According to researchers, the bird flu virus is transmitted to mice through milk

Untreated milk containing the H5N1 bird flu virus can cause illness in mice, according to a study published Friday. The scientists also reported that infectious viruses remain detectable in raw milk for up to five weeks. In addition, heat treatment of the milk significantly reduces the concentration of infectious viruses, but does not eliminate them.

Since March, federal, state and local authorities have been monitoring the spread of bird flu among cattle in the United States. According to the U.S. Department of Agriculture, the virus has so far been detected in 58 herds in nine states. The first reported transmission of this virus from cattle to humans occurred in April. A second suspected case of cattle-to-human transmission was reported by the CDC on Wednesday. Both cases likely occurred through direct contact between humans and an infected animal. Although the virus has been detected in the milk of infected dairy cows, the risk of transmission through milk is not yet fully understood.

To investigate this risk, virologists examined contaminated milk samples from a dairy 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 linked to human infection in Texas.

To determine whether contaminated milk could transmit the virus, the researchers experimentally vaccinated mice orally with 50 microliters, or about one drop of milk. They observed the mice for four days and then killed them. The researchers found that all of the mice survived the four-day period, but showed signs of the disease, such as lethargy, one day after vaccination.

After the mice were euthanized, the researchers measured the amount of virus in various organs. They found high levels of virus in the lungs and trachea. They also discovered viruses in other organs, including the liver, kidneys, spleen and mammary glands.

To study the stability of the virus in milk, the researchers took two approaches. First, they monitored the presence of the virus over a period of time in milk stored at 4 degrees Celsius, or about 39 degrees Fahrenheit. After five weeks, the amount of infectious virus in the samples decreased, but only slightly. In other words, the virus remains present when the milk is refrigerated.

Second, they examined the effects of heat treatment on the virus in the milk. Contaminated milk samples were incubated at 63 degrees Celsius (about 145 degrees Fahrenheit) for various periods of time ranging from 5 to 30 minutes. After these incubations, no infectious viruses could be detected in the milk. Alternatively, milk samples were incubated at 72 degrees Celsius (about 161 degrees Fahrenheit) for 5 to 20 seconds. After these incubations, the amount of infectious virus was significantly reduced, but not eliminated.

So what do these studies tell us? They reveal three main findings. First, the H5N1 avian influenza virus can be transmitted orally to mammals via contaminated milk. Second, the virus remains stable in untreated milk for long periods of time when the milk is refrigerated. Third, heat treatment of milk reduces the infectious virus but does not eliminate it.

There are some important caveats, however. The mouse transmission studies were conducted on a specific type of mouse (six-week-old female Balb/cJ mice). This level of standardization is typical in scientific experiments, but it raises questions about how generalizable the results can be. From these experiments, we cannot confirm that people who drink milk contaminated with H5N1 will become ill. The heat inactivation studies are also very informative, but may not be generalizable. As the report authors note, “laboratory experiments do not replicate 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 viruses from milk. However, the potential impact of this virus on dairy cows and humans remains a major concern.