreading from person to person.
That happened with swine flu in 2009, when pigs became simultaneously infected with avian influenza and human influenza. The two viruses exchanged their genetic material inside the pigs, allowing the bird flu to use the genetic blueprint from the human flu to spread among people.
Such a pandemic cannot be predicted because this exchange of genetic material is a random event.
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“If anything, the odds are against it,” Schaffner said, noting that bird flu strains are circulating all the time and do not pose a risk to humans. Although the strain has infected some mammals - including mink, causing an outbreak at a Spanish farm in October 2022 - “that doesn’t necessarily mean it’s going to pick up the capacity to spread to humans,” he said.
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Can dogs and cats catch bird flu?
Dogs have contracted avian influenza strains in the past, said Carol Cardona, chair of avian health at the University of Minnesota. But because the family dog or cat is not typically in contact with infected birds like wild animals are, their risk is low, she said.
Still, experts warned against letting dogs or cats eat dead birds for a variety of health reasons.
“If you are keeping your cat indoors and you’re keeping your dog on a leash, I don’t see any reason that you would be expecting to see an infection,” Cardona said.
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Is there an impact to the nation’s dairy supply?
Federal agencies are reassuring the public about the nation’s dairy supply after the bird flu was recently found in dairy cows.
Since milk products are pasteurized, “there continues to be no concern about the safety of the interstate commercial milk supply,” said the Food and Drug Administration in a notice on its website.
Only milk from healthy animals is authorized to be distributed into interstate commerce for humans to consume, the agency said, adding that “pasteurization has continually proven to inactivate bacteria and viruses, like influenza.” That means there hasn’t been much impact on the country’s supply of milk nor the cost of dairy products.
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Can you get bird flu from eating chicken or eggs?
No. Humans cannot become infected by eating eggs or fully cooked poultry. However, according to the CDC, “uncooked poultry and poultry products (like blood) could have been the source of a small number of bird flu virus infections in people in Southeast Asia.”
The CDC advises that “it is safe to eat properly handled and cooked poultry in the United States.”
Still, on commercial farms, Cardona said infected poultry - and other chickens or turkeys that are exposed to those infected poultry - are typically killed to protect the U.S. food supply. “It’s good to know that they don’t make it into any food products,” she said. “They don’t even make it into your pet food.”
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What about my backyard chickens?
People who own their chickens should be vigilant, experts said.
Backyard chickens could come into contact with wild birds either directly or through their bodily secretions, putting them at risk for potential infection. Experts recommended keeping backyard chickens inside an enclosure to keep them away from any migratory birds that may be carrying the disease.
Chickens that become infected may become unusually quiet, stop producing eggs or produce fewer of them and develop respiratory symptoms and diarrhea before they start dying.
People who have backyard chickens should take precautions and avoid touching infected chickens or their eggs, or contaminated surfaces, experts said.
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Does risk vary by state or country?
Some locations have been more affected than others, but there is no place that is entirely risk free, said Munir Iqbal, head of the avian influenza virus group at the Pirbright Institute in England. “The virus is very much endemic now in wild birds, and the wild birds are going all around the world, so therefore the threat to the poultry has become constant and unpredictable,” Iqbal said.
Highly pathogenic avian influenza was first identified in geese in Guangdong, China, in 1996. It spread widely in Asia between 2003 and 2005, mutating into different strains that have since caused periodic outbreaks around the world.
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Can birds be vaccinated against bird flu?
China, Egypt and some other nations that experienced prior deadly outbreaks of bird flu have focused on vaccinating flocks of birds to decrease bird mortality. But vaccination for bird flu is also costly, and there is no universal agreement on whether it is an effective strategy to manage outbreaks.
The United Kingdom, where systematic vaccination of domesticated birds is banned in most cases, states in its official guidelines that vaccines are too difficult to administer properly over time, have not been proven to work in all affected bird species and are complicated to develop and adapt because the virus mutates so quickly.
In the United States and Europe, authorities monitor bird populations and, when they detect infections, cull flocks, including healthy birds who may have been exposed. This is costly because many governments compensate farmers when their healthy birds are killed.
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Should you worry birds in your area are spreading avian flu?
Wild birds - such as ducks, geese and swans - are most likely to be infected with this strain of bird flu, according to the CDC. It can easily be spread to poultry, like chickens and turkey.
However, the most common songbirds or other birds found in backyards - like cardinals, robins and pigeons - don’t usually carry bird flu viruses dangerous to poultry or humans. Generally, people should avoid direct contact with wild birds, the CDC states.
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What precautions are federal and state health officials recommending?
Federal and state health officials are recommending that people avoid unprotected exposures to sick or dead animals including wild birds, poultry, other domesticated birds, and other wild or domesticated animals, including cattle, as well as with animal carcasses, raw milk, feces, litter or materials contaminated by birds or other animals with confirmed or suspected HPAI virus infection.
People should not prepare or eat uncooked or undercooked food or food products, including unpasteurized (raw) milk or cheeses, from animals with confirmed or suspected bird flu.
CDC is recommending farmers and poultry, backyard flock, and livestock owners wear personal protective equipment, such as N95 masks, eye protection and gloves, as well as wash their hands thoroughly when in direct contact - or entering any buildings - with sick or dead birds or other animals, carcasses, feces or litter from potentially infected animals.
People exposed to birds or other animals with confirmed or suspected HPAI virus infection should be monitored for symptoms of illness for 10 days after the last known exposure, even if they were wearing recommended personal protective equipment.
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