US public health officials have been monitoring dairy cow herds, as well as beef and milk products around the country since the first outbreak in cows was reported in March. So far, the US is the only country to have reported bird flu in cattle, but there are fears that it could pose a serious threat to humans, too.

In April, a Texas farm worker contracted the virus in what is believed to be the first-ever transmission of the virus from a mammal – in this case, cattle – to a human. Thankfully, he suffered only mild conjunctivitis – an infection in the eye – and has since made a full recovery. The US’s first-ever bird flu case in a human was reported in 2022 when a poultry farm worker in Colorado was exposed to infected chicken. He reported only fatigue as the main symptom.

However, bird flu can be extremely dangerous for humans. Since the virus – also known as Avian Influenza – was first detected nearly three decades ago in China, some 860 people have been infected by birds in 23 countries, including China, Egypt, Vietnam and Turkey, according to the World Health Organization (WHO). Of those, 463 people died from the virus, giving a huge 52 percent death rate.

So far, the Centres for Disease Control and Prevention (CDC) has detected the virus in more than 200 cows, 9,000 wild birds and some 90 million chickens across the US.

On Friday last week, Canada began implementing stricter import checks for US cattle, following similar measures in Colombia.

So how is bird flu spreading, and could it become a more serious threat to humans?

What is bird flu?

Bird flu is a viral infection that primarily affects birds. Some strains of bird flu can, however, also be infectious to other animals, including humans.

Bird flu belongs to the Influenza A group, one of four types of influenza, and the only one known to cause flu pandemics.

There are also subtypes of avian influenza. Three of these – H5N1, H5N6 and H7N9 – have been found to cause illness in humans as well. While the letters in the names of these subtypes refer to a combination of proteins found in bird flu, the numbers indicate the subtype.

The first and best-known subtype of bird flu is H5N1, which was first identified in domestic geese in the Guangdong province of southern China in 1996. This is the subtype of bird flu which is currently affecting birds and cattle in the US.

In birds, the virus is likely to cause severe disease and death. Cows, particularly older ones, have displayed symptoms such as reduced appetite and lactation when infected.

“This particular strain of the H5N1 virus has been causing a global pandemic for animals – the term is an epizootic disease – and it has affected a lot of domestic poultry production,” said Meghan Davis, associate professor at the Johns Hopkins Bloomberg School of Public Health and trained dairy veterinarian.

How widespread is bird flu among cattle in the US?

Since late March, H5N1 has been reported in around 200 animals in 36 dairy cattle herds in the US states of Colorado, South Dakota, Kansas, Michigan, North Carolina, Idaho, Texas, Ohio and New Mexico.

Some experts suspect that the outbreak may have begun to spread to cows before March – even as early as late 2023 – but was not reported.

“What we don’t know is just how widespread this might be”, said Davis. “This is something where we need lots of surveillance, lots of eyes on this, lots of different ways of looking at the challenge.”

Could bird flu spread to cattle in other countries?

Although there have been no confirmed cases of bird flu in cows beyond US borders so far, the World Health Organization (WHO) has warned that there is a risk of it spreading internationally through the movement of migratory birds.

“With the virus carried around the world by migratory birds, certainly there is a risk for cows in other countries to be getting infected,” said Wenqing Zhang, head of WHO’s Global Influenza Programme, at a news briefing in Geneva in late April.

How dangerous is bird flu for humans?

H5N1 is described by the CDC as “highly pathogenic”, meaning it is “strongly able to cause disease”, in birds. But it has also proved to be deadly to humans in past cases. Only 860 people around the world have contracted the virus since it was first discovered in 1996, but more than half of those have died from it, according to the WHO.

Farm workers or others in direct contact with infected animals are most at risk. Beyond that, humans are unlikely to be able to transmit the virus to each other, although there was one suspected case of human-to-human transmission in Indonesia in 2006.

In humans, bird flu can cause a range of symptoms within two to eight days after infection. This ranges from fever, cough, sore throat and muscle aches, to more severe effects such as pneumonia and organ failure.

How do we treat bird flu in humans?

There is no vaccine for bird flu in humans. Infected people are typically prescribed antiviral medication such as oseltamivir or zanamivir to manage the symptoms. Health authorities including the CDC recommend administering such drugs as soon as symptoms begin.

How do farmers deal with outbreaks of a virus?

In most parts of the world, farmers cull animals which have been exposed to an outbreak of a deadly virus such as bird flu. Early in April, for example, a poultry farm in Texas destroyed 1.5 million chickens to curtail the spread of bird flu.

So far, there have been no reports of cattle being culled. This is because bird flu has so far not been as deadly in cows as it has in chickens and turkeys, said Andrew Stevens, assistant professor of agricultural and applied economics at the University of Wisconsin-Madison.

“In this setting, culling would be an extreme and costly step to prevent further spread of the disease. This is especially true because we don’t yet have as good a sense of how contagious the bird flu virus is between cattle,” he said.

