Jack Frost and influenza: partners in crime?

21 April, 2010, Lux Fatimathas

The effect of the weather on Influenza transmissibility

As the final few leaves fall off deciduous trees and the carpet of leafy mulch is worn away, a new season beckons. The close of autumn signals the start of flu season, better known as winter. Growing up in England meant that around the same time every year the airwaves would fill with the news of nationwide flu jabs for the old and infirm. Medical certificate pads would run thin as work place attendance dropped and the atmosphere would grow heavy with the general malaise that engulfs the populous as every other person around you begins to sport a familiar sounding sniffle.[frax09alpha]

In the sunnier climes of Singapore where I now reside, the winter blues has all but vanished. It seems utterly logical to me that the cold and blustery climate of England in the winter would be conducive to the spread of influenza, but when I asked myself why I seemed so certain of this I was stumped. My reasoning appeared to be based on word of mouth – the mouth tending to be my mother’s telling me to wrap up warm or I’d catch something. The scientist in me just wasn’t sated. Enter PubMed – the online search engine for all things life science related.

Image: American Centre for Disease Control A microscopic image of swine flu. These tiny spheres, averaging 100nm in size, encase viral proteins and RNA (the template for making proteins) and express the proteins hemagglutinin and neuraminidase on their surface, depicted here as small bumps on their surface.

In 2007 an American group of scientists, led by Professor Peter Palese, pre-empted my query and published a study addressing the effect of the weather on transmission of the influenza virus (PLoS Pathog 3(10)). To test this they exposed guinea pigs to human flu virus. This may seem like a rather strange choice of animal model compared to the more widely studied laboratory mouse. However mice are unable to efficiently spread the human flu viruses that we are often infecting each other with. Fortunately for us, and perhaps rather unfortunately for the guinea pigs, human strains of influenza are able to infect these less lauded rodents, allowing us to better probe its transmission. Following this eureka moment, the Palese lab went on to infect a guinea pigs with the H3N2 flu virus.

H3N2 belongs to the same family of flu viruses as the now infamous H1N1, which hit headlines worldwide for crossing the species barrier from pigs to humans and killing over six thousand people in the process. These flu strains belong to the influenzavirus A family and are named according to the subtypes of proteins expressed on their surface – Hemagglutinin and Neuraminidase.

Image: Unknown via Shiply.com A sneeze in slow motion. A sneeze, also known as a sternutation, is conservatively estimated to travel at 12 mph and packs quite a punch when the spread of infection is considered.

Aerosol transmission (another way of referring to that invisible cloud of sputum you helplessly try to avoid breathing in when someone sneezes right next to you) was shown to be enhanced in both cold and dry conditions. The study reported that the virus transmitted more often at 5°C than at 20°C or 30°C and that transmission was most effective at lower humidity levels (around 20-35%) and blocked at high humidity levels (80%).

The jury is still out on the precise mechanism of how these conditions aid transmission, but the Palese lab at least seems convinced of its importance. Perhaps I really am safer living on my new island nation, swimming through its moist air and steaming under its brilliant sun. Safer at least from the traditional winter bug, but lest we not forget that H5N1 (also known as bird flu) is thought to have originated in Hong Kong. These alphanumeric viral packages are here to stay, but the more we understand their transmission the better chance we have at containing their spread and limiting the misery they inflict.

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