The U.S. Food and Drug Administration (FDA) announced on Tuesday that it had detected viral particles of H5N1 avian influenza in milk purchased at grocery stores across the country. Despite this finding, the agency maintains that pasteurized dairy products are still safe for consumption, as the viral particles were likely remnants of viruses killed during the pasteurization process. However, additional tests are being conducted to ensure the particles are not capable of infecting people.
The discovery comes in the wake of an ongoing outbreak of bird flu in cattle, which was first confirmed by the U.S. Department of Agriculture (USDA) in late March. Since then, infected cows have been identified in more than 30 farms across eight states. The FDA's detection of H5N1 viral particles in milk on grocery store shelves suggests that the outbreak may be more widespread than previously known.
"The dissemination to cows is far greater than we have been led to believe," said Dr. Eric Topol, founder of the Scripps Research Translational Institute, in an email on Tuesday. "The FDA assurance that the dairy supply is safe is nice, but it's not based on extensive assessment yet, which they acknowledge, and won't engender trust and confidence because it comes in the wake of USDA mishandling."
Scientists have criticized the USDA for sharing too little information about the virus and doing so too slowly. The infected cows exhibit symptoms such as reduced appetite and thickened, discolored milk. Veterinarians have found that while the animals' milk is teeming with the virus, their lungs show little evidence of infection, suggesting that the virus may be directly infecting the cows' mammary glands, possibly through shared milking equipment.
Researchers are still working to determine how the cows initially became infected and how the virus is spreading between animals. Cats living on the same farms as infected cows have died, potentially after exposure to the infected milk.
The FDA stated that milk from sick cows is being "diverted or destroyed," while milk sold in interstate commerce is being pasteurized to kill microbes. "Even if virus is detected in raw milk, pasteurization is generally expected to eliminate pathogens to a level that does not pose a risk to consumer health," the agency noted. However, it acknowledged that pasteurization does not render milk completely sterile.
To assess the safety of the milk supply, the FDA has been running tests on milk samples collected at grocery stores and collaborating with the food safety group at the U.S. Centers for Disease Control and Prevention (CDC). The agency is using "egg inoculation tests," considered the gold standard for determining the viability of a virus, to confirm whether the detected viral particles can still infect people.
Efforts to track the spread of the current outbreak have relied on voluntary reporting, as the USDA's Animal and Plant Health Inspection Service does not require reporting of avian influenza in cattle, unlike in poultry and wild birds. "No one ever thought it was going to be in cows," said Dr. Richard Webby, director of the World Health Organization's coordinating center for studies on the ecology of influenza at St. Jude's Children's Research Hospital.
Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota, expressed concern about the outbreak potentially spreading to pigs, as farms that keep cows often keep other types of animals as well. Pigs have sialic acid receptors similar to both birds and humans in their respiratory tracts, which could provide the H5N1 virus with an opportunity to adapt to human sialic acids if a large outbreak occurs in pigs.
The USDA has encouraged farmers and ranchers to report cases of sick cattle to state animal health officials, but widespread screening of cows, people who work or live around them, and wastewater in affected areas is not currently being conducted. The CDC is monitoring emergency department data and flu testing data in areas where H5N1 viruses have been detected in dairy cattle for any unusual trends in flu-like illness, flu, or conjunctivitis. To date, 23 people with exposures to H5N1 have been tested, with only one person in Texas previously testing positive.
