Highly Pathogenic Avian Influenza (HPAI) is a disease that is highly contagious and often deadly in poultry, caused by highly pathogenic avian influenza A (H5) and A (H7) viruses; it is also known as bird or avian flu. HPAI viruses can be transmitted by wild birds to domestic poultry and other bird and animal species. Although bird flu viruses do not normally infect humans, sporadic human infections have occurred. It is important to note that “highly pathogenic” refers to severe impact in birds, not necessarily in humans.
Ongoing Work to Ensure Continued Effectiveness of the Federal-State Milk Safety System
The FDA, along with our federal partners at the U.S. Department of Agriculture, is announcing results from a first-of-its-kind study using the process typically used by commercial milk processors. The intention of this study was to further confirm that pasteurization is effective at inactivating Highly Pathogenic H5N1 Avian Influenza (H5N1 HPAI) virus in fluid milk and other dairy products made from pasteurized milk.
The study – the only one to date designed to simulate commercial milk processing – found that the most commonly used pasteurization time and temperature requirements were effective at inactivating the H5N1 HPAI virus in milk. These results complement the FDA’s initial retail sampling study in which all 297 samples of dairy products collected at retail locations were found to be negative for viable H5N1 HPAI virus.
Collectively, these studies provide strong assurances that the commercial milk supply is safe.
“This commercial milk processing simulation study is notably different than some recent benchtop studies published by other institutions which have shown mixed results regarding the sensitivity of the virus to thermal treatment,” said Stephen Walker, Ph.D., P.E., Consumer Safety Officer, in the FDA’s Center for Food Safety and Applied Nutrition. “Other recent studies relied on benchtop equipment that was likely not sufficient to accurately simulate High Temperature Short Time (HTST) processing conditions. In contrast, 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.”
For this study, researchers first needed to understand the level of virus in unpasteurized (raw) milk that is intended for commercial processing. To do this, researchers tested 275 raw milk samples obtained from multiple farms in four affected states. The sampling was intentionally focused on regions with infected herds and the results are not nationally representative. Individual farms were eligible to be sampled multiple times during the sampling period. One-hundred and fifty-eight of the raw milk samples were positive for viral fragments and, of those, 39 were found to have infectious virus with an average concentration of 3.5 log10 EID (egg infectious doses)50 per milliliter – which is about 3,000 virus particles per milliliter.
Next, researchers had to determine if the continuous-flow processing of the milk at 161°F (72°C) for 15 seconds was effective at eliminating the levels of virus found in raw milk. These treatment conditions, often referred to as “high-temperature-short-time” (HTST) or “flash pasteurization,” represent the pasteurization treatment (time/temperature combination) required by the Code of Federal Regulations (CFR) and the Pasteurized Milk Ordinance (PMO) that is most commonly utilized by the dairy industry.
In the study, scientists used homogenized raw whole milk that was artificially contaminated with a higher concentration of virus than was found in any raw milk samples – an average concentration of 6.7 log10 EID (egg infectious doses)50 per milliliter (or approximately 5 million virus particles per milliliter). High numbers of organisms are typically used when conducting inactivation studies to document high levels of inactivation. The levels are also comparable to those used in benchtop experiments.
When these spiked milk samples were processed in a HTST continuous flow pasteurization system (illustrated below), which was designed to closely simulate commercial processing conditions, in each of the total of nine repeated experiments, the virus was completely inactivated.
Milk samples that were collected mid-process indicate that the virus was inactivated very quickly, and the FDA’s process engineering experts have extrapolated that HTST pasteurization conditions are likely eliminating at least 12 log10 EID50 per milliliter (about 1 trillion virus particles per milliliter). These results establish that HTST pasteurization is effective at eliminating the virus from milk with a large margin of safety.
“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 and further provides broad assurance that pasteurized milk and dairy products made from pasteurized milk are safe,” said Nathan Anderson, Ph.D., Director, Division of Food Processing Science and Technology in the FDA’s Center for Food Safety and Applied Nutrition.
Scientists at both the FDA and USDA submitted a manuscript with the study details to the Journal of Food Protection for peer review prior to publication in the journal.
The results from this and previous studies are clear about the safety of the commercial milk supply. The FDA is continuing additional surveillance activities, retail sampling of additional dairy products and locations, studies to further characterize the relationship between different time and temperature combinations provided in the PMO, studies to characterize the thermal inactivation kinetics of this virus, and examination of levels of virus in raw, unpasteurized milk. The agency is committed to producing gold-standard science and data and will share additional results as soon as possible. The FDA will continue to work in closely partnership with USDA on many of these efforts.
Importantly, the FDA continues to emphasize its longstanding recommendations regarding the consumption of unpasteurized (raw) milk because it has the potential to be contaminated with pathogens that cause illness and it has been linked to numerous foodborne illness outbreaks in the past. Based on the limited research and information available, we do not know at this time if the HPAI H5N1 virus can be transmitted to humans through consumption of raw milk and products made from raw milk from infected cows. Pasteurization is a proven process with a 100-year history of protecting public health and is highly effective at eliminating the dangers associated with consuming raw milk.