May 24, 2024

expert reaction to study looking at heat inactivation and infectivity cow’s milk containing avian influenza A(H5N1) virus in mice

A study published in The New England Journal of Medicine looks at cows milk containing avian influenza virus A (H5N1)

Dr Ruth Harvey, Deputy Director of the Worldwide Influenza Centre at the Francis Crick Institute, said:

“This is good quality research with conclusions backed up by solid data. The limitations of the experimental set-up for the heat inactivation part of the research are clearly stated: that the conditions used are not identical to large-scale industrial treatment of raw milk. The findings that viable virus was found in the milk samples after 5 weeks of storage at 4C, and that mice could be infected through drinking milk containing the virus, back up the conclusions that drinking raw milk could pose a risk. It’s worth noting that the Food Standards Agency in the UK advises against drinking raw milk already.”

Prof Rowland Kao, Professor of Veterinary Epidemiology and Data Science, University of Edinburgh, said:

“The discovery of cattle infected with Influenza A H5N1 was unexpected, as Influenza A has not previously been known to infect the species. The widespread dissemination of this particular cattle-infecting lineage of virus across much of the continental US (with infected cows identified in at least nine states, and evidence of virus in commercial milk across many others) and evidence of substantial live virus in some milk samples, has led to concerns over risks to human health. Two individuals thus far in the US have been identified with mild infection with this virus type, however it is likely there are more. Thus far, infections have only been confirmed in the US, albeit across not only cattle but other mammals and with ‘reverse’ spillover, back into wild and domesticated birds, raising the possibility of continued circulation of this lineage, including the possibility of it being brought to the UK by migrating birds.

“Thus this study is important in two ways – first, it shows that milk samples containing live virus can cause systemic infection in mice (and by extension that there is likely to be a similar risk to persons drinking infected milk).  As H5N1 is already known to have the capability to have a substantial health impact, including a high mortality rate for clinically infected persons, these findings help to confirm a potentially important risk pathway as no human infections due to infected milk have yet been identified.  Second, standard approaches to pasteurising milk are likely to inactivate any live virus, making that milk safe for consumption.

“An important consideration is that the consumption of raw, unpasteurised milk is becoming increasingly popular both in the US and the UK.  While this study shows that mice can become systemically infected due to ingesting infected milk, this does not prove that the same is true for humans, though it does increase the possibility in those drinking raw, unpasteurised milk.  Should human systemic infections occur, there would be an increased although still low risk of a human pandemic, as additional changes in the virus would have to occur first, for example through recombination with human seasonal flu.  As such, this evidence should be viewed as a warning for the need for increased preparedness, but is not evidence of an immediate threat. Preparedness includes research and also surveillance in birds, cattle and humans as appropriate and plans for containment in case human cases are detected here in the UK. Plus ensuring milk-safe practices are in place including restrictions on unpasteurised milk, which can still be sold commercially in England and Wales (but not in Scotland).”

Cow’s Milk Containing Avian Influenza A(H5N1) Virus — Heat Inactivation and Infectivity in Mice’ by Lizheng Guan et al. was published in The New England Journal of Medicine at 15:00 UK time on Friday 24 May. 

DOI: 10.1056/NEJMc2405495

Declared interests

Dr Ruth Harvey: None to declare. 

Prof Rowland Kao: I am currently part of a large funded consortium researching avian influenza, I chair Defra Science Advisory Council’s committee on Exotic and Emergent Infections, contribute to the UKHSA’s SPI-M modelling group, and to a UKHSA technical advisory groups on avian influenza.

For all other experts, no response to our request for DOI’s was received.