Influenza virus is very stable in unpasteurised milk

Even as the Atlanta-based CDC has cautioned people to avoid drinking raw milk, especially in the States where H5N1 has been reported in cattle herds, there has been increased demand for raw milk. From the day H5N1 (bird flu) was confirmed in cattle in mid-March this year to mid-May, the sales of raw milk has increased 21-65% compared with the same period last year. However, no human case of H5N1 infection in people who consume raw milk has been reported so far in the U.S. despite the increase in raw milk consumption.

“The asymptomatic avian influenza surveillance programme in the U.K. has found that human infection can be very asymptomatic,” says Dr. Vinod Scaria, a senior consultant at Vishwanath Cancer Care Foundation told The Hindu. Two people in the U.K. who had worked on an infected poultry farm in England and tested positive for H5N1 did not show any symptoms and were detected only because of the asymptomatic surveillance programme. “Testing of people, including those who consume raw milk that may be infected with H5N1 has been very low in the U.S.,” says Dr. Scaria. According to the CDC, there has been “no sign of unusual influenza activity in people, including in syndromic surveillance”.

On May 30, the CDC reported a third case of H5N1 infection in a farm worker in Michigan who had direct exposure to infected cows. Eight days earlier, CDC reported the second case of H5N1 infection in a farm worker in the U.S., and the first in the State of Michigan. The first human case in the U.S. was also in a farm worker in Texas. Similar to the first two cases, the latest case of H5N1 infection had eye discomfort with watery discharge. But unlike the earlier two cases, the case reported on May 30 had “more typical symptoms of acute respiratory illness associated with influenza virus infection, including A(H5N1) viruses”. In the second case reported on May 22, two specimens were collected — upper respiratory tract and eye specimens. While the nasal specimen tested negative at both the State health department and the CDC, the eye specimen sent to the CDC for testing was positive for H5N1. The route of transmission has not been ascertained, though it is likely to be airborne.

Meanwhile, researchers from the University of Wisconsin-Madison and Texas A&M Veterinary Medical Diagnostic Laboratory carried out a study to assess the risk that H5N1-containing milk poses to animals and humans and fed droplets of raw milk from infected dairy cattle to five mice. The mice showed signs of illness one day after they were fed raw milk containing the virus. The researchers found high virus titers in the respiratory organs of the mice and moderate virus titers in several other organs. Since the mice were only fed raw milk, the infection in the nasal passages, trachea and lungs, suggests that infection may have occurred through the pharynx, the researchers note in a Letter published in The New England Journal of Medicine. The H5N1 virus was also found in the mammary glands of two mice even though these mice were not lactating.

In another study posted as a preprint(which is yet to be peer-reviewed), researchers found H5N1 virus in unpasteurised milk remains infectious on milking equipment for over one hour. The researchers then used H1N1 as a surrogate for the H5N1 virus found in cattle to study the viral persistence in raw milk. They found the H1N1 virus in unpasteurised milk persisted for at least three hours on rubber and at least one hour on stainless steel. “These results indicate that influenza virus is very stable in unpasteurized milk and that deposited H5N1 on milking equipment could remain infectious for long periods of time,” they write.

Almost all human influenza virus infections are primarily respiratory infections unlike in some other mammals where gut infections have been reported

Even while no case of H5N1 infection in humans has been reported after consuming raw milk, scientists believe that the virus can be transmitted via an infected cow’s milk that has not been pasteurised. The lone reported human case in the current outbreak in cattle spread across 49 herds in nine States in the U.S. has been a dairy farm worker who had direct exposure to cows and developed conjunctivitis. The Atlanta-based CDC has advised people to desist from consuming uncooked dairy food products such as raw, unpasteurised milk, or raw cheeses from animals with suspected or confirmed H5N1 bird flu virus infection.

While FAO noted that H5N1 virus was detected in “high concentrations in milk from infected dairy cattle and at levels greater than that seen in respiratory samples”, there are no reports of viable H5N1 virus being found in raw milk samples. “I have not come across any report of actual isolation of influenza viruses from raw milk. Most reports on viral loads in milk are based on molecular sequencing data rather than actual isolation of the virus from milk. It will be RT-PCR or sequencing of fragments,” immunologist Dr. Vineeta Bal, a visiting faculty at IISER Pune says in an email to The Hindu.

Despite CDC warning people to avoid consuming raw milk, sales of raw milk in the U.S. has increased sharply since the H5N1 outbreak in cattle was first reported there on March 25 this year. Yet, there has not been any reported case of human infection after raw milk intake. Even as it makes eminent sense to avoid consuming raw milk especially in places where H5N1 outbreak in cattle has been reported in the U.S., how likely are people to be infected with H5N1 via raw milk? “We need to remember that viruses outside a cell do not have life of their own. The biological plausibility of transmission through milk is very limited,” Dr. Chandrakant Lahariya, physician epidemiologist and a public policy and health systems expert says in an email to The Hindu. 

“There are very few human cases reported so far and almost all of those were in farm workers who had come in close contact with infected animals. In these cases, the likely route of transmission was airborne. Raw milk from H5N1 infected cows may not cause illness in humans as the route of introduction of the virus is oral rather than nasopharyngeal,” says Dr. Bal. “Theoretical possibility of a human being catching H5N1 infection from infected raw milk is very low. Till date, there has been no documented evidence of H5N1 transmission from animals to humans via the oral route.”

Dr. Bal goes further to underscore the odds of H5N1 infection in humans through consumption of raw milk. “Almost all human influenza virus infections are primarily respiratory infections unlike in some other mammals where gut infections are reported. Based on that it can be stated that consumption of heavily infected H5N1 containing raw milk is unlikely to cause illness in human beings.”

There are many reports of cats being infected with H5N1 in the current outbreak in the U.S. In a recent study, domestic cats fed with raw, unpasteurised colostrum and milk from affected cows developed fatal systemic influenza infection, thus demonstrating the oral route of transmission in cats. “Some birds (poultry) as well as cats do show susceptibility to influenza virus infections via oral route. So far humans have not been infected by faeco-oral route even if the virus is shed by humans in the excreta. We know this from the SARS-CoV-2 example too,” says Dr. Bal.

Despite the CDC’s warning, there is a growing misconception among people in the U.S. that immunity against H5N1 virus can be developed by drinking raw milk containing viable viruses. These notions have no scientific basis whatsoever. “There is no basis for this assumption at all. In fact, drinking raw milk might prove more risky due to the presence of many other pathogens which are known to be present in raw, unpasteurised milk,” Dr. Bal cautions.