Researchers have investigated the spread of H5N1, a contagious and highly pathogenic avian influenza (HPAI), at nine farms in the US states of Texas, New Mexico, Kansas and Ohio — and found that "epidemiologic and genomic data revealed efficient cow-to-cow transmission."

In a study, rush-released by the journal Nature on July 25 after an initial peer review, the researchers reported a "spillover of HPAI H5N1 virus in dairy cattle herds […]."

They were also able to detect cattle-to-cat and cattle-to-raccoon transmission, highlighting, as they put it, "the virus' ability to cross species barriers."

Their evidence suggested the mammary gland in cattle was "the main site of virus replication, resulting in substantial virus shedding in milk." Some infected cows had produced discolored and/or curdled milk.

The researchers wrote that while "risk of human-to-human transmission remains low," the virus' "continuous and widespread circulation" posed "a significant threat to animal and public health."

In May 2024, DW reported that US health authorities had confirmed HPAI H5N1 was spreading among dairy cattle in nine US states.

The virus was found in raw milk from infected cows, including in some samples of milk sold in grocery stores.

US health agencies said milk products were safe to consume when pasteurized because the pasteurization process kills the virus. They recommended against consuming raw milk products.

H5N1 bird flu adapted to spread among other animals

The H5N1 virus is primarily adapted to infect birds, but outbreaks have been common in wild birds in the past two decades. There have been sporadic outbreaks in poultry farms around the world since 2022, including the United States and Europe.

Outbreaks in cows and other mammals, including foxes, bears and domesticated minks, cats and dogs, demonstrate it is possible for H5N1 to adapt and transmit to other species.

"Since 2022, the H5N1 virus spread particularly quickly and transmitted to other animals. Every time it transmits to a mammal, it gives the virus the opportunity to adapt and transmit between animal species," said Andrew Pekosz, an immunologist at Johns Hopkins Bloomberg School of Public Health in the US, in a media briefing on May 15.

Can humans get H5N1 bird flu?

The US Centers for Disease Control and Prevention (CDC) said in an update on May 14 that there was no solid evidence to suggest that bird flu was spreading among people. It also said the current public health risk was low.

It is even rare for H5N1 to spread from one infected person to a close contact, because the virus hasn't adapted to replicate inside human cells.

When infections in humans have occurred since the virus was first identified in 1997, the virus did not spread to many people. Since then, there have been fewer than 1,000 human cases of Asian HPAI H5N1 around the world.

Two people have been confirmed to be infected with H5N1 virus in the current outbreak in the US. The first was in 2022, while a second case was confirmed in April 2024 after exposure to dairy cattle, presumably infected with bird flu.

"The current risk to humans is highest for those with occupational exposure to susceptible animals. This would include those who work on farms (livestock, swine, poultry, etc.) and with wild animals," said Matthew Miller, an immunologist at McMaster University in Canada.

How is bird flu spreading?

Scientists think the virus spreads through a variety of ways, including saliva, mucus or feces from birds.

"It seems likely that contaminated milking equipment may be a source, given the high amount of virus found in the udders and milk — but other routes of transmission may also be possible," Miller told DW via email.

Transmission of the virus between cows could cause it to adapt to be able to replicate in humans, but there isn't evidence that this has happened yet.

"Every time a human is infected, the virus has a 'lottery ticket' that gives it an opportunity to 'learn' how to spread in humans efficiently. Therefore, really robust measures should be put in place to protect those with known high occupational exposure risk," said Miller.

Is there a vaccine for H5N1?

There are vaccines for the H5N1 virus. Vaccination of poultry against H5N1 is widespread in some countries, such as France.

Several vaccines have been approved for use in humans. Some governments have stockpiled various influenza vaccines, which could be moved into production quickly in the event of H5N1 outbreaks in human communities.

"For many years, we've been looking at avian [influenzas] as a potential for the next pandemic. We're much better prepared to react and respond to a potential H5N1 pandemic compared to COVID-19," said Pekosz.

The CDC recommends people avoid unprotected exposure to sick or dead animals, including wild birds, poultry and domesticated animals. 

It also recommends people avoid preparing or eating uncooked or undercooked food or related uncooked food products, such as unpasteurized (raw) milk.

Edited by: Zulfikar Abbany

This article was originally published May 16, 2024, and updated July 25, 2024, to include new findings from investigations at nine US farms as published by the journal Nature: Spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle; published by Caserta, L.C., Frye, E.A., Butt, S.L. et al. https://doi.org/10.1038/s41586-024-07849-4