The strain of H5N1 bird flu isolated from a dairy worker in Texas was 100 percent fatal in ferrets used to model influenza illnesses in humans. However, the virus appeared inefficient at spreading via respiratory droplets, according to newly released study results from the Centers for Disease Control and Prevention.
The data confirms that H5N1 infections are significantly different from seasonal influenza viruses that circulate in humans. Those annual viruses make ferrets sick but are not deadly. They have also shown to be highly efficient at spreading via respiratory droplets, with 100 percent transmission rates in laboratory settings. In contrast, the strain from the Texas man (A/Texas/37/2024) appeared to have only a 33 percent transmission rate via respiratory droplets among ferrets.
"This suggests that A/Texas/37/2024-like viruses would need to undergo changes to spread efficiently by droplets through the air, such as from coughs and sneezes," the CDC said in its data summary. The agency went on to note that "efficient respiratory droplet spread, like what is seen with seasonal influenza viruses, is needed for sustained person-to-person spread to happen."
In the CDC's study, researchers infected six ferrets with A/Texas/37/2024. The CDC's data summary did not specify how the ferrets were infected in this study, but in other recent ferret H5N1 studies, the animals were infected by putting the virus in their noses. Ars has reached out to the agency for clarity on the inoculation route in the latest study and will update the story with any additional information provided.
All six of the infected ferrets developed severe disease and died. To test how well the virus could spread among the ferrets, the CDC scientists set up experiments to test transmission through direct contact and respiratory droplets. For the direct transmission test, three healthy ferrets were placed in the same enclosures with three experimentally infected ferrets. All three healthy ferrets became infected.