A new study has revealed a spillover of highly pathogenic avian influenza (H5N1) virus from birds to cattle, which has led to mammal-to-mammal transmission between cows and cats and a raccoon.
“This is one of the first times that we are seeing evidence of efficient and sustained mammalian-to-mammalian transmission of highly pathogenic avian influenza H5N1,” said Diego Diel, associate professor at Cornell’s Department of Population Medicine and Diagnostic Sciences.
The study, “Spillover of Highly Pathogenic Avian Influenza H5N1 Virus to Dairy Cattle,” published July 25 in Nature, shows the virus has a high capability to infect the mammary gland and is shed in high amounts in the milk of affected animals.
Researchers used whole genome sequencing, modeling and epidemiological information to confirm cow-to-cow transmission when infected cows from Texas were moved to an Ohio farm. Sequencing also revealed that the virus spread to cats, a raccoon and wild birds found dead on affected farms. The cats and raccoon likely became ill from drinking raw milk from infected cows.
Fortunately, pasteurization kills the virus, ensuring a safe milk supply, the study said.
According to Diel, whole genome sequencing of the virus did not reveal any mutations that would enhance H5N1 transmissibility in humans. However, the data does show mammal-to-mammal transmission, which is concerning because the virus may adapt in mammals.
According to the Centers for Disease Control and Prevention (CDC), 13 cases have been reported in humans in the U.S. since March 24, and over 230 people have been tested after exposure to infected animals.
Diel said recent patients fell ill with the same strain circulating in dairy cows, leading researchers to suspect that the virus likely originated from dairy farms in the same county. While the virus can infect and replicate in people, the efficiency of those infections is low.
“The concern is that potential mutations could arise that could lead adaptation to mammals, spillover into humans and potential efficient transmission in humans in the future,” Diel said.
According to Diel, it is crucial to continue monitoring the virus in affected animals and any potentially infected humans. Diel emphasized that early testing, enhanced biosecurity and quarantines in case of positive results are necessary to contain any further virus spread.
A separate study conducted by the University of Texas Medical Branch (UTMB) at Galveston and not yet peer-reviewed, showed two workers from a dairy operation in Texas had the antibodies to influenza H5N1 in early April and were not included in the CDC total.
“I am very confident there are more people being infected than we know about,” senior author Gregory Gray, a UTMB infectious disease researcher, told NPR.
USDA study
The USDA is also conducting a study showing that infecting dairy cows with the H5N1 virus in a laboratory setting can trigger clinical signs of disease similar to those of naturally infected animals on dairy farms, highlighting the importance of understanding the virus’s infection and progression in cows to develop effective therapeutics such as vaccines.
The research was conducted at USDA’s Agricultural Research Service (ARS) National Animal Disease Center in Ames, IA. The team evaluated two infection routes: respiratory and intramammary.
In the respiratory study, four yearling heifers were exposed to an aerosol mist containing the virus, resulting in minimal clinical signs, such as sporadic nasal discharge and lesions in one heifer. In the intramammary study, the teats and udders of two lactating cows were inoculated, showing symptoms within 48 hours, including decreased appetite, reduced milk production and thick, yellowish milk.
These findings and other unpublished data suggest that respiratory transmission is not a primary infection route within farms, and additional studies are ongoing.
“At the time we initiated these experiments, the route of infection and transmission between cows was unknown,” said ARS study leader Amy Baker. “Transmission between farms was linked to movement of live lactating cows, yet within-farm spread to resident cows was observed within days or weeks following movement without a clear pattern of transmission consistent on all farms.”
Infections from H5N1 were first detected in January 2022, and according to the CDC, over 100 million poultry were affected. The virus associated with the dairy cow outbreak was first detected in a Texas dairy cow on March 25 and has affected 177 dairy herds in 13 states. — Charles Wallace, WLJ contributing editor
