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More Evidence Pasteurization Inactivates H5N1 Avian Flu Virus in Milk

Researchers with the US Department of Agriculture (USDA) and US Food and Drug Administration (FDA), in a new preprint study, show that the H5N1 avian flu virus is inactivated through traditional pasteurization used in commercial milk processing. 

In describing the findings, the FDA said the study compliments an initial study the agency did this spring testing 297 samples of commercial dairy products—all found to be negative for viable (live) highly pathogenic avian influenza (HPAI) H5N1 virus. 

Virus highly sensitive to heat treatment

"The results of the study announced today strongly indicate that the virus is much more sensitive to heat treatment with commercial pasteurization equipment than other studies might suggest," said Stephen Walker, PhD, a consumer safety officer at the FDA's Center for Food Safety and Applied Nutrition (CFSAN), in an FDA press release.

The experiments used 275 raw milk samples obtained from multiple farms in four states currently experiencing H5N1 outbreaks among dairy cattle. Of the 275 samples, 158 were positive for viral fragments. Of those, 39 had infectious virus at an average of 3,000 virus particles per milliliter.

The samples were then subjected to "high-temperature-short-time" (HTST) or "flash pasteurization," the most commonly used US pasteurization technique. It consists of heating the milk to 161°F (72°C) for 15 seconds in a continuous flow. The researchers also contaminated some raw milk samples with a higher concentration of virus, 5 million virus particles per milliliter. 

HTST eliminated the virus in all samples tested. 

"While testing finished product post-pasteurization is one strategy to detect potential problems in finished products, validating the effectiveness of the pasteurization parameters critically demonstrates that commercial milk processing is capable of controlling the HPAI virus," said Nathan Anderson, PhD, also from CFSAN.  

Pasteurized milk contains no live virus 

In related news, another new preprint study from researchers at Ohio State University, St Jude Children's Research Hospital, and the University of Illinois shows H5N1 avian flu RNA, but not live virus, in pasteurized retail milk.

The study found evidence of H5N1 viral nucleic acid in 36.3% of samples (61 of 168) from pasteurized milk bought from retail stores across the United States.

"None of the retail milk samples included in this study contained viable, infectious A(H5N1) virus per in vitro and in vivo assays, which provides evidence that pasteurization is sufficient for inactivation of pathogens in the commercial milk supply," the authors wrote.

Neither preprint study has been peer-reviewed.

Currently the USDA's Animal and Plant Health Inspection Service has detected H5N1 in 137 dairy herds from 12 states. Over the weekend the USDA confirmed new outbreaks in Colorado (3) and Iowa (1). 

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