How is bird flu transmitted to farm animals and humans?

Bird flu can be spread by wild birds and waterfowl such as ducks and geese through their droppings or secretions such as saliva.

When poultry or other animals scavenge environments which have been contaminated or have come into contact with infected birds, they can catch the virus too. Infected wild birds may also be present on farms and come into contact with water or feed that cows consume, although researchers are still studying such potential modes of transmission, said Davis.

In cattle, it is still not fully understood how the virus passes from one cow to another, but traces of the virus in milk suggest a heavy viral load can often be found in the mammary glands. Higher viral load in milk compared with respiratory tracts of cows may also point to the routes of exposure of the virus.

Scientists suspect, therefore, that the virus spreads when cows are milked, as equipment may become infected or the virus may become aerosolised – suspended in the air – during cleaning.

Humans who have close or prolonged contact with farm animals or wild birds can become infected when the virus enters their system via the eyes or mouth, or when droplets or small aerosol particles are inhaled via the nose.

Can you contract bird flu from milk and beef products?

Since April, the US Food and Drug Administration (FDA) has tested 297 milk products, including milk, cottage cheese and sour cream. Traces of the virus were found in 20 percent of an initial 96 samples but additional testing has confirmed that pasteurisation – a heating process used by farmers to screen out harmful pathogens and bacteria from farm products – kills the virus, removing the danger for consumers.

Experts and public health authorities including the CDC strongly advise people to only consume pasteurised dairy products, not just because of avian flu but also to avoid bacterial infections such as salmonella and brucella.

Beef samples tested by the US Department of Agriculture (USDA) have so far tested negative for bird flu, but official advice from the FDA remains against consuming raw beef.

Aren’t all dairy products pasteurised?

There is a growing movement against food regulation, seen by some in Western countries such as the US as a symbol of freedom. In 2015, for example, Wyoming passed the Food Freedom Act which allows farmers to sell raw milk and other farm goods directly to customers.

Australia, the only country apart from Canada with a blanket ban on raw milk sales, also has a growing “Raw Milk Movement” which advocates for regulated distribution of unpasteurised products. Such regulations would include hygienic production and packaging of the milk, as well as testing for active harmful pathogens.

While 30 US states allow sales of raw milk, federal law prohibits the distribution of unpasteurised milk across state lines. Additionally, milk from sick cows must be discarded completely under federal law. In cases where raw milk has been packaged before cows are found to be ill, however, products with traces of the virus could still make it to store shelves.

Although such a study has not been conducted on humans due to research ethics, US studies of cats which consumed raw milk infected with H5N1 showed that the cats became sick or even died as a result.

How are the authorities in the US responding to the bird flu outbreak?

Experts and public health authorities such as the CDC say they have been studying bird flu for decades now, and that the COVID-19 pandemic has led to positive shifts in terms of preparing for outbreaks.

Regulatory authorities are also stepping up surveillance of viruses. In late April, the USDA began to require all dairy cattle moving between states to be tested for bird flu.

However, testing at farms within state boundaries is still only done on a voluntary basis, which may not be enough to curb the spread of the virus, said Davis.

“What I would like to see happen is a switch from what we would consider to be more passive surveillance where we are waiting on farmers and workers to tell us that they’re having a problem, to a more active surveillance where we’re really trying to understand the scope of the problem,” she said.

One way to do this is through antibody serology testing, she said, whereby samples of blood or saliva are taken from a human and tested to detect whether their immune system has been exposed to the virus recently.

Another form of testing is polymerase chain reaction (PCR) testing, which typically uses more invasive nasal swabs to determine if a person is currently infected and may not be best suited for those who are asymptomatic or have not had very high exposure to a virus, said Davis.

Rural populations in the US may especially be at a disadvantage during the outbreak, as they can lack access to healthcare services and personal protective equipment, she added. Medical centres may be far away and these populations may be unable to afford services. “And if they [farmers] are not interested in having cows tested, then would the workforce even have the opportunity to get tested? That’s unclear,” she said.

How are other countries reacting to the US bird flu crisis?

In late April, Colombia became the first country to limit trade with the US because of bird flu in cows. It has restricted imports of beef and beef products from states where cows have tested positive for the virus.

Mexico, a major market for US beef and dairy products, has also increased surveillance of cattle entering the country for any sign of respiratory distress, the agriculture ministry said.

On Friday last week, the Canadian Food Inspection Agency also tightened import controls relating to US cattle, introducing a requirement for exporters to provide negative bird flu test results for lactating dairy cattle as well as mandatory testing of retail milk to check for traces of the virus.

Stevens said such measures are important for the long-term sustainability of supply chains. “Although increasing import checks of cattle and dairy products from the US may slightly increase the relative costs of US exports, these costs are small relative to the potential impacts of an import ban or moratorium.”