Untreated milk containing the avian influenza virus H5N1 can cause disease in mice, according to a study published on Friday. The scientists also reported that infectious virus remains detectable in raw milk for up to five weeks. Additionally, heat-treatment of milk significantly reduces the concentration of infectious virus, but does not eliminate it.
Since March, federal, state, and local officials have been monitoring the spread of avian flu among cattle in the United States. The virus thus far has been detected in 58 herds in nine states, according to data provided by U. S. Department of Agriculture. The first reported transmission of this virus from cattle to a human occurred in April. A second presumed case of cattle-to-human transmission was reported by the CDC on Wednesday. Both cases probably occurred via direct contact between the person and an infected animal. Although the virus has been detected in milk produced by infected dairy cows, the risk of transmission via milk has not been fully explored.
To investigate this risk, virologists studied contaminated milk samples obtained from a dairy cow herd in New Mexico. Sequence analysis of the viruses present in the milk confirmed that they were HPAI H5N1 viruses closely related to the virus previously associated with a human infection in Texas.
To determine if contaminated milk could transmit the virus, the researchers experimentally inoculated mice orally with 50 microliters, or about a drop, of milk. The mice were observed for four days, and then euthanized. The researchers noted that all mice survived for the four-day period, but began showing some signs of disease, like lethargy, a day after the inoculation.
The amount of virus in various organs was determined after the mice were euthanized. High levels of virus were detected in the lungs and trachea. Virus also was detected in other organs, including the liver, kidney, spleen, and mammary glands.
To investigate the stability of the virus in milk, the researchers took two approaches. First, they monitored the presence of virus over time in milk stored at 4oC, or about 39oF. After a five-week period, the amount of infectious virus in the samples declined, but only slightly. The virus, in other words, remains present when milk is refrigerated. Second, they explored the effects of heat-treatment on the virus in milk. Contaminated milk samples were incubated at 63oC (approximately 145oF) for various periods of time ranging from 5 to 30 minutes. Following these incubations, no infectious virus could be detected in the milk. Alternatively, milk samples were incubated at 72oC (approximately 161oF) for 5 to 20 seconds. Following these incubations, the amount of infectious virus was significantly reduced, but not eliminated.
So, what do these studies tell us? There are three main takeaways from this report. First, the H5N1 avian influenza virus can be transmitted to mammals orally via contaminated milk. Second, the virus remains stable in untreated milk for extended periods of time, if the milk is refrigerated. Third, heat-treatment of milk reduces, but does not eliminate, the infectious virus.
But there are some important caveats. The mouse transmission studies were conducted in a specific type of mice (six-week-old, female Balb/cJ mice). This degree of standardization is typical in scientific experiments. However, it does raise the question of how generalizable the results may be. From these experiments, we cannot confirm that humans who drink H5N1 contaminated milk will become sick. The heat-inactivation studies also are very informative, but perhaps not generalizable. As the authors of the report note, “benchtop experiments do not recapitulate commercial pasteurization processes.”
We know that the H5N1 avian influenza virus can be detected in milk. We now have evidence that contaminated milk can cause disease in mammals. Current evidence suggests that pasteurization effectively eliminates infectious virus from milk. But the potential impact of this virus in dairy cows and humans remains an important concern.
