New research published today (Monday, June 3) by UK and Scottish scientists has confirmed that pasteurisation should “effectively inactivate” H5N1 influenza (bird flu) viruses in cows’ milk leaving it safe to consume.
The study details that pasteurisation temperatures, if applied for industry standard times, effectively eliminate “any influenza virus” in cows’ milk.
The research – a collaboration between the MRC-University of Glasgow Centre for Virus Research, the University of Edinburgh’s Roslin Institute, the Pirbright Institute and the UK Animal & Plant Health Agency – was in response to an outbreak of bird flu in American dairy cattle.
In the study, the researchers mixed a panel of influenza viruses with milk – both unpasteurised raw milk and commercially-available whole milk – in laboratory conditions, and heated them to pasteurising temperatures of 63°C or 72°C.
To gain a general understanding of the response of influenza viruses to heating they tested a panel of different viruses, including human and avian influenza A viruses, the group of viruses that H5N1 belongs to, as well as an influenza D virus – a different influenza virus that naturally infects cattle.
“Heating in a laboratory to mimic the times and temperatures reached by industrial pasteurisation effectively killed all of the influenza viruses in the study,” researchers concluded.
The scientists followed this up with experiments conducted in a high-containment laboratory using the bird flu virus and these also confirmed that pasteurisation could effectively kill this virus in milk – even though the remains of its genetic material could still be detected.
However the research also showed that if milk is not pasteurised it is an “effective carrier of infectious influenza viruses”, including bird flu.
Ed Hutchinson, senior lecturer at the MRC-University of Glasgow Centre for Virus Research, and the corresponding author of the study, said:” Given the worries surrounding the H5N1 outbreak in dairy cattle in the US, and the discovery of the virus in cows’ milk, we urgently needed to answer the question of whether pasteurisation made milk safe.
“We have now rigorously tested this, showing that the temperatures used in pasteurisation should rapidly inactivate all influenza viruses.
“However, we also found that influenza viruses remained infectious in raw – unpasteurised – milk. Raw milk is already known to carry a variety of pathogens, and we would particularly caution people against drinking it in areas of the USA where cattle might be infected with H5N1 influenza”.
The study, ‘Pasteurisation temperatures effectively inactivate influenza viruses in milk“’ is a pre-print published on medRxiv and has not yet been through peer review.
Bird flu
The U.S. Department of Agriculture, Food and Drug Administration, Centers for Disease Control and Prevention, and State veterinary and public health officials are continuing to investigate confirmed cases of bird flu in domestic livestock.
The first cases – which marked the first time a virus of this sort has spread in cattle – were detected in Kansas and Texas several months ago.
The CDC last week confirmed a third human case of bird flu associated with the ongoing multi-state outbreak of bird flu in U.S. dairy cows.
The centre said that, as with the previous two cases – one in Texas, one in Michigan – the person “is a dairy farm worker with exposure to infected cows, making this another instance of probable cow-to-person spread”.
According to the UK and Scottish scientists who published their research today the “current outbreak of H5N1 avian influenza is the worst on record and since 2020 it has spread rapidly to birds around the world”.
“There have been repeated spillover infections of mammals, but these were generally not in close proximity to humans and the virus did not spread effectively in those species.
“This changed in late 2023, when H5N1 influenza infected cattle in the USA and began to spread among dairy herds.
“So far this outbreak has remained confined to the USA, but it is concerning, as it means that this potentially dangerous virus is now circulating in mammals who have close contact with humans,” they warned.
Last week the U.S. Department of Agriculture launched a new $824 mllion in project to “Protect Livestock Health” and a voluntary “H5N1 Dairy Herd Status Pilot Program” – the main benefit is that once they confirm their herds are free from bird flu they can “ship their cows at the time they prefer and without testing individual animals”.