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.

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.

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.

For the respiratory transmission test, three healthy ferrets were placed in enclosures next to enclosures containing the experimentally infected animals. The infected and uninfected ferrets shared air, but did not have direct contact with each other. Of the three healthy ferrets, only one contracted the H5N1 virus (33 percent). Additionally, that one respiratory transmission event seemed to have a one- to two-day delay compared with what's seen in the same test with seasonal influenza viruses. This suggests further that the virus is inefficient at respiratory transmission.

The CDC called the overall results "not surprising." Previous ferret experiments with H5N1 isolates—collected prior to the current bird flu outbreak among US dairy cows—have also found that H5N1 is often lethal to ferrets. Likewise, H5N1 isolates collected from Spain and Chile during the current global outbreak also found that the virus was inefficient at spreading via respiratory droplets among ferrets—with rates ranging from 0 percent to 37.5 percent.