NIH- Assessing avian influenza in dairy milk, June 4, 2024

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June 4, 2024 Assessing avian influenza in dairy milk

At a Glance

• H5N1 avian flu virus survived in raw dairy milk kept under refrigerated conditions for at least five weeks.
• When mice consumed infected raw milk, they showed signs of illness, suggesting that drinking raw milk may pose a risk of transmission to people.
• Heating milk in ways similar to pasteurization significantly reduced levels of the virus.

An outbreak of H5N1 avian flu virus in dairy cows has been widespread this year. StockMediaSeller / Shutterstock

Certain influenza viruses are found in wild and domestic birds, earning the disease they cause the name “bird flu.” Over the last two decades, about 900 people have been infected with an avian influenza virus called H5N1. Most of them have had close contact with infected birds.

Past H5N1 strains have proven deadly for people, killing about half of those they’ve infected. However, to date, these H5N1 viruses haven’t spread effectively from person to person, limiting their potential to cause a pandemic.

In the past few years, a highly pathogenic avian H5N1, called HPAI H5N1, has spread from birds to infect more than 50 other animal species, including many kinds of mammals. In late March of this year, the first outbreak of HPAI H5N1 virus in dairy cows in the U.S. was reported. Three infections of people working with dairy cows have been picked up by virus surveillance methods. So far, symptoms have been mild.

HPAI H5N1 hasn’t yet gained the ability to spread directly between people. But researchers are concerned that infections could happen through the consumption of raw milk. Dairy milk purchased in the grocery store has been pasteurized—heated to a level high enough and long enough to kill most viruses or bacteria in the milk. However, 30 states in the U.S. allow the sale of raw dairy milk in some form.

In a new NIH-funded study, researchers from the University of Wisconsin-Madison, Wisconsin Veterinary Diagnostic Laboratory, and Texas A&M Veterinary Medical Diagnostic Laboratory used mice to test whether HPAI H5N1 found in raw milk can cause infections. Results were published on May 24, 2024, in the New England Journal of Medicine.

Three influenza A (H5N1/bird flu) virus particles (rod-shaped; purple). CDC and NIAID

The researchers collected milk samples from cows in New Mexico that had been infected with HPAI H5N1. The samples contained a total of 8 different HPAI H5N1 viruses.

The team first heated infected milk at 63°C (145°F) for 5, 10, 20 or 30 minutes. All these experiments reduced the virus to levels that couldn’t be detected by a standard test. When using higher heat—72°C (161°F)—for briefer periods of 5, 10, 15, 20 or 30 seconds, levels of the virus were reduced but not eliminated.

These temperatures mimicked those used in milk pasteurization. The experiments suggest that heat may be able to neutralize HPAI H5N1 in dairy milk. However, additional work is needed to directly test whether industrial pasteurization methods kill the virus.

The researchers also stored raw milk infected with H5N1 in refrigerated conditions (4°C, or 39°F) for 5 weeks and found only a small decline in virus levels. This shows that the virus can likely remain infectious in raw milk when maintained at refrigerated temperatures.

Finally, the scientists fed milk from a cow infected with an HPAI H5N1 strain to five mice. All animals showed signs of illness by 1 day after exposure to the milk. When the team examined the animals’ organs 4 days after infection, they found HPAI H5N1 throughout their bodies, including the nasal passages and lungs. The results suggest that consumption of raw milk may pose a risk for H5N1 infection.

“We must emphasize that the conditions used in our laboratory study are not identical to the large-scale industrial treatment of raw milk,” notes lead researcher Dr. Yoshihiro Kawaoka of the University of Wisconsin-Madison. “It’s important to remember that our heat-treatment findings may not perfectly translate to real-world conditions.” However, to date, there hasn’t been evidence that commercially pasteurized milk poses an infection risk. Related Links

• Research in Context: Progress Toward Universal Vaccines
• Using mRNA Technology for a Universal Flu Vaccine
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• Is It Flu, COVID-19, Allergies, or a Cold?
• Information on Bird Flu (CDC)(link is external)
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References: Cow's Milk Containing Avian Influenza A(H5N1) Virus - Heat Inactivation and Infectivity in Mice. Guan L, Eisfeld AJ, Pattinson D, Gu C, Biswas A, Maemura T, Trifkovic S, Babujee L, Presler R Jr, Dahn R, Halfmann PJ, Barnhardt T, Neumann G, Thompson A, Swinford AK, Dimitrov KM, Poulsen K, Kawaoka Y. N Engl J Med. 2024 May 24. doi: 10.1056/NEJMc2405495. Online ahead of print. PMID: 8785313.

