How do people catch bird flu?
As bird flu infections rise in dairy cattle and chickens, human cases are ticking up too, leaving many people to wonder whether they might be at risk from this recently arrived virus.
As bird flu infections rise in dairy cattle and chickens, human cases are ticking up too, leaving many people to wonder whether they might be at risk from this recently arrived virus.
Bird flu infections are rare in people. Sixty-one human cases have been confirmed in the US this year, according to the US Centers for Disease Control and Prevention, and all but three have been in people who work on poultry or dairy farms.
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As the name suggests, avian influenza viruses prefer to infect birds. They break into cells by latching onto sugars that stick up from their surfaces called sialic acids. H5N1, the bird flu virus behind the ongoing outbreak in the US, has really only demonstrated an affinity for the types of sialic acid receptors that are most plentiful in the respiratory tracts of birds.
But flu viruses can also mutate quickly, and since 2022, H5N1 has been infecting a growing variety of mammals, including dairy cattle.
That has scientists on alert because the more it circulates in animals, the better it gets at finding new hosts.
A study published last week in the journal Science showed that just one key change to the virus’ genetic material would allow it to attach to the kinds of sialic acids that are most common in the nose and lungs of people. But it’s nearly impossible to predict when that could happen — or if it ever will.
Animal-to-human spillover events
When humans have become infected with bird flu, it’s almost always been through contact with infected animals. All but one of these so-called spillover infections have been mild.
The United States’ first severe case was announced this week in a person in Louisiana who remains hospitalized in critical condition. The CDC said Wednesday the person was exposed to sick and dead birds on their property, not from commercial poultry.
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No one who has gotten H5N1 in the US is known to have given the infection to anyone else. For that reason, the CDC estimates that the current risk to the public is low, but there are certain occupations and situations that can increase a person’s risk of catching bird flu.
The two groups of people who are most at risk are farm workers who work with cows or poultry and people who keep backyard flocks, said Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.
Birds shed the virus through their saliva, mucus and feces, and it can become airborne when their litter and feathers are churned up in barns, particularly during culling operations.
“It can be in the air,” Osterholm said. “So it’s not even just contact touching the birds but just the dander and all the dust that occurs when you’re dealing with birds.”
The virus also homes in on the udders of milking cows, and studies have found high concentrations of bird flu virus in milk that is raw or unpasteurized.
The milking parlors of dairies are wet environments, and workers can be infected if they get a splash of raw milk in their eyes or if they get milk on their hands and then rub their eyes. Droplets of virus-laden milk can also become airborne if they spray from milking equipment.
One of the most prominent symptoms in infected farm workers has been red, irritated eyes, or conjunctivitis. That may be because human eyes share the same types of sialic acid receptors that are most common in birds.
Risks from raw milk
What about drinking raw milk?
Cats have been infected after drinking raw cow’s milk. There have also been reports of calves getting sick after drinking infected milk.
“We don’t have those same data for humans,” Osterholm said.
No human infections have been linked to raw milk consumption, although a toddler in California recently tested positive for influenza after drinking a large amount of raw milk. The CDC wasn’t able to confirm whether the infection was bird flu, however, so this child is listed as a suspected case.
But there’s plenty of data on other pathogens that can be found in raw milk, even without specific evidence for H5N1. In 2023 and 2024, outbreaks of E. coli and salmonella infections were traced back to raw milk. It can harbor other pathogens like listeria and Campylobacter, too.
Pasteurization kills all these harmful germs, including H5N1, a recent government study shows.
However, refrigeration does not. A recent study from Stanford University that involved lacing raw milk with flu virus and testing it on cells in a Petri dish found that the virus could still infect cells for up to five days after being refrigerated.
Staying safe
The best way to protect yourself from bird flu is to avoid sources of exposure, according to the CDC.
People can catch bird flu when enough virus gets into their eyes, nose or mouth or when they breathe it in. This usually takes close, prolonged exposure to infected animals.
For that reason, the CDC recommends that people stay away from sick or dead birds or other animals, and avoid touching surfaces contaminated with feces or saliva, such as litter or bedding.
