H5N1 Influenza: An Avian Influenza Virus Transmissible Through the Air Among Mammals

An article in the journal *Nature* describes the mode of transmission of influenza A (H5N1) and how the virus spreads through the air. The researchers used devices to collect viral particles from the air to continuously sample influenza viruses shed by ferrets that had been experimentally infected. They found that the viral load in the air corresponded to the dynamics of virus transmission through contact between animals.  An increase in cases of highly pathogenic H5N1 avian influenza virus transmission to mammals suggests the existence of a viral mutation that facilitates infection among these animals. The scientists sought to understand how the H5N1 influenza virus spreads in the form of airborne viral particles, particularly among ferrets, which are often used as a model for human influenza infections. The amount of infectious virus shed into the air was directly linked to the virus’s ability to spread from one ferret to another. The more virus there is in the air, the more efficient the transmission. The virus’s genetic makeup also influences the efficiency of airborne transmission. This study has important implications for the surveillance and control of avian influenza outbreaks. Understanding the mechanisms of airborne transmission can help develop strategies to limit the spread of the virus, such as stricter biosecurity measures on poultry farms. These findings also raise concerns about the evolution of influenza A (H5N1 virus) toward adaptation to mammals, and possibly to humans. The study also helps explain the absence of airborne transmission in previous studies, which was likely due to low levels of infectious virus shedding rather than the absence of adaptive mutations necessary to trigger infection in a new host. The infectious zoonotic A (H5N1) virus from 2005 and the bovine A (H5N1) virus from 2024 were not detected in the air. In contrast, shedding was observed in one out of four ferrets infected with a 2022 European polecat A (H5N1) virus and a 2024 A (H5N1) virus isolated from a dairy farm worker. In other words, the newly emerging viruses exhibit a higher level of infectious viral shedding into the air compared to older viruses. Given the ongoing epizootic outbreak in cattle and the high risk of exposure for farmers, dairy producers, and domestic or wild mammals to infected cows and contaminated milk, it is essential for effective epidemic control and public health safety to understand how this virus spreads among cattle, its potential to adapt to mammals, and its capacity for airborne transmission.