Study confirms mammal-to-mammal H5N1 spread

USA – A recent study by Cornell University has revealed that avian influenza (H5N1) has spread from birds to dairy cattle across several U.S. states and has now demonstrated mammal-to-mammal transmission, affecting cows, cats, and a raccoon. 

This marks a significant development in understanding the virus’s transmission capabilities.

“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 of virology and director of the Virology Laboratory at the Animal Health Diagnostic Center (AHDC) in the College of Veterinary Medicine.

Diel is the co-corresponding author of the study “Spillover of Highly Pathogenic Avian Influenza H5N1 Virus to Dairy Cattle,” which was published in the journal Nature. 

The study’s whole genome sequencing of the virus showed no mutations that would enhance H5N1’s transmissibility in humans. Yet, it highlighted the virus’s adaptation in mammals, raising concerns about future mutations that could affect human transmission.

As of July 24, 11 human cases have been reported in the U.S. since April 2022, all with mild symptoms. 

Four cases were linked to cattle farms and seven to poultry farms, with a recent outbreak in Colorado. The same strain circulating in dairy cows was identified in these human cases, suggesting a common origin from dairy farms in the same county.

While the virus can infect and replicate in humans, the current efficiency of these infections is low. 

However, the potential for future mutations that could enhance mammalian adaptation and human transmission remains a significant concern.

Continuous monitoring of the virus in animals and humans is crucial. The U.S. Department of Agriculture (USDA) has funded H5N1 testing programs at no cost to producers. Early testing, enhanced biosecurity, and quarantines are essential measures to contain the virus’s spread, according to Diel.

The H5N1 infections were first detected in January 2022, leading to the deaths of over 100 million domestic birds and thousands of wild birds in the U.S. 

The Cornell AHDC and Texas A&M Veterinary Medical Diagnostic Laboratory were among the first to report the virus in dairy cattle herds, which wild birds likely infected. 

Symptoms in cattle included reduced appetite, changes in fecal matter consistency, respiratory distress, and abnormal milk production.

The study found a high tropism of the virus for the mammary gland, with significant viral loads shed in the milk of affected animals.

Whole genome sequencing, modeling, and epidemiological data confirmed cow-to-cow transmission when infected cows from Texas were moved to a farm in Ohio. 

The virus was also transmitted to cats, a raccoon, and wild birds found dead on affected farms. The cats and raccoons likely contracted the virus from drinking raw milk from infected cows. 

At the same time, the wild birds may have been infected through environmental contamination or aerosols during milking or cleaning processes.

The AHDC, the Ohio Animal Disease and Diagnostic Laboratory, the Texas A&M Veterinary Medical Diagnostic Laboratory, and the USDA funded the study.

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