Flu Virus Fortified In Colder Weather

A new finding may explain why the flu virus is more infectious in cold winter months than during warmer seasons.

Flu is caused by a group of viruses known as influenza viruses. They usually spread from person to person through coughs and sneezes. Flu symptoms can include fever, chills, body aches, runny nose, sore throat, headache and extreme exhaustion. Healthy people aren't usually seriously threatened by seasonal flu, but it can be severe for the very young, the elderly and those with a weakened immune system.

In October 2007, researchers found that guinea pigs sick with the flu were more likely to get other guinea pigs sick at colder rather than warmer temperatures. Meanwhile, a team of researchers from NIH's National Institute of Child Health and Human Development (NICHD) and National Institute on Alcohol Abuse and Alcoholism (NIAAA) were performing experiments that would lend insight into this phenomenon.

The outer membrane of the influenza virus is made chiefly of molecules known as lipids. Lipids—which include oils, fats, waxes and cholesterol—don't mix with water. The NIH researchers used a sophisticated technique called magic angle spinning nuclear magnetic resonance, which was developed and previously tested in NIAAA's laboratories, to investigate how the virus's outer membrane responds to variations in temperature. Their findings were published online on March 2, 2008, in Nature Chemical Biology.

The researchers discovered that at temperatures slightly above freezing and below, the virus's lipid covering solidified into a gel. At about 70 degrees Fahrenheit, much of the lipid was still in gel form. At warmer temperatures, however, the gel melts to a liquid phase. At temperatures of about 105 degrees and higher, the coat was all in liquid form.

The virus's rubbery outer coat, the researchers believe, allows it to withstand cooler temperatures and travel from person to person. In the respiratory tract, the body's warmth causes the covering to melt so that the virus can infect the cells of its new host.

“Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract,” explained Dr. Joshua Zimmerberg, chief of NICHD's Laboratory of Cellular and Molecular Biophysics and the study's senior author. “It's only in this liquid phase that the virus is capable of entering a cell to infect it.”

The liquid phase, presumably, isn't tough enough to protect the virus against the elements, and so the virus loses its ability to spread from person to person in warm air. As the weather warms in spring, the flu viruses dry out and weaken, and the flu season wanes.

“The study results open new avenues of research for thwarting winter flu outbreaks,” NICHD Director Duane Alexander said. “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.”