Q&A: FDA confirms traces of bird flu virus in pasteurized milk
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Key takeaways:
- Traces of “bird flu” virus have been identified in pasteurized cow milk.
- Eight states have reported infected herds: Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas.
The FDA announced this week that testing has identified genetic traces of highly pathogenic avian influenza virus in pasteurized cow milk samples collected in the United States.
Noting that the pasteurization process is expected to inactivate viruses, the FDA said there is currently no indication that the presence of influenza A(H5N1) viral particles in milk samples is a threat to human health.
The particles were identified through quantitative PCR (qPCR) testing, which is able to detect only the presence of a pathogen’s genetic material — not whether it remains intact or infectious. For that, the FDA is conducting additional testing to determine if milk samples contain viable virus that remains infectious — the first time that scientists have studied the effect of pasteurization on highly pathogenic influenza viruses like H5N1, which is also referred to as “H5N1 bird flu.”
“To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the FDA said.
Still, the finding raised questions about the tracking of pathogens in the nation’s milk supply and how widespread the outbreak of H5N1 is in dairy cows. As of Monday, eight states had reported infected herds, according to the U.S. Department of Agriculture (USDA): Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas. One human case has also been reported, in Texas.
The CDC has advised states to prepare response plans for avian influenza, although it says the risk to human health remains low.
On Wednesday, the USDA announced several steps to limit the spread of H5N1 among cattle, including a requirement that dairy cattle receive a negative test for influenza A before crossing state lines. Owners of herds that include infected cows will have to provide information on the herd’s movements, and labs and veterinarians will be required to report positive influenza A tests to the USDA.
To gain a better understanding of the public health implications of the H5N1 outbreak in cows, we checked in with Andrew S. Pekosz, PhD, professor of molecular microbiology and immunology and vice chair of the department of molecular microbiology and immunology at the Johns Hopkins University Bloomberg School of Public Health, and Richard Webby, PhD, a faculty member in the department of infectious diseases at St. Jude Children’s Research Hospital and director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds.
[Editor’s note: An earlier version of this Q&A was updated to include responses from Webby.]
Healio: In simple terms, what does finding particles of highly pathogenic avian influenza in pasteurized milk mean?
Pekosz: For the general public, concern should still be low because the tests are most likely picking up “dead” viruses in the milk.
Webby: The finding of H5 RNA in store milk simply means that some milk — which is where most of the H5 virus is found in cows — from infected cows is getting through and into milk destined for consumption.
Healio: Is it unexpected?
Webby: No, not really. There is a high level of virus in infected cow milk, and with many infectious diseases, there is often asymptomatic or mildly affected animals. Therefore, even when milk from obviously impacted cows is diverted, it is absolutely expected that some will get through.
Healio: Is there anything about it that alarms you?
Pekosz: What concerns me is that this result means that milk from H5N1 infected cows did make it into the milk supply. Early reports about the milk from an infected cow being discolored and very thick [shows it can be] easily identified and taken out of the supply chain. Finding virus in the milk may mean that it is present in milk that looks otherwise normal and therefore wasn’t caught by this visual inspection.
Webby: In itself, no. I am not at all concerned about consuming pasteurized milk. Perhaps the most informative part of this study was the finding of how many milk samples were positive. It does suggest the virus is in more herds than officially reported.
Healio: Does finding genetic particles of H5N1 in milk signal danger to the American public?
Pekosz: No.
Webby: Absolutely not. We have been unable to detect any viable viruses in supermarket milk. The presence of RNA in the sample does not mean there is any virus still there. In this instance, we are sure it’s simply a footprint that says the virus was there once.
Healio: Is there any reason for people to stop drinking milk?
Pekosz: No.
Webby: With the exception of unpasteurized milk, which should not be consumed right now, everything we have says pasteurized milk is safe.
Healio: The remnants were found in pasteurized milk. There are a lot of reasons not to drink raw milk, which the CDC says is among the riskiest foods to consume. Would you expect there to be live H5N1 virus in raw milk? Would that pose a risk to someone drinking it?
Pekosz: The safety of pasteurized milk compared with raw milk is clear, and in many ways, that validates how effective pasteurization is at killing lots of different microbes, many of which are more “hearty” or difficult to kill than an influenza virus. I would expect there to be live virus in the raw milk, given what we have heard about very strong signals for virus in the milk of cows that show severe symptoms of avian flu infection.
It’s important to re-emphasize that there are a lot of other things in raw milk that can make people sick, so pasteurized milk is the safe way to consume milk. It would be nice to have data showing how well pasteurization works against this specific H5N1 virus — and that data should be coming soon.
Webby: Yes, we know there is lots of H5 virus in an infected cow’s milk. While the most severely affected milk is clearly “off” and unlikely to make it any further toward human consumption, the presence of H5 RNA in supermarket milk does show not all infected milk is being diverted. So, there is definitely a risk from drinking raw milk and raw milk products.
Healio: What does finding H5N1 genetic material in milk mean in terms of how widespread infections are in cattle in the United States?
Pekosz: It will be important to do much more widespread testing of dairy cattle and milk produced on dairy farms to understand what the source of H5N1 in the milk is. It may be that the milk has virus in it before the cows show symptoms of infection like reduced milk production or thick, discolored milk. It wouldn’t surprise me if an infected cow had virus in its milk for a day or two before changes in the milk quality could be identified.
Another theory is that cows that don’t show symptoms of infection may be shedding virus into their milk. Detecting this would take a lot of testing of apparently healthy cows and their milk. Determining if cows can be infected and shed virus in milk when they show no, or only very mild symptoms will be critical.
Webby: As mentioned, this does suggest there are more infections than officially reported. The new directives from USDA requiring reporting of any positive tests will also be informative here.
Healio: Do you believe that public health agencies in this country are acting appropriately to respond to the situation?
Pekosz: FDA testing and communication around its testing has been quite good. CDC is ready to respond to any potential infections in humans, so I think these public health agencies are responding well.
Webby: Currently, the public health agencies in this country are a bit hamstrung. They cannot currently mandate that everyone gets tested who is in contact with the infected herds. They have to wait until someone seeks medical care. So, the response to date isn’t ideal, but is not the fault of the agencies but rather what they can and can’t do.
References:
- FDA. Updates on highly pathogenic avian influenza (HPAI). https://www.fda.gov/food/alerts-advisories-safety-information/updates-highly-pathogenic-avian-influenza-hpai. Accessed April 24, 2024.
- USDA. USDA actions to protect livestock health from highly pathogenic H5N1 avian influenza. https://www.usda.gov/media/press-releases/2024/04/24/usda-actions-protect-livestock-health-highly-pathogenic-h5n1-avian. Published April 24, 2024. Accessed April 25, 2024.
- USDA. Highly pathogenic avian influenza (HPAI) detections in livestock. https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/livestock. Accessed April 24, 2024.
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