Tuesday, 22 July 2014

Norovirus is a social butterfly

Every writer needs a muse. Today, mine is diarrhoea. Inspired by the recent norovirus outbreak at the Glasgow Commonwealth Games I thought I’d write a few words on this intestinal disruptor. There will be images, and they will be beautiful. I promise.

Norovirus is a social butterfly. It spreads readily from person to person, casually disregarding social etiquette and even Michelin stars. In the spring of 2009 it descended upon Heston Blumenthal’s £200 a head restaurant The Fat Duck. It was the largest ever norovirus outbreak at a UK restaurant, with at least 240 people gushing out their haute cuisine. In the aftermath it became apparent that the most likely source was a batch of contaminated oysters.

 "Fresh Oysters" Credit Urville86 
  Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons
A fact not to be shared over a romantic dinner is that this alleged aphrodisiac often harbours norovirus. A 2011 study by the Food Standards Agency found that 76% of British oysters tested were contaminated. Like us, oysters unwittingly consume the virus from human sewage. As filter feeders they churn huge volumes of water through their bodies to extract food and during this process they can accumulate any viruses and bacteria contaminating the water. There are industrial techniques that aim to purge these contaminants from oysters by leaving them in clean, UV irradiated water for a few days before harvesting. However it’s unclear how effective this approach is for norovirus. On the plus side, even eating norovirus contaminated oysters won’t necessarily result in harm- that depends on the health of the person eating them and the concentration of viral particles. Nevertheless, you’d be forgiven if this undermines your faith in Cosmopolitan’s assertion that swallowing this raw grey bivalve mollusc “really can spark randiness”.

Colour-enhanced electron micrograph of Norovirus.
                   Credit David Gregory&Debbie Marshall, Wellcome Images Creative Commons by-nc-nd 4.0
The conduit for the recent Glasgow Commonwealth Games norovirus outbreak was less glamorous: a temporary toilet. So far there have been 53 suspected cases amongst the staff, who are a bathroom-hovering microcosm of a global problem- every year norovirus causes 267 million infections worldwide. Inside the guts of the Glasgow staff norovirus will have set to work with its toolkit, entering cells and hijacking their machinery to make new norovirus particles. What makes this heist particularly impressive is that the virus has just nine protein-coding genes in its repertoire. For contrast fruit flies have 13,000. By leveraging the cells’ own resources the norovirus is able to set up an epic production line that creates billions of new viral particles. Then with an efficiency that would make even Amazon envious, norovirus sets about dispatching from its human factory to the outside world. Which is where things get unpleasant. We don’t know how, but the virus hijacks the nervous system and sends signals to the stomach to make it contract violently and vomit. This generates tiny droplets, which float through the air, dispersing the virus onto surrounding surfaces. The next unsuspecting host has to transfer just 18 viral particles to their mouth to become ill.

Not content with one exit strategy, norovirus also causes diarrhoea. It achieves this by increasing the amount of fluid escaping through the lining of the intestine. In a healthy gut, the cells that line the intestine are held together by protein linking-structures called tight junctions. These are demonstrated as continuous blue lines in the left-hand image below. Norovirus disrupts these tight junctions, which makes the gut leaky and causes watery poo. E. coli takes a similar approach, and in the right-hand image we can see how the red dots (E. coli) have proliferated and the tight junctions have been broken-down. The remnants of the tight junctions exists as discontinuous blue dots, which are no longer capable of stopping water leaking between the cells and out into the intestine.

How E. coli causes diarrhoea.
Credit S. Schuller, Wellcome Images Creative Commons by-nc-nd 4.0

In its quest for a steady stream of diarrhoea, norovirus also increases the number of chloride ions pumped by cells into the intestine. Water follows the lead of the ions in a process called osmosis. This potent combination of leaky tight junctions and increased osmosis enables norovirus to generate diarrhoea.

It’s been estimated that a single gram of poo from an infected person contains 100 billion viral particles. That means there are ten times more norovirus particles in a gram of infected poo, than there are people on the planet. Which perhaps allows Glasgow and Heston to take comfort from the fact that they were beaten by a virus that’s disgustingly good at spreading itself.