Saturday, August 24, 2013

Bats as a possible reservoir for Middle Eastern Respiratory Syndrome

Middle Eastern Respiratory Syndrome , or MERS, is a lethal viral pneumonia which has only been known to epidemiologists since 2012.  The causative agent of MERS is a coronavirus, a member the same virus family responsible for Severe Acute Respiratory Syndrome (SARS) and for several animal diseases.  There have been about 100 confirmed cases of MERS so far, all clustered around the Middle East, with more than 40% mortality.  The key question for researchers of this new disease agent is whether this virus has the potential to cause an epidemic, as SARS did in 2003.

A phylogeny, or family tree, of coronaviruses, based on
amino acid sequences of an enzyme encoded in their
 genome. The virus which causes MERS is labeled in this figure
 with HCoV-EMC/2012 for Human Coronavirus (HCoV) and the 
case  number, and is highlighted in grey. This virus clusters very 
tightly with two coronaviruses  isolated from bats. Note that the 
SARS virus (labeled SARS-CoV) also has a close relationship to 
MERS with respect to this sequence being compared. 
The coronavirus which causes MERS has been isolated, and its genome resembles  several coronaviruses known to be present in bats. Thus the leading theory about this newly discovered virus is that it is a zoonotic virus, that is, a virus which can be transmitted from humans to non-human vertebrates or vice-versa.  Outbreaks of MERS in humans would become more likely in communities which interact frequently with nearby infected animals, and it becomes important to identify the non-human species which harbor the virus. And just this month, there has been a report of isolation of a MERS sequence from bat samples, which would be consistent with the sequence similarity of MERS coronaviruses to other bat coronaviruses.
These results are very preliminary, with a short MERS  fragment identified in only a single bat. Specifically, because bats harbor related coronaviruses, some of which are only now being discovered, researchers would like to see more genomic sequence, and isolates from more bats to be sure that virus that causes MERS is indeed harbored by bats.  Finally, since bats and humans generally avoid one another, it remains to be seen whether other animals are also part of the infection cycle.

Overall, then, MERS is a deadly infectious disease that can spread from person to person and likely from animal to person. Moreover, mutations in the virus causing MERS could change its infectious properties, and trigger an epidemic of viral pneumonia.

Update: A nice write-up of the detective work involved with these viruses is at Understanding Evolution.

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