Wednesday 16 October 2013

Middle East respiratory syndrome coronavirus: how tough is it?

Slide tweeted by @HZowawi captured from my talk on H7N9
and MERS-CoV presented at the Royal Children's Hospital,
Brisbane, Queensland, to the local Serology/Virology &
Molecular Special Interest Group of the Australian Society
of Micorbiology. 15th October, 2013.
This publication is nearly a month old so apologies if you know of it already.

For the rest of you, Doremalen, Bushmaker and Munster recently wrote in Eurosurveillance about the results of their experiments to discover how resilient MERS-CoV is on surfaces at different humidities and temperatures and how it survives in an aerosol. They also used MERS-CoV (the EMC/2012 strain) to influenza A (H1N1)pdm09 (Mexico/4108/2009 strain) virus for comparison.

Some key findings below (you can read the entire article yourself because this excellent journal has open access):

  • Plastic and steel surfaces behaved similarly for these viral survival studies
    • MERS-CoV was still infectious after 48-hours at 20°C in 40% relative humidity (RH; low humidity similar to indoors). 
    • MERS-CoV remained viable for 8-hours at high temperature (30°C)/high humidity (80% RH) and 24-hours at high temperature/low humidity (30% RH)
    • (H1N1)pdm09 was inactivated after 4-hours at any of those conditions
  • Viruses were aerosolised and the amount of viral RNA and viral infectivity compared at 20°C/40%RH or 20°C/70%RH, after the aerosol was impinged into tissue culture medium.
    • MERS-CoV viability dropped 7% at 40%RH and 89% at 70%RH - both at 20°C. Viral genome copies did not drop significantly.
    • (H1N1)pdm09 dropped 95% and 62% respectively.
  • SARS-CoV reportedly survives for 5-days at 22-25°C and 40-50%RH
The authors go on to conclude that MERS-CoV remains viable in the air and on surfaces for longer than a pandemic influenza virus. When you consider that a pandemic results largely from efficient transmission, of which virus stability is a component, this is a significant study.

These are pure preparations of virus under experimental conditions so it's fair to say that things would be different "in the wild". Rougher environmental conditions may accelerate viral decay although, if larger droplets were expectorated during coughing fits, extra material may act to prolong the survival of virus.

Transmission through fomites (e.g. door handles, glass screen phones, other hard surfaces, cups, utensils, clothing) is a possible route that now has some data to support it-although the current high temperatures (30s-40°C) in the Kingdom of Saudi Arabia suggest survival on surfaces, away from air conditioning, won't be for long.

What would be nice to know next, is whether mild and moderate cases of MERS are also capable of producing aerosolised virus. And what about asymptomatic cases? What about the mysterious animal sources? Could infected animal excreta further prolong viral survival? So many questions.

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