A farm worker in New York State is milking a cow. Image source: Nature website
In the past three weeks, the number of states in the U.S. where dairy cows have been infected with avian flu has increased from 6 to 9. A report published on April 16 in a preprint website revealed that the avian flu virus was found in raw milk from infected cows. Federal authorities in the U.S. also announced on April 24 that the virus was detected in the lung tissue of a seemingly healthy cow.
With dairy cows infected with avian flu, is the milk supply safe, and what threat does it pose to human health? A recent report on the Nature website highlighted that pasteurized milk may not pose a threat to human health, but droplets of fresh milk on milking equipment could potentially spread the virus within the cow population. Experts emphasize the need to enhance inspections and monitoring on dairy farms to reduce the spread of the epidemic.
Virus detected in retail milk
The outbreak of avian flu in dairy cows is caused by the H5N1 strain. Officials in the U.S. confirmed on April 24 that genetic material of the H5N1 strain was detected in milk sold in stores. Diego Diel, a food scientist at Cornell University, suggested that this finding indicates the avian flu outbreak in dairy cows may be "more widespread than initially thought."
It is currently unclear how many milk samples the Food and Drug Administration (FDA) has tested and where these samples were collected. The agency stated that more information will be released in the coming weeks.
After leaving the farm, milk undergoes pasteurization to deactivate pathogens before being shelved. To detect H5N1, the FDA used a method called quantitative polymerase chain reaction (qPCR). This method can extract virus RNA but only detects fragments of the virus genome, making it unable to distinguish between live and residual dead viruses.
Detecting virus material in milk about to be sold is significant. Brian Wasik, a virologist at Cornell University, suggested two possibilities: either the avian flu outbreak is more common than farmers realize, with milk from infected animals entering commercial supply, or asymptomatic cows are secreting the virus into the milk without detection.
Federal regulations in the U.S. require discarding milk from infected cows, but it is still unclear if cows start spreading the virus before showing signs of illness or abnormal milk production. An article published on April 16 on a preprint website mentioned that milk from infected cows is thicker and yellower than regular milk, and infected cows eat and produce less milk than usual. However, this article has not yet undergone peer review.
Uncertainty regarding the threat to human health
Does milk containing genetic material of H5N1 pose a threat to human health?
Wasik stated that there is no definitive evidence that pasteurization can kill H5N1, but this method can kill viruses that are more resilient than the flu virus and replicate in the intestines. In general, the flu virus is relatively unstable, sensitive to heat, and the temperature for pasteurizing milk is higher than for eggs. While pasteurization may struggle to kill viruses in milk at relatively high concentrations, more experimental data is needed to confirm this. In the absence of clear answers, excluding milk from infected cows from commercial supply is crucial.
FDA public affairs specialist Yanel Goodwin mentioned that the agency is closely collaborating with the U.S. Department of Agriculture to collect and evaluate data and information related to H5N1.
Virus transmission through milk droplets
Can avian flu virus spread through milk? The answer may be yes.
Researchers from the U.S. Department of Agriculture tested nasal swabs, tissues, and milk samples from affected dairy cow herds and found the highest virus concentration in milk, suggesting that the virus may spread through milk droplets.
Tais Queken, a virologist at Erasmus University Medical Center in Rotterdam, Netherlands, suggested that milking equipment could be a "culprit" if this is the case. The teat cups of milking machines can transfer milk residues containing the H5N1 virus from one cow to another. Even with cleaning and disinfection of these devices, the virus concentration in milk from infected cows remains high, indicating the possibility of virus transmission through equipment.
Reducing the frequency of cow transfers is crucial
How can the spread of avian flu in dairy cows be contained? The FDA announced that cows must test negative for avian flu before being transferred to other states. Industry experts believe this measure may help contain the outbreak.
Wasik explained that animals in the U.S. dairy industry frequently move: calves are transferred to dairy farms, and cows are sold by farmers once they stop producing milk. This movement could be a "major driver" of the outbreak.
Researchers hope to monitor bulk milk samples from farms. Wasik suggested that wastewater testing and environmental sampling could also be useful, especially around farms where outbreaks occur or where cows are transferred. Additionally, when cows are moved to new farms, a quarantine or observation period of at least 24 hours is necessary. These monitoring measures aim to buy time, slow down the outbreak, and allow researchers and relevant agencies to better manage the situation.
