Note: This program first aired on January 19, 2013.
If you have been following the news lately, you are probably well aware that it is flu season, and influenza has been affecting Americans in officially epidemic proportions. To be clear, the seasonal flu refers the illness caused by the influenza virus, and includes fever and chills, fatigue and body ache, and severe respiratory symptoms. Viruses are tiny pieces of biological material, that most scientists define as not actually being alive (in that they do not exhibit the accepted characteristics of life like growing or multiplying on their own). They exist as a bit of genetic material, encased in a protein capsule. To reproduce and become biologically active, they have to get their genetic material into a living host cell, which they then hijack. Influenza is an RNA virus, meaning that its genetic material is in the form of RNA or ribonucleic acid, instead of the more familiar DNA. Much is made in the media (and at the Center for Disease Control and Prevention) about the strain of the flu (H1N1, H2N3 etc). These numbers simply refer to the different glycoprotiens that are found on the capsule or “envelope” that the viral genetic material is in.
“Flu season” is refered to regularly in the media but no explanation is given as to why there is a strong seasonality to it. I wondered, as you may have, why winter is flu season. Is the virus around all the time, and is it something about us or our behavior that makes us more susceptible to it in winter, or is it something about the virus and its life span or cycles of mutation that make it come and go with such regularity? Is there a flu season in the tropics?
It turns out there has been some research on this very topic, epidemeologists are keen to figure out why the flu strikes in winter, so they can come up with better ways of combating it. The answer seems to have to do with aspects of the virus itself as well as the physical characteristics of life in winter. First researchers were looking for an animal they could conduct transmissibility trials on, but all the normal test subjects weren’t susceptible to influenza. Then a researcher discovered (after reading an account of the 1918 influenza pandemic) that guinea pigs also get and transmit the flu and a study was born. They found that guinea pigs are much more likely to transmit the flu to each other when the temperature is cold. When the temperature rises into the 80’s, guinea pig to guinea pig transmission rates dropped to zero. Likewise, when humidity was at 20% the virus was easily transmitted, and when the humidity reached 80%, transmission stopped. A relationship between cold dry air and flu transmission was clearly established. The flu’s favorite way to spread itself around is in “respiratory droplets” that people sneeze and cough out. Researchers speculate that in the cold dry air, these droplets stay smaller and travel further than they would in warm air. Warm air has more water vapor in it, and the droplets will quickly absorb the water vapor and increase in size, and become too large to stay suspended in the air.
A second study of the influenza virus itself provided more insight into the virus’s relationship with nature. It turns out that in colder temperatures, the capsule that the virus is contained in develops a thick fatty persistent coating that protects the virus and apparently enables it to exist in the environment for longer periods of time. At warmer temperatures, this tough coat melts, which is good for the virus if it is in a host body, but bad if it is on a sandy tropical beach or any other external environment. The naked virus can not persist in the environment. This seems to confirm why it is so hard to spread the flu in the summer. Its not that it doesn’t exist then, but it is unlikely to cause an epidemic because it is so much harder to infect masses of people due to the virus’s lack of persistence in the warm environment.
Is it really this simple? No, probably not. For example while there isn’t a specific flu season in the tropics, these latitudes are not totally immune to outbreaks of the flu. The findings we‘ve discussed can’t really address that, which tells me there is more to the story. But that is ok, we’ve got some of the pieces of the puzzle, important pieces about the characteristics of a pathogen that is smaller than our cells by several orders of magnitude. Scientists will keep working, and the flu will keep mutating and changing, keeping this area of science alive and vibrant for years to come. Stay well everyone.
References:
Everything you wanted to know about flu statistics, from the Center for Disease Control http://www.cdc.gov/flu/weekly/
Lots of good material in Influenza here, at the Rapid Reference website: http://www.rapidreferenceinfluenza.com/chapter/B978-0-7234-3433-7.50009-8/aim/introduction
The University of California Museum of Paleontology to the rescue again: http://www.ucmp.berkeley.edu/alllife/virus.html
An article about the study from the National Institute of Child Health and Human Development that might explain it all: http://www.reuters.com/article/2008/03/02/us-flu-winter-idUSN0228175320080302
A New York Times article about a guinea pig (literally!) study that looks at specific aspects of winter air and their effect on “spreadability” of flu virus: http://www.nytimes.com/2007/12/05/health/research/05flu.html?_r=0
