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  1. Alerts, Advisories & Safety Information

Updates on Highly Pathogenic Avian Influenza (HPAI)

Updates on Highly Pathogenic Avian Influenza (HPAI)

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

What's New | Previous Updates 

Background | U.S. Agency Response | Testing Results | Additional Resources


What's New

June 28, 2024

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.

Inactivation of highly pathogenic avian influenza virus with high temperature short time continuous flow pasteurization and virus detection in bulk milk tanks

Previous Updates


The U.S. Department of Agriculture (USDA), the U.S. Food and Drug Administration (FDA), and the Centers for Disease Control and Prevention (CDC), along with state partners, continue to investigate an outbreak of highly pathogenic avian influenza (HPAI) virus impacting dairy cows in multiple states. Infection with the virus is causing decreased lactation, low appetite, and other symptoms in affected cattle.

The FDA and USDA have indicated that based on the information currently available, our commercial milk supply is safe because of these two reasons:

1) the pasteurization process and

2) the diversion or destruction of milk from sick cows.

The pasteurization process has served public health well for more than 100 years. Pasteurization is a process that kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time to make milk safer. Even if virus is detected in raw milk, pasteurization is generally expected to eliminate pathogens to a level that does not pose a risk to consumer health. However, pasteurization is different than complete sterilization; sterilization extends shelf life but is not required to ensure milk safety. While milk is pasteurized, not sterilized, this process has helped ensure the health of the American public for more than 100 years by inactivating infectious agents.

Nearly all (99%) of the commercial milk supply that is produced on dairy farms in the U.S. comes from farms that participate in the Grade “A” milk program and follow the Pasteurized Milk Ordinance (PMO), which includes controls that help ensure the safety of dairy products. Pasteurization and diversion or destruction of milk from sick cows are two important measures that are part of the federal-state milk safety system.

In our May 10 update, we announced that all 297 samples from the FDA’s initial survey of retail dairy products were found to be negative for viable Highly Pathogenic H5N1 Avian Influenza (H5N1 HPAI) virus. Today, for continued transparency, the FDA is providing additional information on our retail sample survey.

The samples taken as part of this survey were collected at retail locations in 17 states by milk specialists in the FDA’s Office of Regulatory Affairs.  USDA Agricultural Research Service’s U.S. National Poultry Research Center (ARS) analyzed these samples using stepwise, scientific methods. This testing included first conducting quantitative real time polymerase chain reaction (qRT-PCR) screening to determine if any of the retail samples contained H5N1 viral nucleic acid. The samples that were found to contain viral nucleic acid during qRT-PCR screening were followed with gold-standard egg inoculation testing conducted by ARS to determine if they contained live virus. None of the samples were positive for live virus. ARS scientists are currently obtaining peer review of their analysis as a first step to publishing these results. The prepublication is available at https://www.medrxiv.org/content/10.1101/2024.05.21.24307706v1.

While the FDA collected the 297 samples at retail locations in 17 states, these retail samples represent products produced at 132 processing locations in 38 states. The information in the first chart below shows the state in which the product was processed. Because the intent of our study was to assess a variety of products, samples were selected to be representative of processors in states that have been reported to have impacted dairy cattle and those that have not. Of note, the location of where milk was processed does not indicate where the milk was produced. This is because milk could be produced from cows on a farm or farms a few states away, processed (pasteurized) in a different state, and then be available for purchase in yet another state.

The charts below provide additional details on the samples taken as part of our survey of retail dairy products.

As noted previously, qRT-PCR-positive results do not necessarily represent live virus that may be a risk to consumers. Therefore, viability testing by egg inoculation was performed on the qPCR samples that were positive for viral nucleic acid. All of these samples did not detect any viable virus. If samples tested by qRT-PCR were negative, no further testing was performed since those samples did not contain HPAI viral nucleic acid. These findings further support our assessment that the milk safety system including pasteurization is effective against this virus and that the commercial milk supply remains safe.

Retail samples were collected between April 18-22 and represent a snapshot in time. This testing did not detect any live, infectious virus.

