New study finds cows can transmit avian flu to other mammals

Photo: chrisroosfotografie/Adobe Stock

A recent study published by Nature has shown evidence of multidirectional mammalian interspecies transmission of the highly pathogenic avian influenza (H5N1). Specifically, the study revealed cattle-to-cat and cattle-to-raccoon transmission.¹

Investigators conducted a clinic epidemiological investigation across 9 farms in Texas, New Mexico, Kansas, and Ohio that reported morbidity events between February 11, 2024, and March 19, 2024. A diagnostic investigation was carried out on samples collected from the 9 farms, with investigators utilizing whole genome sequencing of characterized viral strains as well as modeling and epidemiological data. They concluded that cattle from the Ohio farm had been infected by cows in Texas after the Texas cattle were transported to the Ohio farm.

The study also revealed H5N1 was transmitted to other mammals including cats and a raccoon. Mortality events were also reported in great-tailed grackles and pigeons. Oropharyngeal swabs from great-tailed grackles and rock pigeons, along with lung and brain tissues from a cat found dead on one of the farms in Texas, tested positive for H5N1.¹

Moreover, a comparable epidemiological situation involving the deaths of both domestic and wild mammals was noted in 4 farms located in Ohio, New Mexico, and Texas. On the Ohio farm, 6 domestic cats died following the disease outbreak in the dairy cows. Additionally, cats found deceased on 2 farms, as well as a raccoon found dead on one of the farms in New Mexico, also tested positive for the virus.¹

“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,” Diego Diel, associate professor of virology in the Department of Population Medicine and Diagnostic Sciences and director of the virology laboratory at the Animal Health Diagnostic Center (AHDC) in the College of Veterinary Medicine at Cornell University in Ithaca, New York, and co-corresponding author of the study, said in a news release.²

The cats and raccoon likely acquired the virus after consuming raw milk from infected cows, according to the study.^1,2 The investigators theorized that the wild birds became infected through environmental contamination or aerosols generated during milking or cleaning activities in the milking parlors, according to a release.² However, the exact cause of this infection is unclear.^1,2

Additional viral sequence analysis gathered from 1 farm in Texas and 1 farm in Kansas—approximately 280 km apart from each other—indicate a connection and potential 2-way virus dispersal between these 2 farms. Further phylogenetic analysis including other H5N1 B3.13 sequences identified 2 additional H5N1 sequences from blackbirds that clustered with viral sequences from the 2 farms. Additionally, the blackbirds were collected 8 km to 12 km away from the farm in Texas. “Together these results suggest both 268 long- and close-range lateral spread and transmission of HPAIV between farms,” wrote the authors of the study.¹

Although whole genome sequencing of the virus did not show any mutations that would increase the transmissibility of H5N1 in humans, Diel addressed the underlying implications and concerns of the study’s recent findings. “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,” he said in a news release.²

The study, funded by the Animal Health Diagnostic Center at Cornell University in New York, the Ohio Animal Disease and Diagnostic Laboratory, the Texas A&M Veterinary Medical Diagnostic Laboratory, and the United States Department of Agriculture,² demonstrated a high tropism of the virus for the mammary gland, with high infectious viral loads present in the milk of affected animals. Still, milk supply remains safe, as pasteurization destroys the virus.

H5N1 infections have led to the deaths of more than 100 million domestic birds and thousands of wild birds in the country. Moreover, 11 human cases of avian influenza have been reported in the US to date, with the first case dating back to April 2022. Four cases were associated with cattle farms, while 7 were connected to poultry farms, including a recent outbreak in Colorado. The recent patients were infected with the same strain identified in the study as being present in dairy cows. This prompts researchers to believe the virus likely came from dairy farms in the same county where the patients were located.^1,2

Recently, the US Centers for Disease Control and Prevention announced a $5 million initiative to provide seasonal flu vaccines to livestock workers. The announcement came amid concern that a new variant of H5N1 might develop in farm workers, who are at increased risk of contracting the virus, which could put the public health in jeopardy.³

State-level public health officials will distribute seasonal flu vaccines to workers at local events and in areas where they frequently congregate, but it will not be mandatory, according to a news release.³

“Preventing seasonal influenza in these workers, many of whom are also potentially exposed to H5N1 viruses, may also reduce the risk of new versions of the influenza A virus emerging,” said Nirav Shah, principal deputy director of the CDC, in a news release.³

References

1. Caserta LC, Frye EA, Butt SL, et al. Spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle. Nature. Published online July 25, 2024. doi:10.1038/s41586-024-07849-4
2. Ramanujan K. Study confirms mamal-to-mammal avian flu spread. Cornell Chronicle. July 24, 2024. Accessed August 1, 2024. https://news.cornell.edu/stories/2024/07/study-confirms-mammal-mammal-avian-flu-spread
3. Christensen J. To prevent supercharged flu virus, CDC pushes seasonal flu vaccines to farmworkers at risk of coinfection with bird flu. CNN. July 30, 2024. Accessed August 1, 2024. https://www.cnn.com/2024/07/30/health/cdc-worry-new-virus-emerge-bird-flu-seasonal-flu-vaccines/index.html