If you do have to handle sick birds, cows or other animals, such as in a farm or rescue setting, the CDC recommends wearing appropriate personal protective equipment, or PPE. This includes:
- Goggles
- Disposable gloves
- An N95 face mask
- Coveralls
- Rubber boots
Because raw milk can carry so many dangerous germs, experts advise avoiding it and sticking to pasteurized dairy products.
But what about raw eggs? The US Food and Drug Administration says it’s unlikely that eggs from an infected flock would make it onto store shelves because birds rapidly become sick from bird flu, and there are safeguards in place, such as inspections, to prevent contaminated eggs from reaching the market.
It’s still a good idea to cook eggs and poultry to a safe temperature and prevent cross-contamination between raw and cooked foods in the kitchen.
Avian Flu Fast Facts
Here’s a look at avian flu.
Avian influenza, also called avian flu or bird flu, is an illness that usually affects only birds.
There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5, H7 and H10 have caused deaths in humans.
The most commonly seen and most deadly form of the virus is called “Influenza A (H5N1),” or the “H5N1 virus.”
Diagnosis/Treatment
Most cases of human bird flu infections are due to contact with infected poultry or surfaces that are contaminated with infected bird excretions: saliva, nasal secretions or feces.
Symptoms of avian flu include fever, cough, sore throat and sometimes severe respiratory diseases and pneumonia.
The CDC recommends oral oseltamivir (brand name: Tamiflu), inhaled zanamivir (brand name: Relenza) and intravenous permavir (brand name: Rapivab) for the treatment of human illness associated with avian flu. See also CDC’s Guidance on Prevention and Treatment.
As of May 17, 2024, the mortality rate is close to 50% for infected humans.
As of November 22, 2024, the CDC reports there are 49 US states with poultry outbreaks.
Timeline
Early 1900s –The avian flu is first identified in Italy.
1961 – The H5N1 strain is isolated in birds in South Africa.
December 1983 – Chickens in Pennsylvania and Virginia are exposed to the avian flu and more than five million birds are killed to stop the disease from spreading.
1997 – Eighteen people are infected by the H5N1 strain in Hong Kong, six die. These are the first documented cases of human infection. Hong Kong destroys its entire poultry population, 1.5 million birds.
1999 – Two children in Hong Kong are infected by the H9N2 strain.
February 2003 – Eighty-four people in the Netherlands are affected by the H7N7 strain of the virus, one dies.
February 7, 2004 – Twelve thousand chickens are killed in Kent County, Delaware, after they are found to be infected with the H7 virus.
October 7, 2005 – The avian flu reaches Europe. Romanian officials quarantine a village of about 30 people after three dead ducks there test positive for bird flu.
November 12, 2005 – A one-year-old boy in Thailand tests positive for the H5N1 strain of avian influenza.
November 16, 2005 – The World Health Organization confirms two human cases of bird flu in China, including a female poultry worker who died from the H5N1 strain.
November 17, 2005 – Two deaths are confirmed in Indonesia from the H5N1 strain of avian influenza.
January 1, 2006 – A Turkish teenager dies of the H5N1 strain of avian influenza in Istanbul, and later that week, two of his sisters die.
January 17, 2006 – A 15-year-old girl from northern Iraq dies after contracting bird flu.
February 20, 2006 – Vietnam becomes the first country to successfully contain the disease. A country is considered disease-free when no new cases are reported in 21 days.
March 12, 2006 – Officials in Cameroon confirm cases of the H5N1 strain. The avian flu has now reached four African countries.
March 13, 2006 – The avian flu is confirmed by officials in Myanmar.
May 11, 2006 – Djibouti announces its first cases of H5N1 – several birds and one human.
December 20, 2011 – The US Department of Health and Human Services releases a statement saying that the government is urging scientific journals to omit details from research they intend to publish on the transfer of H5N1 among mammals. There is concern that the information could be misused by terrorists.
July 31, 2012 – Scientists announce that H3N8, a new strain of avian flu, caused the death of more than 160 baby seals in New England in 2011.
March 31, 2013 – Chinese authorities report the first human cases of infection of avian flu H7N9 to the World Health Organization. H7N9 has not previously been detected in humans.
