There’s a good chance the milk in your fridge is contaminated with bird flu
Key Takeaways
Avian flu viral particles were found in 20% of retail milk samples recently tested by the FDA.
The pasteurization process in the United States is able to destroy H5N1, making commercial milk safe for consumers.
Following the report of at least one case of the transmission of bird flu to humans, the CDC is closely monitoring the current outbreak.
Experts have been keeping an eye on the H5N1 bird flu virus since it was first reported in United States dairy herds in March 2024.[] To date, the FDA has confirmed cases in 33 herds across 8 states.[]
On April 25, 2024, the FDA reported that 20% of retail milk samples from around the country tested positive for genetic particles of the virus.[]
To some experts, this signals that bird flu among American dairy cows may be more widespread than current testing has shown.
Prevalence across the US
The United States Department of Agriculture (USDA) has reported infected cattle herds in nine states: Colorado, Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota, and Texas.[] Bird flu infections among these herds have spread on farms and infected poultry flocks and barn cats.[] In North Carolina, a herd tested positive but showed no symptoms of the virus.[]
To date, there has only been one known human infection in the US associated with this outbreak.[]
In March 2024, a dairy worker in Texas contracted the virus. The worker’s only reported symptom was eye redness, and they were successfully treated with oseltamivir.[]
How do we know our milk is safe?
The CDC has not elevated the public health risk for bird flu.[] Donald N. Forthal, MD, a professor of medicine, molecular biology, and biochemistry and the chief of the infectious disease department at the University of California, Irvine, says that detecting fragments and not intact viruses doesn’t always mean there’s not a risk, but that the pasteurizing process provides a layer of safety.
“Detecting viral fragments doesn’t indicate whether or not there is [an] intact, infectious virus present. However, the pasteurization process appears to be adequate to make milk safe,” Dr. Forthal tells MDLinx.
The genetic fragments the FDA found in milk last week were not live, viral material. Additionally, the commercial pasteurization process for US dairy products can destroy H5NI.
In studies, H5N1 was destroyed when exposed to the pasteurization process used on US eggs, which occurs at lower temperatures than those used for milk pasteurization.
This was confirmed on April 26, 2024, when the FDA released an additional statement on the H5N1 fragments found in milk samples and revealed that further testing confirmed that pasteurization inactivated the virus.[] The statement reads, in part: “[P]reliminary results of egg inoculation tests on quantitative polymerase chain reaction (qPCR)-positive retail milk samples show that pasteurization is effective in inactivating HPAI. This additional testing did not detect any live, infectious virus."
"These results reaffirm our assessment that the commercial milk supply is safe."
— US Food and Drug Administration
Determining the scope of the outbreak
The presence of H5N1 traces in milk has some experts concerned about the size of this outbreak.[] The FDA stated that a greater proportion of positive results came from milk in areas with infected herds.
However, according to 2022 Census of Agriculture data, over 24,000 dairy farms sold milk that year, and over 17,000 of those farms have herds of at least 50 cows.[] Milk from farms around the nation is collected for mass distribution. While it’s unlikely that 20% of milk currently in-transit to grocery store shelves contains H5N1 fragments, it’s also unlikely that 33 herds could explain why 20% of the sampled milk contained H5N1 fragments.
The possibility of asymptomatic infections, as suggested by the herd in North Carolina, could make it difficult to find the additional infected herds and track the virus.
On April 24, the USDA issued a federal order requiring that dairy cows be tested for H5N1 before being transported across state lines.[]
H5N1 and human transmission
As there has only been one known human case associated with this outbreak, there’s no real data about how this virus might behave if it spreads further among humans.
Often, bird flu is linked to mild symptoms such as eye redness and flu-like upper respiratory symptoms; however, it can also cause severe infections and death in a not-insignificant percentage of cases.[]
As is true of many diseases, factors such as the specific strain of infection, coupled with the available health resources in the area of an outbreak, can impact death rates.
And while the Texas farmworker had only mild symptoms and recovered without incident, it's difficult to draw meaningful conclusions from a single case. Infections in animals are also not a reliable source of data for predicting how the virus could behave in humans. For instance, the asymptomatic herd in North Carolina does not suggest that humans will also present asymptomatically. Dr. Forthal agrees: “That doesn’t tell us anything about its severity in other species,” he says.
The CDC is closely monitoring the outbreak and will update its guidance if needed.
What this means for you
The FDA has confirmed cases of avian bird flu (aka, H5N1) in 33 dairy herds across 8 states, and 20% of retail milk samples recently tested positive for genetic particles of the virus. Fortunately, the virus fragments found in milk were not live, as pasteurization effectively inactivated the virus. Despite concerns about the outbreak's scope, the CDC has not raised the public health risk, citing the safety of pasteurized milk. While there has been only one known human infection, the severity of potential human transmission remains uncertain, as bird flu symptoms can vary.