Raw milk can carry bird flu – stick with pasteurized
NIH/National Institute of Allergy and
Infectious Diseases
The amount of infectious H5N1 influenza viruses in raw milk rapidly declined with heat treatment in laboratory research conducted by scientists at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.
However, small, detectable amounts of
infectious virus remained in raw milk samples with high virus levels when
treated at 72 degrees Celsius (161.6 degrees Fahrenheit) for 15 seconds -- one
of the standard pasteurization methods used by the dairy industry.
The authors of the study stress, however,
that their findings reflect experimental conditions in a laboratory setting and
are not identical to large-scale industrial pasteurization processes for raw
milk. The findings were published today in the New England Journal of
Medicine
In late March 2024, United States officials
reported an outbreak of highly pathogenic avian influenza virus called HPAI
H5N1 among dairy cows in Texas.
To date, 95 cattle herds across 12 states
have been affected, with three human infections detected in farm workers with
conjunctivitis.
Although the virus so far has shown no
genetic evidence of acquiring the ability to spread from person to person,
public health officials are closely monitoring the dairy cow situation as part
of overarching pandemic preparedness efforts.
Given the limited data on the susceptibility of avian influenza viruses to pasteurization methods used by the dairy industry, scientists at NIAID's Rocky Mountain Laboratories sought to quantify the stability of H5N1 virus in raw milk when tested at different time intervals at 63℃ (145.4 degrees Fahrenheit) and 72℃, the temperatures most common in commercial dairy pasteurization processes.
The scientists isolated HPAI H5N1 from the
lungs of a dead mountain lion in Montana.
Then they mixed these viral isolates with
raw, unpasteurized cow milk samples and heat-treated the milk at 63℃ and 72℃
for different periods of time.
The samples were then cell-cultured and
tested to determine if live virus remained and if so, how much.
They found that 63℃ caused a marked
decrease (1010-fold) in infectious H5N1 virus levels within 2.5
minutes and note that standard bulk pasteurization of 30 minutes would
eliminate infectious virus.
At 72℃, they observed a decrease (104-fold)
in infectious virus within five seconds, however, very small amounts of
infectious virus were detected after up to 20 seconds of heat treatment in one
out of three samples.
"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℃ if the initial virus levels were
sufficiently high," the authors note.
The scientists stress that their
measurements reflect experimental conditions, should be replicated with direct
measurement of infected milk in commercial pasteurization equipment and should
not be used to draw any conclusions about the safety of the U.S. milk supply.
Additionally, a limitation of their study
was the use of raw milk samples spiked with H5N1 virus, whereas raw milk from
cows infected with H5N1 influenza may have a different composition or contain
cell-associated virus that may impact heat effects.
The authors conclude that although
gastrointestinal infections with HPAI H5N1 virus have occurred in several
species of mammals, it remains unknown whether ingesting live H5N1 in raw milk
could cause illness in people.
To date, the U.S. Food and Drug
Administration concludes that the totality of evidence continues to indicate
that the commercial milk supply is safe.
While laboratory benchtop studies provide
important, useful information, there are limitations that challenge inferences
to real-world commercial processing and pasteurization. The FDA conducted an
initial survey of 297 retail dairy products collected at retail locations in 17
states and represented products produced at 132 processing locations in 38
states.
All of the samples were found to be
negative for viable virus. These results underscore the opportunity to conduct
additional studies that closely replicate real-world conditions. FDA, in
partnership with USDA, is conducting pasteurization validation studies --
including the use of a homogenizer and continuous flow pasteurizer. Additional
results will be made available as soon as they are available.
Story Source:
Materials provided
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Institute of Allergy and Infectious Diseases. Note:
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Journal Reference:
Franziska Kaiser, Dylan H. Morris, Arthur
Wickenhagen, Reshma Mukesh, Shane Gallogly, Kwe Claude Yinda, Emmie de Wit,
James O. Lloyd-Smith, Vincent J. Munster. Inactivation of Avian
Influenza A(H5N1) Virus in Raw Milk at 63°C and 72°C. New England
Journal of Medicine, 2024; DOI: 10.1056/NEJMc2405488