Saturday, 24 June 2017

Another canary in the same coalmine - mild MERS may be bad news...

Sometimes, the Middle East respiratory syndrome coronavirus (MERS-CoV) is detected in a person who is not ill. 

Weird huh? 

Not really. This is the result of laboratory testing of contacts of a known and infected person during the process of containing a potential outbreak.


For me personally, this is one big question about new or emerging viral infections or infections we are still learning about - like new influenza viruses, MERS-CoV, ebolaviruses and Zika virus. Do we really know how often a laboratory-confirmed infected person with mild or no illness can spread virus to a new person - an uninfected potential host? Are our tools up to the job of detecting what's happening and are we using them properly?

Conventional wisdom is that truly asymptomatic but virus infected people do not infect others around them, or if they do, it's a pretty rare event. Because the risk is seen as low, studies around this issue are often down the list of research priorities.

The importance of this issue lies in whether mild or asymptomatic people need to be more closely considered as having a role in spreading virus and contributing to community or hospital outbreaks.

Emerging from the 2015 South Korean MERS-CoV outbreak, a recent report described the findings from laboratory testing of 82 contacts of an asymptomatic healthcare worker.[1] No other person became MERS-CoV positive. I have some issues with the fact that the nurse herself does not seem to have been tested to show that she developed antibodies to MERS-CoV and there also isn't a lot of discussion about how the PCR testing for MERS-CoV can be a bit "flaky" when sampling once from the upper respiratory tract. Although, there aren't any sampling details in this paper either (I'll blog about this paper another day)!

But I digress. 

I've plotted the all the publicly available mentions of asymptomatic MERS-CoV infections, by week, in the graph below (the bottom panel). 

Click on image to enlarge.
The yellow peaks show that cases without illness usually correlate with healthcare workers in the graph above, during hospital and healthcare facility outbreaks (see my previous post describing the pink graph in the top panel).[2] 

This isn't too surprising. The majority of disease associated with MERS-CoV infection arises in older males who already have an underlying disease including diabetes mellitus, cirrhosis and various lung, renal and cardiac conditions. Healthcare workers however are usually younger and do not have, or have not yet developed, such comorbidities. 

MERS-CoV is often a shown to be a bit of a bully when challenged by a healthy younger host's immune system. Although, when hit with a larger primary dose of virus from an infected camel, even the healthy can get hit very hard.

Healthcare workers can be the 'canary in the coalmine', except singing about a healthcare outbreak rather than a gas leak. Similarly, laboratory confirmed MERS-CoV infection manifesting with only mild or no signs and symptoms of disease, also serve this role as a sentinel of hospital, rather than camel-to-human, transmission of MERS-CoV.

References...