In laboratory research, scientists at the National Institute of Allergy and Infectious Diseases (NIAID), a National Institutes of Health division, found that heat treatment rapidly reduced the proportion of infectious H5N1 influenza viruses in raw milk. On the other hand, when treated at 72 degrees Celsius (161.6 degrees Fahrenheit) for 15 seconds one of the common pasteurisation procedures employed by the dairy industry small, measurable quantities of infectious virus persisted in raw milk samples with high viral levels. The authors emphasise that their results are based on laboratory experiments and do not indicate large-scale industrial pasteurisation procedures for raw milk.
The findings were published in the New England Journal of Medicine.
US officials announced in late March 2024 that dairy cows in Texas were experiencing an epidemic of the highly pathogenic avian influenza virus or HPAI H5N1. There have been three cases of human illnesses among agricultural workers who have conjunctivitis, and 95 cattle herds in 12 states have been impacted thus far. Public health experts are actively watching the dairy cow issue as part of broader pandemic preparedness measures, even though the virus has not yet demonstrated genetic indications of acquiring the capacity to transfer from person to person.
In light of the scant information available regarding the vulnerability of avian influenza viruses to the pasteurisation techniques employed by the dairy sector, researchers at NIAID’s Rocky Mountain Laboratories set out to measure the H5N1 virus’s stability in raw milk when subjected to varying time interval tests at 63°C (145.4°F) and 72°C, the temperatures most frequently used in commercial dairy pasteurisation procedures. In Montana, a deceased mountain lion’s lungs had HPAI H5N1, which the scientists managed to identify. The milk was then heat-treated at 63°C and 72°C for varying lengths of time after the virus isolates were combined with raw, unpasteurized cow milk samples. After that, the samples were grown in cells and examined to see how much live virus was still present.
They discovered that, after 2.5 minutes, 63°C significantly reduced the amount of infectious H5N1 virus (1010-fold), and they observed that, after 30 minutes of bulk pasteurisation, the infectious virus would be eradicated. They saw a 104-fold reduction in infectious virus at 72°C in five seconds; but, in one out of three samples, extremely minute levels of infectious virus were still present for up to 20 seconds of heat treatment. The scientists write, “If the initial virus levels were sufficiently high, 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°C.”
The researchers emphasise that any conclusions on the safety of the milk supply in the United States should not be made based solely on their data, which represent experimental settings and should be repeated with direct measurement of contaminated milk in commercial pasteurisation equipment. Their investigation was further limited by the use of raw milk samples that had been tainted with the H5N1 virus; in contrast, raw milk from cows that had contracted the H5N1 influenza may have a different composition or carry cell-associated viruses that could affect the effects of heat. The authors draw the conclusion that whereas HPAI H5N1 virus-related gastrointestinal diseases have been reported in a number of animal species, it is still unclear if consuming live H5N1 in raw milk may make people sick.
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The U.S. Food and Drug Administration (USFDA) has concluded that there is still sufficient evidence to support the safety of the commercial milk supply. The constraints of laboratory benchtop research make it difficult to draw conclusions about commercial processing and pasteurisation in the real world, even if they offer valuable and significant information. In its initial study, the FDA looked at 297 retail dairy products that were produced at 132 processing facilities spread across 38 states and were gathered at retail outlets in 17 states. It was discovered that there was no live virus in any of the samples. These findings highlight the need for more research that closely mimics real-world circumstances. Studies on pasteurisation validation, including the use of homogenizers and continuous-flow pasteurizers, are being carried out by the FDA and USDA in collaboration.
Source: National Institute of Allergy and Infectious Disease News
Journal Reference: Kaiser, Franziska et al. “Inactivation of Avian Influenza A(H5N1) Virus in Raw Milk at 63°C and 72°C.” The New England journal of medicine, 10.1056/NEJMc2405488. 14 Jun. 2024, DOI :10.1056/NEJMc2405488
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