More Evidence to Support Russian Theories of the Black Plague?

When I first started reading about the plague several years ago, there were actually conflicting theories concerning bubonic plague. American researchers believed (and probably still do) that there was but a single strain of y. pestis bacteria responsible for bubonic plague. This raised the issue, though, of why certain plagues (the Justinian plague and the Black Death) were so deadly and virulent compared to later outbreaks of plague. It was not too long ago that some researches argued that the Black Death was not caused by y. pestis, but was some form of hemorrhagic fever. That issue has been settled by genetic testing which has shown that victims of the Black Death had died of y. pestis. Later research showed that the y. pestis that killed victims in the Black Death was unrelated to modern strains of plague. 

Earlier this year, scientist confirmed that Justinian's plague, in the 5th and 6th centuries, was also due to y. pestis. Yesterday, the Daily Mail ran an article noting research had shown that Justinian's plague was a different strain from that of today and that of the Black Death. From the article:

Tiny samples of the plague bacteria were taken from skeletons belonging to two victims of the Justinian plague who were buried in Bavaria, Germany. 

Fragments of DNA were found in their 1,500-year-old teeth and used to recreate the bacteria’s whole genetic code.

Researchers compared it with a database of hundreds of modern plague pathogens, some of which still kill thousands every year.

The study, published in The Lancet Infectious Diseases, shows the strain responsible for the Justinian outbreak was an evolutionary ‘dead-end’ and distinct from strains involved later in the Black Death and subsequent pandemics. 

A third pandemic, which spread from Hong Kong across the globe is also likely a descendant of the Black Death strain and thus much more successful than the one responsible for the Justinian Plague. 

Ancient DNA expert Professor Hendrik Poinar said: 'The research is both fascinating and perplexing, it generates new questions which need to be explored, for example why did this pandemic, which killed somewhere between 50 and 100 million people die out?'

At first glance, this research actually seems to be further confirmation of the Russian model. Under both the Russian and American models, y. pestis resides in certain rodent populations, which provide a reservoir for the disease. