Cows can become infected with the highly pathogenic avian influenza H5N1 by inhaling virus-laden aerosols, according to a preprint source 1 on bioRxiv. However, scientists say this mode of transmission is unlikely to be driving the current outbreak among cattle in the United States.

H5N1 was first identified in cattle in Texas in March 2024. Since then, the virus has been detected in 157 herds and 4 people in contact with cattle in 13 US states. Studies have suggested that the main route of transmission in cows is through infected milk that contaminates milking equipment.

Before the outbreak, researchers didn't know that influenza A viruses like H5N1 could spread in cows. Scientists have raised concerns that if H5N1 spread effectively in cows through the respiratory system, it would be more difficult to control and the risk of transmission to humans would increase, given cows' close contact with humans.

To better understand infections in cows, several teams conducted challenge studies in which they experimentally infected animals with H5N1. The first results of one of these studies were published on July 13 and have not yet been peer-reviewed.

Virus-laden fog

Amy Baker and her colleagues at the U.S. Department of Agriculture (USDA) in Ames, Iowa, infected cows and calves with the specific H5N1 strain isolated from cattle in Texas early in the outbreak. They exposed four year-old female calves to virus-laden mist using a mask that covered the animals' noses and mouths. All infected animals produced neutralizing antibodies against the virus, confirming that they were infected.

The infected calves showed mild symptoms, and researchers isolated infectious virus in the upper respiratory tract of two of the four calves. The study results suggest that the virus could be spread through the respiratory route in an environment where hundreds of animals are kept in close contact.

But since the animals did not shed high levels of virus in their respiratory tract, it is unlikely to be a major source of spread, says Thomas Peacock, a virologist at Imperial College London. However, it is possible that cows may be infected indirectly through the respiratory route through aerosolized virus secreted from the udder, he adds.

Wendy Barclay, a virologist at Imperial College London, agrees that because of the low amount of infectious virus in the animals despite their high H5N1 exposure, airborne transmission is unlikely to be efficient and does not "explain what is currently happening."

Instead, the study adds to previous work that suggests the virus is spread primarily through infected milk, researchers say. “Udder-to-udder transmission appears to be the most likely primary route at this time,” says Peacock. “But that doesn’t mean the virus can’t change if this outbreak continues at the current pace,” he says. “What we need to do now is monitor the virus carefully.”

The study primarily examined cows' susceptibility to infection, not transmission, making it difficult to draw conclusions about viral spread, says Jürgen Richt, a veterinary virologist at Kansas State University in Manhattan. Richt has conducted his own cattle experiments with colleagues in Germany, and the results are expected to appear in a preprint within weeks.

Curdled milk

Baker and her colleagues also infected lactating cows with the virus through teats in the mammary glands of two three-year-old cows. Two days after infection, the cows showed signs of inflammation of the mammary glands, which lasted for two weeks. They started producing less milk and the milk also became yellow and thick.

The researchers detected infectious virus in the mammary glands during the three weeks of the study and in the milk for up to two weeks. These findings “suggest that the mammary gland and milk are the primary sources of virus spread within and between dairy herds,” the preprint source says.

Richt says the study reflects what has been observed on farms. However, given the small number of animals included in the studies, respiratory transmission cannot be ruled out, he says.