Scientists who know about the types of pathogens — E. coli and Salmonella among them — that can be transmitted in raw milk generally think drinking unpasteurized milk is a bad idea. But right now, they believe, the danger associated with raw milk may have gone to a whole new level.
“If I were in charge, for the moment I would forbid the selling of raw milk,” said Thijs Kuiken, a pathologist in the department of viroscience at Erasmus Medical Center in Rotterdam, the Netherlands, who has done research on H5N1 and the damage it inflicts for about two decades.
H5N1 bird flu has been circulating in dairy cow herds in multiple parts of the country, likely for months now. Testing of milk from infected cows shows the virus is present in concentrations that have taken scientists by surprise. They worry that if a raw-milk consumer inadvertently drank milk from infected cows, the results could be bad — potentially really bad.
“I absolutely wouldn’t go anywhere near raw milk in terms of consuming it,” said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital in Memphis, Tenn., whose laboratories have been involved in testing to see if evidence of H5N1 RNA can be found in commercially purchased milk — it can — and whether live virus can be grown from pasteurized milk containing H5N1 RNA. So far it looks like the answer to that question is no.
While the Food and Drug Administration bans the interstate sale and distribution of raw milk, rules surrounding its use in an individual state are set by the state legislature. Some ban the sale of raw milk for human consumption; others allow it under a variety of circumstances. (A state-by-state rundown is here.)
Kuiken said his concern about the risk that infected raw milk poses is not so much that the practice might somehow help the virus to mutate in ways that would allow it to spread easily to and among people — in other words, trigger a pandemic. But he believes it would likely seriously sicken people who drink raw milk from an H5N1-infected cow. Reports of the amount of virus present in infected udders is higher than anything he’s seen in studies where he’s experimentally infected animals with H5N1 to chart the illness the virus wreaked, Kuiken said.
Jürgen Richt, a veterinarian and director of the Center of Excellence for Emerging and Zoonotic Animal Diseases at Kansas State University’s College of Veterinary Medicine, spoke with a note of disbelief in his voice about the amount of dead viruses or viral particles being found in commercial milk that tested positive for the virus.
“From [results] I have seen, I wouldn’t want to drink raw milk,” Richt said. “And I wouldn’t feed it to my cats, nor my dogs, nor my calves, if I’m on a farm.”
The FDA is urging consumers not to drink raw milk or eat raw milk cheeses. That is a position the agency has long held, because of the other health risks these products hold, but it has re-emphasized it in the current context.
It has also recommended the dairy industry not to “manufacture or sell raw milk or raw milk products, including raw milk cheese, made with milk from cows showing symptoms of illness, including those infected with avian influenza viruses or exposed to those infected with avian influenza viruses.”
Kuiken said he is less concerned about raw milk cheeses, saying the various processes involved in cheesemaking are “not conducive to survival of infectious virus.” He did suggest, though, that raw milk cheesemakers could be at risk, if they were inadvertently using milk laced with H5N1 virus.
Whether herds owned by farmers who sell raw milk have been infected by the virus isn’t publicly known. While authorities and scientists believe outbreaks are occurring over a much broader swathe of the country than has been detected, the U.S. Department of Agriculture has only confirmed infections of 33 herds in nine states — Texas, Kansas, Michigan, New Mexico, Idaho, Ohio, South Dakota, North Carolina, and Colorado. It has not given any details about the operations on which infected animals were found.
But the USDA has admitted some farmers have been refusing to test their animals. And analysis of the genetic sequences of viruses retrieved from cows combined with evidence of H5N1 RNA in commercial milk found in a number of U.S. markets — the FDA said Thursday that about 1 in 5 samples tested for H5N1 from across the country have been positive — bolster the argument that this has been going on for longer than has been recognized and likely involves far more herds than have tested positive.
The testing of commercial milk was done by polymerase chain reaction, or PCR. In PCR testing, the concentration of a pathogen is estimated by how many cycles the test has to run to find it. The lower the cycle threshold — known as a “Ct value” — the higher the concentration. Anything with a cycle threshold of 29 or lower is considered a strongly positive result. Some milk testing has shown a Ct value of below 10, Kuiken said.