Table 1: Breakdown of Retail Sample Results by State Where Milk Was Processed

State Where Milk Was Processed (May Not Relate to Where Milk Was Produced) Detection of Live Virus in Retail Product(s) Number of Retail Product Samples Tested Retail Product Samples Negative for Viral RNA
(qRT-PCR Screening -)
Retail Product Samples Positive for Viral RNA
(qRT-PCR Screening +)
Retail Product Sample Results for Live Virus (Viability Testing by Egg Inoculation)
AR No 5 0 5 0
AZ No 5 4 1 0
CA No 21 21 0 Not Performed (Negative qRT-PCR)
CO No 8 5 3 0
CT No 2 2 0 Not Performed (Negative qRT-PCR)
FL No 10 9 1 0
GA No 8 8 0 Not Performed (Negative qRT-PCR)
IA No 11 11 0 Not Performed (Negative qRT-PCR)
ID No 4 4 0 Not performed (Negative qRT-PCR)
IL No 5 5 0 Not Performed (Negative qRT-PCR)
IN No 9 8 1 0
KS No 7 1 6 0
KY No 4 1 3 0
MA No 4 4 0 Not Performed (Negative qRT-PCR)
ME No 2 2 0 Not Performed (Negative qRT-PCR)
MI No 13 9 4 0
MN No 16 13 3 0
MO No 10 7 3 0
NC No 5 4 1 0
ND No 2 2 0 Not Performed (Negative qRT-PCR)
NE No 3 3 0 Not Performed (Negative qRT-PCR)
NH No 1 1 0 Not Performed (Negative qRT-PCR)
NJ No 3 3 0 Not Performed (Negative qRT-PCR)
NV No 4 4 0 Not Performed (Negative qRT-PCR)
NY No 38 38 0 Not Performed (Negative qRT-PCR)
OH No 8 5 3 0
OK No 12 2 10 0
OR No 10 10 0 Not Performed (Negative qRT-PCR)
PA No 2 2 0 Not Performed (Negative qRT-PCR)
SC No 3 0 3 0
TN No 3 3 0 Not Performed (Negative qRT-PCR)
TX No 26 13 13 0
UT No 5 5 0 Not Performed (Negative qRT-PCR)
VA No 6 6 0 Not Performed (Negative qRT-PCR)
VT No 2 2 0 Not Performed (Negative qRT-PCR)
WA No 8 8 0 Not Performed (Negative qRT-PCR)
WI No 11 11 0 Not Performed (Negative qRT-PCR)
WV No 1 1 0 Not Performed (Negative qRT-PCR)

Table 2: Breakdown of Retail Sample Results by Product Type

Number of Retail Product Samples Detection of Live Virus in Retail Product Retail Product Samples Negative for Viral RNA
(qRT-PCR Screening -)
Retail Product Samples Positive for Viral RNA
(qRT-PCR Screening +)
Percent of Retail Product Samples Positive for Viral RNA
(via qRT-PCR screening)
Retail Product Sample Results for Live Virus (Confirmatory Virus Culture)
Skim Milk 36 No 32 4 11.1% 0/4
1% Milk 28 No 19 9 32.1% 0/9
2% Milk 58 No 42 16 27.6% 0/16
Whole Milk 68 No 52 16 23.5% 0/16
Cottage Cheese 21 No 20 1 4.8% 0/1
Cream 17 No 14 3 17.6% 0/3
Half and Half 25 No 19 6 24.0% 0/6
Sour Cream and Similar 30   No   25 5 16.7% 0/5
Yogurt 14 No 14 0 0 NA
Total 297 None 237 60 20.2% 0/60

This retail sampling study was designed to assess the effectiveness of the PMO milk safety system; it was not designed to assess the prevalence of H5N1 in dairy herds. It is important to underscore that milk purchased for the retail study in a particular state does not mean that it was produced or processed in that state. Commercial milk is typically pooled from many dairy farms, pasteurized in bulk and distributed to a variety of states. Even if a sample was collected in one particular state, the milk in a consumer package could have come from cows on several farms located in several states, pasteurized in a different state from the states where the milk was produced, and available for purchase in yet another state.

To further validate pasteurization effectiveness against the recently identified H5N1 virus, we are undertaking a pasteurization study designed to better replicate real-world conditions. Preliminary results from this work are expected in the near future.

Data Considerations

Multiple tests are used to assess the safety of food items. Understanding how and why different methodologies are used and work, as well as how results fit into the larger picture, is critical to interpret any findings.

  • Quantitative polymerase chain reaction (qRT-PCR) is a screening tool used to determine the presence or absence of an organism’s genetic material in a sample. A positive qRT-PCR means that the genetic material from the targeted pathogen was detected in the sample, but that does not mean that the sample contains an intact, infectious pathogen. That’s because qRT-PCR tests will also detect the residual genetic material from pathogens killed by heat, like pasteurization, or other food safety treatments. Importantly, additional testing is required to determine whether intact pathogen is still present and if it remains infectious, which determines whether there is any risk of illness associated with consuming the product.
  • Embryonated Egg Viability Studies are considered the “gold standard” for sensitive detection of active, infectious virus. These studies are one of the types of additional tests necessary following PCR testing. These studies are done by injecting an embryonated chicken egg with a sample and then evaluating to see whether any active virus replicates. While this provides the most sensitive results, it takes a longer time to complete than other methods.
  • Madin-Darby Canine Kidney (MDCK) Cell Culture is different type of additional test used following PCR testing to detect live, infectious virus. This is done by injecting a sample into specific tissue cells to determine whether any live virus is present and replicates. This method can usually be done more quickly than embryonated egg viability studies, but it is not as sensitive and may provide false negative results when the amount of virus in the sample is very low.

Additional Resources




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