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.
