A joint risk assessment from FAO/WHO/WOAH in April 2024 considers the public health risk and risk of virus spread among animals for the A(H5N1) viruses of highly pathogenic avian influenza (HPAI). The assessment coincides with FDA reports that “initial results from a nationally representative commercial milk sampling study” revealed that “about 1 in 5 of the retail samples tested” were qPCR-positive for HPAI viral fragments. This update caused global concern, despite the agency’s caution that “additional testing is required” to understand the risk to consumers.
The risk assessment
The joint risk assessment puts the current outbreak in a (recent) historical context; in 2020, highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b viruses arose from “previously circulating” influenza A(H5Nx) viruses. These have spread “predominantly” through migratory birds to parts of Africa, Asia, and Europe, causing “unprecedented” deaths in wild birds and outbreaks in domestic poultry.
By the end of 2022, the viruses were detected in North and South America. To add to growing concerns, there were “increased detections” of A(H5N1) viruses in non-avian species. Furthermore, 28 detections of A(H5N1) in humans have been reported to WHO.
“Avian influenza A(H5N1) viruses, especially those of clade 2.3.4.4b, continue to diversify genetically and spread geographically.”
The infection of a “broader range” of wild bird species has provoked “deleterious ecological consequences” and “mass die-offs in some species”. Some wild mammal species have suffered “significant mortality events”. The assessment highlights that there have been “limited reports” of transmission between mammals, despite an increase in mammalian infections. However, since March 2024, A(H5N1) detections in 33 dairy cattle herds in 8 states in the USA suggest that “lateral transmission among cattle” was “likely”.
The assessment refers to 28 detections of A(H5N1) in humans since the beginning of 2021, 13 of which have been associated with the clade 2.3.4.4b viruses. The geographical distribution of these is:
- China (2)
- Chile (1)
- Ecuador (1)
- Spain (2)
- The UK (5)
- The USA (2)
The cases in Europe and North America were “asymptomatic or mild”, but the cases in China were severe, with one resulting in death. In Chile and Ecuador both cases had severe symptoms and recovered. All human cases, excluding the case in Chile, had exposure to infected animals through participation in outbreak response activities or direct exposure in farm, backyard holdings, or live bird market environments. It is believed that the case in Chile was caused by “environmental exposure”.
The assessment suggests that WHO assesses the overall public health risk posed by A(H5N1) to be “low”, for those with exposure to infected birds or animals or contaminated environments to be “low-to-moderate”. However, this requires “close monitoring”.
US agencies: commercial milk supply is safe
Amid the US outbreak of HPAI across dairy farms in “multiple” states, the FDA and USDA have drawn on “currently available information” to conclude that the commercial milk supply is safe because of two reasons:
- The pasteurisation process
- The diversion or destruction of milk from sick cows
The emphasis on pasteurisation is clear:
“The pasteurisation process has served public health well for more than 100 years. Pasteurisation is a process that kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time to make milk safer…pasteurisation is generally expected to eliminate pathogens to a level that does not pose a risk to consumer health.”
The FDA states that “nearly all (99%)” of the commercial milk supply from dairy farms in the US is provided by farms on the Grade ‘A’ milk programme that follow the Pasteurised Milk Ordinance. This emphasis on pasteurisation addresses reports that the presence of the virus has been detected in raw milk. While pasteurisation is “likely to inactivate the virus”, it is not expected to “remove the presence of viral particles”.
Experts’ opinions
STAT News’ Helen Branswell spoke to several pathogen experts about the “danger associated with raw milk”. One such expert is Professor Thijs Kuiken from Erasmus Medical Centre in the Netherlands, who has been engaged in H5N1 research for “about two decades”.
“If I were in charge, for the moment I would forbid the selling of raw milk.”
Equally averse to raw milk consumption is US-based St Jude Children’s Research Hospital’s Dr Richard Webby:
“I absolutely wouldn’t go anywhere near raw milk in terms of consuming it.”
Dr Jürgen Richt, Director of the Centre of Excellence for Emerging and Zoonotic Animal Diseases at Kansas State University’s College of Veterinary Medicine, echoes this concern.
“I wouldn’t want to drink raw milk. And I wouldn’t feed it to my cats, nor my dogs, nor my calves, if I’m on a farm.”
Professor Florian Krammer of the Icahn School of Medicine at Mount Sinai, New York, considered the possibility of starting infections from the mid gut less likely than other options, as stomach acids could inactivate the virus. However, he too is cautious about the risks.
“Maybe it does something very unexpected. There’s just not enough data to say anything conclusive. So, I think people should just be watchful.”
As the H5N1 situation continues to evolve and infectious disease management remains a primary global concern, we will share insights and updates in our newsletters, so do make sure that you have subscribed here.
