Influenza summers far from home

Influenza, long thought to lie dormant in its epidemic territory during its summer off-season, in fact migrates far and wide during that time, according to a study published last week in PLOS Pathogens. The results suggest that strains use their annual migration to swap genetic material with other strains from around the world.

"The findings here show what most of us have long believed - the interchange between the influenza populations in the northern and southern hemispheres is important to viral evolution," James Cherry of the University of California, Los Angeles, who did not participate in the work, told The Scientist.

Influenza evolves seasonally, flummoxing the human immune system and leaving people vulnerable to reinfection year after year. Previous research on strains from New York revealed that the virus does not generate mutations in genes recognized by the immune system during winter epidemic periods. This led researchers to question whether the virus undergoes such antigenic drift during the non-epidemic season instead, and if so, where this evolution takes place.

Researchers have debated what happens to a viral strain between epidemics. A long-standing theory proposes that viral strains enter a latent state during the summer off-season, to be activated again in the autumn and winter by climate conditions such as the level of solar radiation. According to an opposing theory, however, viral strains migrate towards the tropics during the off season and then move back to their epidemic territory during the winter.

To address this debate, Eddie Holmes of the Pennsylvania State University and collaborators analyzed eight consecutive years of genetic material from 900 influenza strains collected from Australia, New Zealand, and the United States. By reconstructing large-scale phylogenetic trees of these data they found that every year, New York viruses were more closely related to that year's far-off strains than to the local strains of the previous year.

"This deals a serious blow to the theory that flu viruses remain latent during the summer in temperate climates," Holmes, the study's senior author, told The Scientist. "The data shows us that the epidemiology of influenza is more complex than we previously thought."

Cherry noted that the results do not discount the possibility that seasonal conditions could also contribute to epidemic spread, but they strongly argue against summer latency, suggesting that future research should be directed at viral migration pathways.

"The source for the seasonal emergence of influenza has been unexplained for a long time and this work highlights the important role of global migration of viruses in a much more detailed way than had been possible before" Scott Dowell of the Centers for Disease Control and Prevention in Atlanta told The Scientist in an Email.

"What remains unexplained is the virtual disappearance of the disease for large parts of each year," said Dowell, who was not a coauthor on the study. "The fact that we now understand that the viruses are continually circulating globally only makes this observation more puzzling."

To fully understand how the virus evolves, Holmes noted, world-wide sampling will be necessary. "We need to know more about the virus in diverse locations like Southeast Asia," he said. "This information is central to designing influenza vaccines that best anticipate the epidemics of the future."

Matt Kaplan
mail@the-scientist.com

Links within this article:

M.I. Nelson et al., "Phylogenetic analysis reveals the global migration of seasonal Influenza A viruses," PLoS Pathogens, September 14, 2007.
http:/www.plospathogens.org

James Cherry
http://www.uclahealth.org/body.cfm?xyzpdqabc=0&id=479&action=detail&ref=4376

B. Borrell, "A new dynamic," The Scientist, May 1, 2007.
http://www.the-scientist.com/article/display/53122

M.I. Nelson et al., "Stochastic Processes Are Key Determinants of Short-Term Evolution in Influenza A Virus," PLoS Pathogens, December 1, 2006.
http://www.the-scientist.com/pubmed/17140286

P. Hunter, "Outsmarting influenza's rapid evolution," The Scientist, June 30, 2003.
http://www.the-scientist.com/article/display/13909/

R.E. Hope-Simpson and D.B. Golubev, "A new concept of the epidemic process of influenza A virus," Edpidemiol Infect, August, 1987.
http://www.the-scientist.com/pubmed/3301379

C. Viboud et al., "Influenza in tropical regions," PLoS Medicine, March 7, 2006.
http://www.the-scientist.com/pubmed/16509764

Eddie Holmes
http://www.cidd.psu.edu/people/bio_holmes.html

C. Holding, "Flu virulence linked to species jump," The Scientist, February 6, 2004. http://www.the-scientist.com/article/display/21979/