Milk on sale in US supermarkets tested positive for severe bird flu

Pasteurized milk sold in supermarkets in the USA is positive for highly pathogenic avian influenza (HPAI) or H5N1. The Food and Drug Administration (FDA) announced that one in five milk samples nationwide had genetic traces of the virus.

Milk samples from areas with infected herds were more likely to return positive results. For analysis, the FDA used quantitative polymerase chain reaction (qPCR) tests to check the milk.

While the findings are concerning, it doesn’t necessarily mean the milk contains a live virus that could cause an infection, explained Nam Tran, senior director of clinical pathology at UC Davis Health.

“With qPCR tests, the genetic material, not necessarily the entire active or infectious virus, is what is detected. In the case of food, the genetic material, the RNA found in supermarket milk samples, may not be the infectious H5N1 virus, but fragments of it.e,” Nam Tran explained.

Beate Crossley, virologist at the UC Davis College of Veterinary Medicine, said: “PCR tests target the genetic material of a virus. The discovery of genetic material does not provide any information about the viability status of the virus“, i.e, “In other words, PCR can detect live and dead viruses.”

Pasteurization kills pathogens

However, based on currently available information, the FDA states that commercial milk is safe.

Commercially available milk is pasteurized, a process that kills harmful bacteria and viruses by heating the milk to a specific temperature for a set period of time. The FDA noted that viral particles detected by highly sensitive qPCR tests were likely remnants of viruses killed during the pasteurization process.

“To date, we have not seen anything that would change our assessment that the commercial supply of milk is safe”, stated the Agency.

To determine whether any active infectious virus remains in milk samples, the FDA will perform egg inoculation tests, considered the gold standard for determining viability.

“With these tests, they inject the sample into the egg to see if the virus replicates or not.”, explained Nam Tran.

Although this provides the most sensitive results, it takes longer to complete than other methods.

“Virus isolation propagates viruses and needs a live virus particle to begin,” said Crossley. “A positive virus isolation result from a sample would indicate that a live infectious virus is present in the sample.”

Spread in dairy cattle

Avian influenza is very contagious and often fatal in bird populations. It was first detected in dairy cattle in the US in March. To date, 33 outbreaks of bird flu have been confirmed in dairy cattle in eight states. Since the outbreak, the FDA has been evaluating milk from affected animals, the processing system and supermarket shelves.

Two human cases of H5N1 have been confirmed in the United States. The Centers for Disease Control and Prevention (CDC) considers the risk to the general public to be low.

Dean Blumberg, chief of pediatric infectious diseases at UC Davis Health said: “Bird flu rarely spreads from person to person. It is most commonly transmitted from animal to human”.

The expert added: “There has been no evidence of sustained human-to-human spread, although there is always a chance that the virus could evolve to transmit more easily between humans.”

Some groups of people with professional or recreational exposure to infected birds or other animals are at greater risk of infection. They must take appropriate precautions to protect themselves against bird flu.

Regarding milk safety, Blumberg noted that the main risk would be raw milk. “A Pasteurization results in a greater than 99.9999% reduction of infectious virus in contaminated milk, likely eliminating transmission, but there may be a risk of transmission if unpasteurized milk from an infected animal is ingested.”

According to the CDC, bird flu symptoms in humans range from mild, such as conjunctivitis (pink eye) and flu-like upper respiratory symptoms, to severe, such as high fever and pneumonia that require hospitalization.

Humans infected with bird flu can be treated effectively with the antiviral oseltamivir (Tamiflu), according to Blumberg.

Although the move to livestock is recent, bird flu has been on Nam Tran’s radar for years. The first human cases were reported in China in 2020.

“We have been working closely with the Division of Infectious Diseases on H5N1 preparations since before the COVID-19 pandemic.” said Nam Tran. “If there are any suspected cases of bird flu in humans, UC Davis Health has tests ready.”