Disproportionate pneumonic cases in Madagascar plague outbreak

The African island nation of Madagascar is reporting an outbreak of plague which has infected 86 people, killing 39. The more deadly--and contagious--pneumonic form of the disease represents 90% of cases.

Madagascar is a well known focus of plague and has recently led the world in cases. Madagascar is also the site where a naturally occuring multi-drug resistant strain of plague was discovered.

The United States reports a few cases per year, the great majority of which are of the bubonic form. These cases are almost exclusively from the western half of the country as the rodent populations which serve as the reservoir for the plague bacillus (Yersinia pestis) seem not to move east of an artificial plague line which falls roughly on the 100th meridian. 

A few months ago, when a squirrel in Los Angeles County tested positive for plague, I was interviewed by Erin Burnett on CNN on this topic.

What is most interesting about the current cases in Madagascar is that they are of the pneumonic form and not the much more common bubonic form. It is crucial to understand the reason behind the disproportionate number of the pneumonic cases.