Feeding mice with raw milk contaminated with bird flu can lead to infection, study spotlights

27 May 2024 ---US researchers have found that mice that consume raw milk infected with the H5N1 virus are getting infected, sparking concerns over its possible spread to humans consuming raw, unpasteurized milk. The study comes as the US is currently grappling with the spread of the bird flu in dairy cattle and the government and F&B industry are finding ways to mitigate the spread.

Raw milk sales in the US continue to rise, up 65% compared with the same periods a year ago, since March 25 when the bird flu virus was first confirmed in US cattle, flags the University of Nebraska Medical Center Global Center for Health Security.

The scientists tested the virus's spread in mice through two methods of heat inactivation on four H5N1-positive milk samples taken from infected cows in New Mexico and Kansas: 63°C for 5-30 minutes and 72°C for up to 30 seconds. 

The former reduced the virus levels more than 99.999%, while the latter inactivated more than 99.99% of the virus within the samples, though “it did not completely inactivate the virus,” says the study.   

“We must emphasize that the conditions used in our laboratory study are not identical to the large-scale industrial treatment of raw milk,” says Yoshihiro Kawaoka, professor in the UW–Madison Department of Pathobiological Sciences and Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, who led the work.

Kawaoka advises consumers against drinking raw milk (Image credit: University of Wisconsin–Madison).“It’s important to remember that our heat-treatment findings may not perfectly translate to real-world conditions.”

The findings are published in the New England Journal of Medicine and also included scientists from the Texas A&M University in the US.

Humans at risk?
The study’s findings suggest that consumers could also risk possible H5N1 infection if they consume raw milk from an infected cow.

“Do not drink raw milk,” warns Kawaoka.

The study found that mice that consumed “untreated milk infected with H5N1” subsequently suffered with influenza and that “small amounts of virus persist in untreated milk for weeks when kept at standard refrigeration temperatures.”

The university highlights that only two people in the US have had confirmed H5N1 infections until now. One is in Texas, and another is in Michigan.

“Both cases were in farm workers who worked closely with dairy cattle, and the primary symptom in both was inflammation of the eye.”

Meanwhile, epidemiologists have not confirmed the mode of transmission. The Centers for Disease Control and Prevention said the “most likely routes” were infected milk splashing into the workers’ eyes or workers touching their eyes with hands contaminated with the virus.

Authorities continue to urge dairy farms to ensure all milk is pasteurized before disposal.Raw milk is legally sold in 30 states in the US but contributes only a small fraction of total milk sales.

What’s being done?
The “highly pathogenic” avian influenza virus (HPAI) of the H5N1 subtype was detected for the first time in nasal swabs and milk of dairy cows in March, notes the study. 

Following the WHO’s warning of bird flu spread, nearly 21 states in the US restricted cattle importations from regions where the virus infected dairy cows.

The US Department of Agriculture’s Animal and Plant Health Inspection Service also recently advised farmers to reduce the movement of cattle, but has not issued federal quarantine orders.

Earlier this month, the US Department of Agriculture (USDA) unveiled a series of measures to mitigate the virus’ impact on US dairy cattle through a cash injection of US$101 million. The funds are aimed at prevention and treatment initiatives, and up to US$28,000 per farm is used to enhance testing and biosecurity measures to curb the virus’s spread.

Meanwhile, stakeholders drew parallels to the COVID-19 pandemic when the virus spread to dairy cows and advised understanding the influenza virus’ transmission, including via mechanical spread.

By Insha Naureen