WASHINGTON (Reuters) - Influenza viruses coat themselves in fatty material that hardens and protects them in colder temperatures -- a finding that could explain why winter is the flu season, U.S. researchers reported on Sunday.

Pedestrians make their way through the snow in New York, February 22, 2008. REUTERS/Keith Bedford

This butter-like coating melts in the respiratory tract, allowing the virus to infect cells, the team at the National Institutes of Health found.

“Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract,” said Joshua Zimmerberg of the National Institute of Child Health and Human Development (NICHD), who led the study.

The NICHD is one of the National Institutes of Health.

“It’s only in this liquid phase that the virus is capable of entering a cell to infect it.”

Experts have long pondered why flu and other respiratory viruses spread more in winter. No one explanation, such as people staying indoors more, or the destructive effect of the sun’s radiation in summer, has fully explained it.

The new report, published in the journal Nature Chemical Biology, could lead to new ways to prevent and treat flu, said NICHD Director Duane Alexander.

“The study results open new avenues of research for thwarting winter flu outbreaks,” Alexander said in a statement.

“Now that we understand how the flu virus protects itself so that it can spread from person to person, we can work on ways to interfere with that protective mechanism.”

Zimmerman’s team used a type of imaging called nuclear magnetic resonance imaging to look at the outer coat of flu viruses.

GOING OUT INTO THE WORLD

Viruses cannot replicate on their own but instead must hijack a living cell. Influenza viruses have a membrane-like outer coating that they fuse to the victim cell.

They inject genetic material into the cell, turning it into a virus factory. Some types of viruses simply explode out of these hijacked cells, but influenza instead “buds” out, and uses lipids such as cholesterol from the cells to make a membrane to help it do so.

“This is the protein we make vaccines against,” Zimmerman said in a telephone interview. The outside envelope protein, called hemagglutinin, gives influenza viruses the “H” in their names.

Inside a nice, warm cell, the hemagglutinin is liquid. But at cooler temperatures it starts a process that resembles crystallization, called ordering.

“It solidifies gradually all the way down from 40 degrees Celsius (104 degrees F) down to 4 degrees C (39 degrees F),” Zimmerman said.

“I believe that this gradualness lets it exist at every temperature.”

In warmer outdoor temperatures this protective coating melts, and unless it is inside a living person or animal, the virus perishes.

The finding could also help scientists find new ways to eradicate influenza. In cold temperatures, the hard lipid shell might withstand certain detergents, making it more difficult to wash the virus off of hands and surfaces.

Influenza and other respiratory viruses are spread in small droplets broadcast by coughing, sneezing and talking and which can also settle onto surfaces, to be picked up on fingertips.