December 6, 2013 – A 73-year-old woman infected with H10N8 dies in China, the first human fatality from this strain.
January 8, 2014 – Canadian health officials confirm that a resident from Alberta has died from H5N1 avian flu, the first case of the virus in North America. It is also the first case of H5N1 infection ever imported by a traveler into a country where the virus is not present in poultry.
April 20, 2015 – Officials say more than five million hens will be euthanized after bird flu was detected at a commercial laying facility in northwest Iowa. According to the US Department of Agriculture, close to eight million cases of bird flu have been detected in 13 states since December. Health officials say there is little to no risk for transmission to humans with respect to H5N2. No human infections with the virus have ever been detected.
January 15, 2016 – The US Department of Agriculture confirms that a commercial turkey farm in Dubois County, Indiana, has tested positive for the H7N8 strain of avian influenza.
January 24, 2017 – Britain’s Department for Environment, Food & Rural Affairs releases a statement confirming that a case of H5N8 avian flu has been detected in a flock of farmed breeding pheasants in Preston, UK. The flock is estimated to contain around 10,000 birds. The statement adds that a number of those birds have died, and the remaining live birds at the premises are being “humanely” killed because of the disease.
February 12, 2017 – A number of provinces in China have shut down their live poultry markets to prevent the spread of avian flu after a surge in the number of infections from the H7N9 strain. At least six provinces have reported human cases of H7N9 influenza this year, according to Chinese state media, Xinhua.
March 5-7, 2017 – The USDA confirms that a commercial chicken farm in Tennessee has tested positive for the H7N9 strain of avian flu, but says it is genetically different from the H7N9 lineage out of China. The 73,500-bird flock in Lincoln County will be euthanized, according to Tyson Foods.
February 14, 2018 – Hong Kong’s Centre for Health Protection announces that a 68-year-old woman has been treated for the H7N4 strain. This is the first case of this strain in a human.
December 2019 – The United Kingdom Department for Environment, Food & Rural Affairs confirms that a case of H5N1 avian flu has been detected at a poultry farm in Suffolk. 27,000 birds are humanely killed because of the disease.
April 9, 2020 – The USDA confirms that a commercial turkey flock in Chesterfield County, South Carolina has tested positive for the H7N3 strain of avian flu.
January 2021 – India culls tens of thousands of poultry birds after avian influenza is detected in ducks, crows and wild geese in at least a dozen locations across the country.
February 18, 2021 – Russian authorities notify WHO that they have detected H5N8 in humans. “If confirmed, this would be the first time H5N8 has infected people,” a WHO Europe spokesperson says in a statement.
June 1, 2021 – China’s National Health Commission announces the first human case of H10N3.
February 2022 – The USDA confirms that wild birds and domestic poultry in the United States have tested positive for the H5N1 strain of avian flu.
April 26, 2022 – China’s National Health Commission announces the first human case of H3N8.
April 28, 2022 – The CDC announces a case of H5 bird flu has been confirmed in a man in Colorado.
March 25, 2024 – The USDA, FDA and CDC announce H5N1 has been identified in US dairy cattle for the first time. The virus has been confirmed in dairy cattle in nine states.
May 3, 2024 – A new study in the New England Journal of Medicine reveals a dairy worker was treated for bird flu after working with dairy cattle in Texas. He appears to be the first known case of mammal-to-human transmission of the virus.
May 28, 2024 – The US Department of Agriculture’s National Veterinary Services Laboratories announces H5N1 has been confirmed in alpacas for the first time. The animals that tested positive were on a farm in Idaho where poultry had tested positive for the virus and were culled in May.
November 22, 2024 – The CDC announces a case of H5 bird flu has been confirmed in a child in California. This is the first reported case in the US of a child infected with H5 bird flu.
December 18, 2024 – The CDC confirms a patient in Louisiana has been hospitalized with a severe case of H5N1 bird flu, the first such case in the US. According to the CDC, the patient was exposed to sick and dead birds in backyard flocks; this the first US bird flu case linked to a backyard flock.
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