In raw milk samples containing large amounts of avian flu virus, small amounts of infectious virus were still detectable after treatment with a standard pasteurization method, researchers said Friday.
These results reflect laboratory experimental conditions and should not be used to draw conclusions about the safety of the U.S. milk supply, according to study authors from the National Institute of Allergy’s Rocky Mountain Laboratories and Infectious Diseases (NIAID) of the US government.
This study was published in the New England Journal of Medicine.
Compared to raw milk spiked with the free-floating virus used in the study, raw milk from cows infected with H5N1 flu may have a different composition or contain the virus inside the cells, which may impact the effects of heat, the researchers said.
American dairy cows were infected with bird flu in March. The Food and Drug Administration examined retail samples of pasteurized milk and estimated that one-fifth of the U.S. milk supply contained strands of the virus. The agency said pasteurized milk is safe to consume.
The virus used in the experiments had been isolated from the lungs of a dead mountain lion, mixed with samples of raw, unpasteurized cow’s milk, and heat-treated at 63 degrees C (145.4 degrees F) and 72 degrees C (161.6 degrees F) for different periods.
After processing at 72 degrees C for 20 seconds – five seconds longer than the industry standard for pasteurization at this temperature – very small amounts of infectious virus were detected in one of three samples, the study found.
“This finding indicates that a relatively small but detectable amount of H5N1 virus can remain infectious in milk after 15 seconds at 72 degrees C if initial virus levels are high enough, the authors note.
Within 2.5 minutes, treatment at 63 degrees C resulted in a marked decrease in levels of infectious H5N1 virus, indicating that standard industrial pasteurization for 30 minutes at this temperature would eliminate infectious virus , according to the researchers.
The researchers point out that their experimental conditions are not identical to large-scale industrial pasteurization processes of raw milk and that their results must be replicated by directly measuring infected milk in commercial pasteurization equipment.
It remains unclear whether ingesting the active H5N1 virus in milk can cause illness in humans, the researchers added. (Reporting by Nancy Lapid; Editing by Will Dunham)
