What do we know about bird flu and its spread among new animals and humans?

On Wednesday, May 22, the Centers for Disease Control and Prevention (CDC) announced a second human case of avian influenza likely linked to an outbreak in cows on U.S. dairy farms.

In addition to the suspicion that these two human cases were infected after exposure to infected cows, the avian influenza virus, called H5N1, has recently infected animals that had not previously been infected, such as goats and cows.

What is the H5N1 bird flu virus ?

The H5N1 virus is a subtype of influenza A virus. It is classified as a form of highly pathogenic avian influenza (HPAI), because it causes severe and fatal disease in poultry. 

The H5N1 virus has been responsible for large outbreaks of bird flu on US poultry farms and on farms in other countries since 2022.

Although highly pathogenic avian influenza A (HPAI) viruses are known to devastate poultry populations, H5N1 also infects wild birds and occasionally infects various mammals, including humans.

In non-bird species, it can still cause fatal disease, but some cases are mild or asymptomatic.

H5N1 infection in mammals in general raises concerns that the virus could evolve to infect humans more easily and possibly spread widely among people, leading to a pandemic, according to the World Health Organization. But scientists have not seen any changes in the virus that would indicate this has happened at this stage.

What animals can the H5N1 virus infect ?

H5N1 most often infects domestic and wild birds, although some wild birds act as “reservoirs” for the virus, meaning they can carry and spread it without getting sick. These vectors include waterfowl, such as ducks and swans, and shorebirds, such as plovers, according to the Centers for Disease Control.

In the past 20 years or so, H5N1 has also been detected in at least 48 species of mammals in 26 countries, including foxes, bears, seals and sea lions, as well as domestic cats, dogs and farmed mink.

This year, the H5N1 virus was detected for the first time in goats, on a farm in Minnesota, where ducks and chickens had previously tested positive for the virus. It was discovered that cows in Texas and Kansas were infected with the virus, and they were also suspected of contracting the infection from infected poultry.

This is the first time that the H5N1 virus has been found in ruminants.

Can the H5N1 virus infect humans, and how dangerous is it?

The H5N1 virus can occasionally infect humans and can be fatal, with about 50% to 60% of reported cases resulting in fatal illness.

More than 880 human cases of H5N1 have been reported worldwide since the late 1990s, according to the Centers for Disease Control and Prevention.

Most often, human infections with H5N1 occur after a person has close or prolonged unprotected contact (without gloves, face mask, or eye protection) with infected birds or with surfaces contaminated with saliva, mucus, or feces of sick birds.

These cases have not been linked to sustained human-to-human spread of the virus, but there is some evidence of very limited spread between humans.

Although H5N1 infection can be fatal, some people have no or only mild symptoms.

Mild symptoms may include eye infections, such as conjunctivitis (pink eye), and upper respiratory symptoms, such as sneezing and coughing. Severe cases can lead to life-threatening pneumonia.

Bird influenza can be treated with antiviral drugs used to treat seasonal influenza, as well as several vaccine candidates in the pipeline, just in case this virus or a closely related virus suddenly spreads among people, as seasonal influenza vaccines do not protect against the H5N1 virus.

Should we be worried about H5N1 in cows?

Finding the H5N1 virus in cows is interesting to scientists because it has never been seen before.

As for cows, there are some early hints that the virus could spread between cattle, with cows in Idaho reportedly contracting the disease after being exposed to cows shipped from Texas. The Michigan cases are also believed to be linked to cows from Texas. However, the ability of H5N1 to spread between cattle still needs to be confirmed.

Regarding humans, “preliminary testing has not found changes in the H5N1 virus that would make it more transmissible to humans,” the USDA noted in a statement in April. She added: “While cases among humans in direct contact with infected animals are possible, this indicates that the current risk to the public remains low.”

The CDC also reported that there were no changes to the virus that would make it resistant to antiviral drugs approved by the Food and Drug Administration (FDA) to treat influenza.

However, experts recommend avoiding sick or dead animals. They should also avoid raw milk, feces, garbage, or other materials contaminated by animals suspected of being infected with the H5N1 virus, and uncooked or undercooked food products from these animals.

The USDA noted that the discovery of the H5N1 virus in cows does not threaten milk supplies. Commercial milk supplies are pasteurized before being put on the market, which kills viruses.

So far, the H5N1 virus has been found in dairy cows, but the USDA has also confirmed that the beef supply is safe and that cooking meat properly kills the viruses.

It is worth noting that there is not yet sufficient evidence to know whether drinking raw milk contaminated with the H5N1 virus can lead to human infection.