Experiments by researchers found that highly pathogenic avian influenza virus (HPAI) of H5N1 subtype can be inactivated when heated to pasteurization temperatures, although a small amount of infectious virus remains.
H5N1 Outbreak in Dairy Farms
In late March 2024, US health officials reported an outbreak of H5N1 among dairy cows in Texas. There are currently 95 cattle herds across 12 states that have been affected, with three human infections discovered in farm workers with conjunctivitis.
So far, the virus has not shown any genetic evidence of acquiring the ability to spread from one person to another. Still, public health officials are closely monitoring the situation of dairy cows as part of pandemic preparedness efforts.
Can Pasteurization Kill Bird Flu Virus?
In a recent study, a team of experts conducted laboratory research at the National Institute of Allergy and Infectious Diseases (NIAID) using limited data on the effect of pasteurization methods used by the dairy industry against avian influenza viruses. The details of their study are described in the paper "Inactivation of Avian Influenza A(H5N1) Virus in Raw Milk at 63°C and 72°C."
Led by Franziska Kaiser, the scientists isolated H5N1 virus from the lungs of a dead mountain lion in Montana. The viral isolates were mixed with raw, unpasteurized cow milk samples and then heated for different periods of time. Finally, the samples were cultured and tested to find out if the virus remained alive and if so, how much.
They tried to quantify the stability of H5N1 virus in raw milk when tested at various time intervals at 145.4 and 161.6 degrees Fahrenheit (63 and 72 degrees Celsius). This is one of the methods used by the dairy industry for standard pasteurization.
The result of the study revealed that 145.4 degrees Fahrenheit caused a significant decrease in the levels of H5N1 virus within 2.5 minutes. The study authors also noted that standard bulk pasteurization of 30 minutes would eliminate the infectious virus.
At 161.6 degrees Fahrenheit, the research team noted a decrease in infectious virus within five seconds. However, very small amounts of infectious virus were found in one out of three samples after 20 seconds of heat treatment.
The researchers stress that the measurements they made reflect experimental conditions, and must be replicated with direct measurements of infected milk in commercial pasteurization equipment. It should not also be used in drawing any conclusions regarding the safety of milk supply in the US.
Gastrointestinal infections with HPAI H5N1 virus have been detected in several species of mammals. It is also unknown whether consuming live H5N1 in raw milk can cause illness in people.
The researchers believe that their findings indicate the potential for a relatively small but detectable quantity of H5N1 virus. According to the authors, their findings demonstrate experimental conditions in a laboratory setting and are not similar to large-scale industrial pasteurization processes for raw milk.
To date, the US Food and Drug Administration (PDA) reports that the evidence indicates that the commercial milk supply is safe.
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