In a recent study by the National Institute of Allergy and Infectious Diseases (NIAID), researchers have found that heat treatment significantly reduces the amount of infectious H5N1 influenza viruses in raw milk. However, small, detectable amounts of the virus remained in samples with high initial virus levels when treated at 72 degrees Celsius (161.6 degrees Fahrenheit) for 15 seconds, a standard pasteurisation method used by the dairy industry. These findings were published in the New England Journal of Medicine.
This research comes amid a backdrop of heightened concern following a recent outbreak of highly pathogenic avian influenza (HPAI) H5N1 among dairy cows in Texas, reported by U.S. officials in late March 2024. The outbreak has since affected 95 cattle herds across 12 states, with three human infections detected in farm workers presenting with conjunctivitis. To date, there is no evidence of human-to-human transmission of the virus, but public health officials are closely monitoring the situation as part of pandemic preparedness efforts.
Given the limited data on the susceptibility of avian influenza viruses to pasteurisation, scientists at NIAID’s Rocky Mountain Laboratories aimed to quantify the stability of the H5N1 virus in raw milk under different heat treatments. The virus was isolated from the lungs of a deceased mountain lion in Montana and mixed with raw cow milk samples, which were then heat-treated at 63℃ (145.4°F) and 72℃. The treated samples were subsequently tested for the presence of live virus.
The study found that heating the milk to 63℃ caused a significant decrease (1010-fold) in infectious H5N1 virus levels within 2.5 minutes, with standard bulk pasteurisation of 30 minutes expected to eliminate the virus. At 72℃, a notable decrease (104-fold) in virus levels was observed within five seconds. However, in one out of three samples, very small amounts of infectious virus were detected after up to 20 seconds of heat treatment.
“This finding indicates the potential for a relatively small but detectable quantity of H5N1 virus to remain infectious in milk after 15 seconds at 72℃ if the initial virus levels were sufficiently high,” the authors noted. They emphasised that these measurements reflect laboratory conditions and should be replicated in commercial pasteurisation settings before drawing conclusions about the safety of the U.S. milk supply.
A significant limitation of the study is the use of raw milk spiked with H5N1 virus, which may not entirely represent the composition of milk from infected cows. Additionally, the potential for gastrointestinal infections in humans from ingesting live H5N1 in raw milk remains unknown.
The U.S. Food and Drug Administration (FDA) reassures the public that the commercial milk supply remains safe. An initial FDA survey of 297 retail dairy products from 17 states found no viable virus. The FDA, in partnership with the USDA, is conducting further studies to validate pasteurisation processes under real-world conditions. Results from these studies will be released as they become available.
As researchers and public health officials continue to monitor the H5N1 outbreak and its implications, this study underscores the importance of ongoing vigilance and research in ensuring the safety of the food supply.
