|NIH Scientists Offer Explanation for Winter
Stability of Virus' Membrane at Cold Temperatures May Ease Winter
A finding by a team of scientists at the National Institutes of
Health may account for why the flu virus is more infectious in
cold winter temperatures than during the warmer months.
At winter temperatures, the virus's outer covering, or envelope,
hardens to a rubbery gel that could shield the virus as it passes
from person to person, the researchers have found. At warmer temperatures,
however, the protective gel melts to a liquid phase. But this liquid
phase apparently isn't tough enough to protect the virus against
the elements, and so the virus loses its ability to spread from
person to person.
The findings were published online March 2 in Nature Chemical
Biology. The study was a collaboration between researchers
at two NIH institutes, the National Institute of Child Health
and Human Development, and the National Institute on Alcohol
Abuse and Alcoholism.
"The study results open new avenues of research for thwarting
winter flu outbreaks," said NICHD Director Duane Alexander. "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."
Influenza viruses are usually spread from person to person through
coughs and sneezes. Infection with flu virus can cause mild to
severe illness, and at times can lead to death.
In October of 2007, researchers working with guinea pigs showed
that animals sick with the flu were more likely to get other guinea
pigs sick at colder temperatures than at warmer temperatures.
In the current study, the NIH researchers used a sophisticated
magnetic resonance technique, developed and previously tested in
NIAAA's Laboratory of Membrane Biochemistry and Biophysics, to
create a detailed fingerprint of how the virus's outer membrane
responded to variations in temperature. The virus's outer membrane
is composed chiefly of molecules known as lipids, explained the
study's senior author, Joshua Zimmerberg, Ph.D., chief of NICHD's
Laboratory of Cellular And Molecular Biophysics. This
family of molecules does not mix with water, and includes oils,
fats, waxes, and cholesterol.
Dr. Zimmerberg and his colleagues found that at temperatures slightly
above freezing, the virus's lipid covering solidified into a gel.
As temperatures approach 60 degrees Fahrenheit, the covering gradually
thaws, eventually melting to a soupy mix.
Cooler temperatures, apparently, cause the virus to form the rubbery
outer covering that can withstand travel from person to person,
Dr. Zimmerberg said. Once in the respiratory tract, the warm temperature
in the body causes the covering to melt to its liquid form, so
that the virus can infect the cells of its new host, he added.
"Like an M&M in your mouth, the protective covering melts
when it enters the respiratory tract," Dr. Zimmerberg said. "It's
only in this liquid phase that the virus is capable of entering
a cell to infect it."
In spring and summer, however, the temperatures are too high to
allow the viral membrane to enter its gel state. Dr. Zimmerberg
said that at these temperatures, the individual flu viruses would
dry out and weaken, and this would help to account for the ending
of flu season.
The finding opens up new possibilities for research, Dr. Zimmerberg
said. Strategies to disrupt the virus and prevent it from spreading
could involve seeking ways to disrupt the virus's lipid membrane.
In cold temperatures, the hard lipid shell can be resistant to
certain detergents, so one strategy could involve testing for more
effective detergents and hand-washing protocols to hinder the spread
of the virus.
Similarly, Dr. Zimmerberg added that flu researchers might wish
to study whether, in areas affected by a severe form of the flu,
people might better protect themselves against getting sick by
remaining indoors at warmer temperatures than usual.
Other authors of the paper were I.V. Polozov and L. Bezrukov,
both of the Laboratory of Cellular And Molecular Biophysics at
NICHD and K. Gawrisch of the Laboratory of Membrane Biochemistry
and Biophysics, National Institute of Alcohol Abuse and Alcoholism.
Magnetic resonance experiments were conducted and analyzed at NIAAA
under Dr. Gawrisch's guidance.
The NICHD sponsors research on development, before and after birth;
maternal, child, and family health; reproductive biology and population
issues; and medical rehabilitation. For more information, visit
the Institute's Web site at http://www.nichd.nih.gov/.
The National Institute on Alcohol Abuse and Alcoholism, part of
the National Institutes of Health, is the primary U.S. agency for
conducting and supporting research on the causes, consequences,
prevention, and treatment of alcohol abuse, alcoholism, and alcohol
problems and disseminates research findings to general, professional,
and academic audiences. Additional alcohol research information
and publications are available at www.niaaa.nih.gov.
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.
The text of this release was changed on Monday, March 3, 2008.