Health authorities have confirmed that a highly pathogenic avian influenza (HPAI) has been spreading among dairy cattle in nine US states since March 2024.
The H5N1 bird flu virus has been found in the raw milk of infected cows, including in 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. But they recommend against consuming raw milk products.
The H5N1 virus is primarily adapted to infect birds. 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, demonstrates 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, US, in a media briefing May 15, 2024.
The US Centers for Disease Control and Prevention (CDC) said in an update on May 14, 2024, 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 (909) 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.
Scientists think the virus spreads through a variety of ways, such as saliva, mucus or feces from birds.
The virus is thought to spread between cows via two ways: Through the milking process via contaminated milk and through respiratory routes. But it's unclear what the most common route of transmission is between animals.
"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.
It's also unclear whether the high levels of virus in wastewater indicate infections in cows, birds or other animals.
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.
But the good news, said Pekosz, is that the current evidence shows the virus hasn't changed quickly after recent outbreaks.
Health authorities are monitoring multiple bird flu indicators, including spread of the virus to, or among, people in areas where the virus has been identified.
Wastewater sampling shows that an influenza A virus has been detected in several sites across the US, including in Alaska, California, Florida, Illinois and Kansas. But it's unclear in which species the virus originated from this analysis.
"Scientists have sequenced the gene of H5N1 virus found in cows and other animals. That data suggests all the outbreaks are highly associated and linked to a single source outbreak, likely at a dairy farm in Texas," said Pekosz, adding that animal transport accounted for "pretty much all the cases in the nine US states outbreaks."
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
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