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Cattle may become a permanent host for bird flu

The recent finding that pasteurized milk in the United States is no longer suspected of harboring the H5N1 avian influenza virus has alleviated some public health concerns. However, the persistence of the bird flu virus in the U.S. cattle population is alarming experts who fear that cattle could become a lasting reservoir for the virus. 

This scenario provides the virus with more opportunities to mutate and potentially jump to humans. Research indicates that the virus can be transmitted between birds and cows, suggesting it could disseminate across extensive geographic regions. 

Detecting bird flu in cattle

Unlike other mammals that succumb to the virus, most cows carrying the virus do not show severe symptoms or die, making it difficult to detect infected animals without specific testing. 

Additionally, a single cow might carry multiple flu viruses, raising the possibility of these viruses exchanging genetic material and creating new strains more capable of infecting humans.

“Eventually the wrong combination of gene segments and mutations inevitably comes along. Whatever opportunity we may have had to nip it in the bud we lost by a really slow detection,” said Michael Worobey, an evolutionary biologist at the University of Arizona.

Escalating concerns about human transmission 

The virus is not new; different forms of H5N1 have been in circulation since the 1990s, with a particularly lethal variant identified in 1996. While this variant has decimated millions of birds and affected various mammals, cows were not previously recognized as hosts until recently. 

Following the discovery of H5N1 in U.S. cattle on March 25, with 36 herds in 9 states testing positive by May 7, concerns have escalated.

“Every time it gets a new mammalian host species, like cows, there’s more risk of human transmission and reduced human immunity,” said Jessica Leibler, an environmental epidemiologist at Boston University.

Effective mixing vessels 

Genomic analysis by the U.S. Department of Agriculture suggests that the virus transitioned from wild birds to cattle in late 2023. Thus, cows appear to be “effective mixing vessels” where the virus can exchange genetic material with other viruses, significantly enhancing the risk of developing a strain that can efficiently infect humans.

“If you have a virus that’s hopscotching back and forth between cows, humans and birds, that virus is going to have selective pressures to grow efficiently in all those species,” said Angela Rasmussen, a virologist at the University of Saskatchewan.

Will bird flu become endemic in cattle?

Gregory Gray, an infectious-disease epidemiologist at the University of Texas, pointed out the potential for H5N1 to become endemic in cows, a situation complicated by the impracticality of culling infected cattle. 

The constant interaction between humans and these vast numbers of animals makes cattle an extremely concerning reservoir for bird flu.

Airborne transmission 

The transmission mechanisms of the virus remain under investigation, with wild birds suspected as a primary vector. Concerns also exist about airborne transmission, which could explain the spread between dairy farms. 

Current regulatory measures include mandatory testing of cows before interstate transport, which helps researchers track the virus’s spread but may not halt it. However, given the potential for airborne transmission, vaccinating cattle against H5N1 might become necessary. 

What is the risk to humans? 

Current information on the transmission of the virus between humans is limited. A recent study published in the New England Journal of Medicine on May 3rd confirmed that a dairy worker in Texas was infected, showing only mild symptoms. However, tests have not been conducted on others who live and work with this individual.

Although there have not been many reported deaths or severe cases among humans, suggesting that the virus may not be highly transmissible or lethal, exposure among farm workers may be quite common. 

Bird flu may be undetected in other species

“When you see symptomatic patients, that’s the tip of the iceberg,” Liebler said. She fears that the virus could remain undetected in various species for an extended period, potentially mutating and setting the stage for a future pandemic. “We have an awareness now from the COVID pandemic of how devastating that could be,” she added.

The experts are advocating for public health initiatives to commence testing on workers and their families to ensure any human transmission of the virus is promptly identified. “H5N1 is with us. It’s not a virus that’s going to disappear by any means,” Liebler concluded.

The continued presence of H5N1 in cattle poses significant risks, necessitating urgent and comprehensive measures to understand and mitigate its impact. As Leibler remarked, “H5N1 is with us. It’s not a virus that’s going to disappear by any means.”


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