With the recent spread of avian influenza to dairy cattle in the US, the government is taking action to avoid further spreading. Is consuming milk still safe?
The United States is currently experiencing an unprecedented outbreak of the highly pathogenic avian influenza (HPAI) in dairy cattle. Since the first reported case in March 2024, the HPAI virus of the H5N1 subtype has spread to 36 dairy farms in nine US states – with one confirmed case spreading to humans in a dairy farm worker, who has most likely been handling contaminated milk.
Previous research has revealed high viral levels of HPAI in milk rather than other bodily fluids, such as mucus of infected dairy cattle. Thus, the Centers for Disease Control (CDC) recommended staying away from raw milk and any raw or undercooked beef products. However, the consumption of pasteurized milk from supermarkets so far has been deemed unproblematic due to the general effectiveness of pasteurization for the inactivation of bacteria and viruses.
To ensure the safety of commercially sold milk and to test the effectiveness of pasteurization for inactivating the HPAI virus the Food and Drug Administration (FDA) in coordination with the US Department of Agriculture (USDA) tested commercially sold dairy products. Retail milk and other milk-derived products such as cottage cheese or sour cream were tested for traces of the HPAI virus using a quantitative PCR (qPCR). Of the initially tested products (96 retail milk samples) one in five samples tested qPCR positive for HPAI fragments. However, the presence of viral particles does not necessitate an infectious potential.
To rule out if viable or intact virus particles remain in the milk after the pasteurization process, 201 qPCR-positive samples were injected into chicken eggs. Preliminary results of these inoculation tests showed no infectious potential of the viral fragments. Thus, confirming the effectiveness of pasteurization for the HPAI virus. Additionally, milk powder marketed as infant formula is being monitored by the FDA. So far, all analyzed samples have been tested qPCR negative.
Whereas consumers might be in the clear for now, the threat of a large-scale outbreak has not been banned yet. With increased numbers of infected cows, viral levels in raw milk could rise – with unknown consequences. Whether pasteurization is still effective at elevated levels of HPAI has yet to be revealed. The FDA will continue to sporadically test pooled raw milk to ensure the safety of dairy products.
The handling and transport of dairy cattle so far follows a less strict regime. Interstate transport of dairy cattle is still ongoing, posing a potential threat of further outbreaks. As of April 26th, 2024 lactating dairy cattle must be tested for HPAI H5 no more than seven days before transport. However, this only applies to 30 animals per herd – thus a small risk of unknowingly transporting infected cattle remains.
Quarantining transported animals or testing of non-lactating cattle is still a recommendation only. Furthermore, no regime for the proper disposal and handling of discarded milk or ban on selling raw milk for human consumption has been put in place. Thus, veterinarians must continue to educate farm workers and owners of cattle about the risk of transport, introducing new animals into a herd, and the infectious potential of raw milk.
Image Source: Anita Jankovic, Unsplash
