wattAshton The U.S. Food and Drug Administration confirmed Tuesday that agency testing of commercially purchased pasteurized milk found genetic evidence of the H5N1 avian influenza virus. But tests using the polymerase chain reaction (PCR) cannot distinguish between live viruses or fragments of the virus that may have been killed by the pasteurization process.
The agency said it had been trying to see whether the virus could be grown from milk containing evidence of H5N1, the gold standard test for whether live viruses are present in products. The lengthy statement released by the agency did not explicitly say that FDA laboratories were unable to find live viruses in the milk samples, but it did say that the findings did not change its belief that commercially pasteurized milk is safe to eat.
“To date, we have seen nothing that would change our assessment of the safety of the commercial milk supply,” the statement said.
The document makes lengthy reassurances but lacks details of what was done or discovered. It did not specify how many commercial samples were collected or in how many markets, nor did it show what percentage of samples were PCR positive for H5N1. The statement did not say whether the tests showed low or high levels of viral genetic material in the milk.
In addition, the statement did not disclose whether the dairy products were purchased in areas of the country where the outbreak occurred or where no infection in dairy cows was found.
The FDA did say it is testing infected animals, processing systems and milk on store shelves with the goal of producing a representative sample of the national milk supply. In addition to assessing the safety of the supply, such samples would allow the agency to gain a more complete understanding of the spread of H5N1 among dairy cows across the country.
The agency stressed that testing of commercially available milk is ongoing and includes efforts to discern potential differences between different dairy products, such as cream and whole milk.
The findings are not surprising, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, who said PCR testing of milk may reveal genetic evidence of a range of other pathogens that may be affected by pasteurization. Kills, for example, listeria.
Osterholm said he still believes the risk of any live viral activity in milk is low, although he noted the FDA’s statement contained too few details to prove that. This is an obtuse document.
The FDA has for some time dodged questions about its ongoing work to assess the safety of the milk supply, saying the commercial investigation is part of its work to evaluate federal state milk safety systems in the context of: Currently, at least eight states across the country An outbreak of H5N1 avian influenza occurred in dairy cows. As of Monday, 33 pig outbreaks have been confirmed.
Some samples collected indicate the presence [H5N1] Using quantitative polymerase chain reaction (qPCR) testing, the FDA wrote in a statement.
It reiterated that it believed the pasteurization process likely inactivated H5N1. While the agency acknowledged that it has not yet published studies on the effects of pasteurization on the H5N1 virus in milk, it said it used milk from dairy cows and other species.
However, some academic centers have already begun this work. John Lucey, director of the Dairy Research Center at the University of Wisconsin-Madison, told STAT in an email that the university’s preliminary tests on milk show pasteurization is effective at killing the virus. But he declined to give further details. He wrote that the USDA is currently conducting its own trials and I hope to hear from them soon on the pasteurization results, which should be considered an official test.
The virus, which has been causing outbreaks around the world for more than a quarter of a century, had never before been known to infect dairy cows.
Pasteurization is the process of heating raw milk to kill problematic microorganisms. There are several ways to do this; some push the milk quickly through a heated, pressurized tube, while others heat the milk in a vat for a long time.
The degree of microbial destruction depends on the combination of temperature and holding time in the pasteurizer, as well as the specific nature of the pathogen and how much of it is present in the milk. The higher the bacteria or virus content, the longer it will take to kill. The minimum temperature and time requirements used by U.S. dairy processing plants are based on research showing how to kill the most heat-resistant pathogen found in milk: Rickettsia burnetii (the bacterium that causes Q fever).
“This is a very stubborn microbe,” said Terry Lehenbauer, a cattle disease epidemiologist and director of the Veterinary Teaching and Research Center at the University of California, Davis. So if we know that pasteurization is effective at killing indicator organisms, then we know that all other less durable pathogens will also be eliminated.
Most milk in the United States is processed using high-temperature, low-time technology, where the milk is heated to 162 degrees Fahrenheit and held there for 16 seconds. Since this is a heat treatment process that does not involve the physical removal of any bacteria or viruses, the dead genetic material left behind can be detected through methods that pick up small fragments of DNA or RNA, such as qPCR.
Jennifer Cruickshank, a genomics expert at Oregon State University, said it doesn’t actually tell you anything about the viability or ability of the virus to infect. It actually just tells you that, in the case of the influenza virus, there is this specific piece of RNA.
To date, no studies have been published on the effectiveness of pasteurization against H5N1 viruses in milk. But researchers interviewed by STAT said that based on previous research, the virus is relatively sensitive to heat and is expected to be inactivated. A 2022 review by German researchers found that influenza viruses, including the H5N1 strain found in chicken, feces and eggs, can be reduced by four orders of magnitude in 30 minutes at temperatures of 140 degrees Fahrenheit.
But milk is a very different substance, a suspension of water, butterfat and protein, and its complex dynamics can sometimes complicate the pasteurization process. Research conducted by the U.S. Department of Agriculture in the 1970s and 1980s found that these butterfats act like a protective bubble wrap around the virus that causes foot-and-mouth disease, making it harder for the heat from pasteurization to penetrate and inactivate the virus. There is no indication that the same is true for the H5N1 virus. Linebauer points out that avian influenza belongs to a group of viruses that are enveloped and, counterintuitively, are much less resistant than non-enveloped viruses such as the one that causes foot-and-mouth disease. But as with any emerging pathogen, it’s worth knowing for sure.
Andrew Pekosz, a molecular microbiologist at the Johns Hopkins Bloomberg School of Public Health who studies respiratory viruses, says pasteurization can kill viruses that are more stubborn than the flu, so we expect it to work effect. But it would be great to have data.
This story has been updated.
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