Webby, who is also the director of the World Health Organization’s Collaborating Center for Studies on the Ecology of Influenza in Animals (located at St. Jude), said drinking raw milk that is infected with H5N1 could deliver a whopping dose of virus to a person’s system — enough to trigger a fatal infection in lab studies where small animals are experimentally infected with H5N1.
“Of course, we don’t know what a lethal dose, or even infectious dose, is in humans. But it makes absolutely no sense to me that you’d want to try,” he said.
We think of flu viruses as pests that plague our upper and sometimes lower respiratory tracts, causing congestion, sneezing, fever, coughing, and in some cases triggering pneumonia. Because of their biology, the influenza A viruses that routinely transmit among people — H1N1 and H3N2 — are effectively limited to infecting the cells in the respiratory tract, said Florian Krammer, an influenza virologist at the Icahn School of Medicine at Mount Sinai in New York.
But H5N1 has more tricks up its sleeve than seasonal flu viruses do; it can infect organs other than the lungs. Kuiken noted that the virus has been seen to move into the liver, the central nervous system and the brain, among other tissues. He saw the latter when he experimentally infected cats in the mid-2000s. Similar results were seen when baby goats were infected with the virus on a Minnesota farm in March. Ten of the kids died; necropsies on five of them showed virus in the brains and other organs.
Many of the animal species infected in the wild have been reported to sustain damage outside of the respiratory tract. It’s believed these animals — bears, raccoons, seals, foxes, and a raft of other mammals — became infected by eating sick or dead wild birds or poultry infected with the virus.
“So many, many species, they get a brain infection and brain disease, not a pulmonary disease,” Kuiken said. “In some cases the disease is missed because people are expecting pneumonia. If the animal is found dead, they don’t see the neurologic signs, and they forget to test the brain.”
Human infection with flu occurs when people breathe viruses emitted by others into their upper respiratory tracts. But the shared real estate of the top of the respiratory and gastric tracts make it impossible to rule out the possibility that ingestion of food or drink that contained high levels of viable H5N1 virus could trigger infection, scientists said.
Krammer said there are several mechanisms that could come into play. “You could get infection of cells in the upper respiratory tract that could go down and cause lower respiratory tract infection. There could be a route where it’s really going to the olfactory bulb,” he said, going from there to the brain. “And there’s the other option that it really makes it through the stomach … and then you start an infection from the mid gut.”
Krammer said he thought that last option was unlikely; he thinks stomach acids would inactivate the virus. But Kuiken doesn’t believe that’s necessarily true, referring to his study of H5N1 in cats. Milk acts as a buffer, he said. “It’s the perfect fluid for [virus] reaching the intestine in an infectious state.”
Maria Van Kerkhove, acting head of the WHO’s department of epidemic and pandemic preparedness, pointed to another outbreak, involving another virus and another animal species — the MERS coronavirus, which spreads to people from camels — as an example of where ingestion of virus-laced milk may have triggered infection of a respiratory disease. Some cases of MERS were traced to the drinking of raw camel’s milk, Van Kerkhove said, though she noted it wasn’t generally possible to tease out whether it was the drinking of the milk or the act of milking that led to the infections. (Influenza viruses and coronaviruses belong to different families.)
She acknowledged that some people take comfort from the notion that stomach acids would kill H5N1, but said it is important to look for evidence-based answers.
The experts STAT spoke to for this story said it would also be prudent for public health authorities looking for possible human cases of H5N1 to cast their net broadly, given that ingestion of the virus might trigger symptoms not typically associated with influenza. The Centers for Disease Control and Prevention have said it is monitoring emergency room data for unusual cases of flu and for conjunctivitis. (There has only been one confirmed case associated with the dairy cow outbreaks to date, a farm worker in Texas who had conjunctivitis.)
“I think that’s why it’s also important to look out for all kinds of weird syndromes that might come up in the areas where there is more exposure and people have more exposure to dairy cattle,” Krammer said. “It could be neurological. It could be basically conjunctivitis. It could be more traditional respiratory infections. But also, something new could come up.”
“In a way, we think we know H5, but the cow trick was completely new and we don’t know what the virus would do when it goes into humans. Maybe it does something very unexpected. There’s just not enough data to say anything conclusive. So I think people should just be watchful,” he said.
Richt agreed. “Expect the unexpected. That’s the situation we deal with here.”