A strain of the H5N1 avian influenza virus was discovered in a Texas dairy worker who contracted it this spring. This strain has been shown to spread through the air among ferrets, albeit not very effectively, and was lethal to 100% of the infected animals during studies conducted by researchers earlier this year.
A strain of H5N1 avian influenza virus discovered in a Texas dairy worker this spring demonstrated the ability to transmit through the air among ferrets, though not efficiently, and resulted in a 100% fatality rate in infected animals during studies by researchers at the University of Wisconsin-Madison conducted earlier this year.
Fortunately, the dairy worker had mild symptoms and made a full recovery, and it seems this H5N1 strain did not continue to spread in the wild.
Nonetheless, these findings underscore the potential dangers posed by a virus that continues to infect dairy cattle and occasionally farm workers. The lead scientist of the study expressed surprise at how easily this strain was able to kill ferrets.
“This is one of the most harmful viruses I’ve encountered in ferrets,” stated Yoshihiro Kawaoka, a professor of pathobiological sciences at UW-Madison, in a study discussed on October 28, 2024, in the journal Nature.
Ferrets are frequently used in research to understand how avian influenza viruses adapt to mammals, a subject that Kawaoka and his team at the Influenza Research Institute at UW-Madison examine due to its potential to trigger an influenza pandemic.
Similar to other types of influenza, H5N1 viruses mutate quickly as they infect new hosts. Occasionally, these mutations enable the viruses to infect and spread among different species more effectively. This is how the current strains, which have been affecting birds globally in recent years, started spreading amongst mammals, especially in North American dairy cattle in 2024.
Kawaoka and his team discovered that the H5N1 virus infecting the Texas dairy worker contained a mutation they initially identified in 2001, which is known for causing severe illness. Fortunately, Kawaoka mentioned that this mutated strain seems to have disappeared.
“This isolate is distinct from other H5N1 viruses currently found in cows,” he explains.
Kawaoka speculates that the H5N1 viruses adopted two different pathways when transitioning from birds to cows, both of which were aided by mutations that enhanced the virus’s ability to infect mammals.
He and his associates believe that one of these pathways led to the more concerning mutation found in the Texas dairy worker, while the other resulted in a less threatening mutation in the same protein.
“Both mutations allow the virus to adjust to mammals, but the positive aspect is that the strain with the more dangerous mutation has not been seen again,” Kawaoka notes. “Thus far, there are no highly pathogenic H5N1 viruses known to be circulating among cows. However, if a currently circulating H5N1 virus among cows acquires that mutation, it could pose a serious issue.”
It’s still uncertain if a virus with such a mutation would be harmful to humans.
“It’s puzzling why the individual who contracted this virus did not suffer from a severe infection,” Kawaoka observes, pointing out several potential explanations.
For instance, prior exposure to seasonal influenza viruses might provide some immunity through antibodies, or the method of infection may play a role; the main symptom for the Texas dairy worker was conjunctivitis, indicating the virus entered through the eye instead of the usual respiratory pathway.
Another explanation could be the enhanced monitoring of influenza cases among American dairy workers since the virus started spreading in farming environments, allowing for the identification of more cases — even mild ones. Additionally, this particular strain may simply be less severe in humans compared to other mammals such as ferrets.
“All of these are possibilities, but we still don’t know,” Kawaoka acknowledges. “We are currently investigating why this virus is so pathogenic in ferrets and what that might imply for human infections.”