Funding: NIH’s National Institute of Allergy and Infectious Diseases (NIAID); Japan Agency for Medical Research and Development; United States Department of Agriculture (USDA); Colorado State University; Wisconsin Department of Agriculture, Trade, and Consumer Protection; APHIS National Animal Health Laboratory Network Enhancement Project.

https://www.nih.gov/news-events/nih-research-matters/assessing-avian-influenza-dairy-milk

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Friday, June 14, 2024 Infectious H5N1 influenza virus in raw milk rapidly declines with heat treatment


The amount of infectious H5N1 influenza viruses in raw milk rapidly declined with heat treatment in laboratory research.
Fresh natural milk on a farm. Given the 2024 multistate outbreak of H5N1 influenza among U.S. dairy cows, federal authorities recommend against drinking unpasteurized (raw) milk. What


The amount of infectious H5N1 influenza viruses in raw milk rapidly declined with heat treatment in laboratory research conducted by scientists at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. However, small, detectable amounts of infectious virus remained in raw milk samples with high virus levels when treated at 72 degrees Celsius (161.6 degrees Fahrenheit) for 15 seconds—one of the standard pasteurization methods used by the dairy industry. The authors of the study stress, however, that their findings reflect experimental conditions in a laboratory setting and are not identical to large-scale industrial pasteurization processes for raw milk. The findings were published today in the New England Journal of Medicine

In late March 2024, United States officials reported an outbreak of highly pathogenic avian influenza virus called HPAI H5N1 among dairy cows in Texas. To date, 95 cattle herds across 12 states have been affected, with three human infections detected in farm workers with conjunctivitis. Although the virus so far has shown no genetic evidence of acquiring the ability to spread from person to person, public health officials are closely monitoring the dairy cow situation as part of overarching pandemic preparedness efforts.

Given the limited data on the susceptibility of avian influenza viruses to pasteurization methods used by the dairy industry, scientists at NIAID’s Rocky Mountain Laboratories sought to quantify the stability of H5N1 virus in raw milk when tested at different time intervals at 63℃ (145.4 degrees Fahrenheit) and 72℃, the temperatures most common in commercial dairy pasteurization processes. The scientists isolated HPAI H5N1 from the lungs of a dead mountain lion in Montana. Then they mixed these viral isolates with raw, unpasteurized cow milk samples and heat-treated the milk at 63℃ and 72℃ for different periods of time. The samples were then cell-cultured and tested to determine if live virus remained and if so, how much.

They found that 63℃ caused a marked decrease (10¹⁰-fold) in infectious H5N1 virus levels within 2.5 minutes and note that standard bulk pasteurization of 30 minutes would eliminate infectious virus. At 72℃, they observed a decrease (10⁴-fold) in infectious virus within five seconds, however, very small amounts of infectious virus were detected after up to 20 seconds of heat treatment in one out of three samples. “This finding indicates the potential for a relatively small but detectable quantity of H5N1 virus to remain infectious in milk after 15 seconds at 72℃ if the initial virus levels were sufficiently high,” the authors note.

The scientists stress that their measurements reflect experimental conditions, should be replicated with direct measurement of infected milk in commercial pasteurization equipment and should not be used to draw any conclusions about the safety of the U.S. milk supply. Additionally, a limitation of their study was the use of raw milk samples spiked with H5N1 virus, whereas raw milk from cows infected with H5N1 influenza may have a different composition or contain cell-associated virus that may impact heat effects. The authors conclude that although gastrointestinal infections with HPAI H5N1 virus have occurred in several species of mammals, it remains unknown whether ingesting live H5N1 in raw milk could cause illness in people.

To date, the U.S. Food and Drug Administration concludes that the totality of evidence continues to indicate that the commercial milk supply is safe. While laboratory benchtop studies provide important, useful information, there are limitations that challenge inferences to real-world commercial processing and pasteurization. The FDA conducted an initial survey of 297 retail dairy products collected at retail locations in 17 states and represented products produced at 132 processing locations in 38 states. All of the samples were found to be negative for viable virus. These results underscore the opportunity to conduct additional studies that closely replicate real world conditions. FDA, in partnership with USDA, is conducting pasteurization validation studies – including the use of a homogenizer and continuous flow pasteurizer. Additional results will be made available as soon as they are available. Article


F Kaiser et al. Inactivation rate of highly pathogenic avian influenza H5N1 virus (clade 2.3.4.4b) in raw milk at 63 and 72 degrees Celsius. The New England Journal of Medicine DOI: 10.1056/NEJMc2405488 (2024). Who


Vincent Munster, Ph.D., and Emmie de Wit, Ph.D., senior investigators in NIAID’s Laboratory of Virology at the Rocky Mountain Laboratories in Hamilton, Mont., are available to discuss the findings. Contact


To schedule interviews, please contact the NIAID News and Science Writing Branch, (301) 402-1663, niaidnews@niaid.nih.gov.

NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID website.

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