Influenza viruses can spread through the air on dust, fibers and other microscopic particles, according to new research from the University of California-Davis (UC-Davis) and the Icahn School of Medicine at Mt. Sinai in New York.

The findings, which have implications for the transmission several viruses besides influenza, were published Aug. 18 in Nature Communications.

"It's really shocking to most virologists and epidemiologists that airborne dust, rather than expiratory droplets, can carry influenza virus capable of infecting animals," said UC-Davis department of chemical engineering professor William Ristenpart, who helped lead the research. "The implicit assumption is always that airborne transmission occurs because of respiratory droplets emitted by coughing, sneezing or talking. Transmission via dust opens up whole new areas of investigation and has profound implications for how we interpret laboratory experiments as well as epidemiological investigations of outbreaks."

Influenza virus is thought to spread by several different routes, including in droplets exhaled from the respiratory tract or on secondary objects called fomites, UC-Davis said, noting that little is known about which routes are the most important.

The answer may be different for different strains of influenza virus or for other respiratory viruses, including coronaviruses such as SARS-CoV-2, UC-Davis said.

In the new study, which was geared toward human health but has implications for animal health, Ristenpart and UC-Davis engineering graduate student Sima Asadi teamed up with virologists led by Dr. Nicole Bouvier at Mt. Sinai to look at whether tiny, non-respiratory particles they call "aerosolized fomites" could carry influenza virus between guinea pigs.

Using an automated particle sizer to count airborne particles, they found that uninfected guinea pigs gave off spikes of up to 1,000 particles per second as they moved around a cage. Particles given off by the animals' breathing were at a constant and much lower rate, the announcement said.

Immune guinea pigs with influenza virus painted on their fur could transmit the virus through the air to other susceptible guinea pigs, showing that the virus did not have to come directly from the respiratory tract to be infectious, UC-Davis said.

Additional co-authors on the paper are Anthony Wexler at UC-Davis and Nassima Gaaloul ben Hnia and Ramya S. Barre at the Icahn School of Medicine at Mt. Sinai.

The work was supported by a grant from the National Institute of Allergy & Infectious Diseases of the National Institutes of Health.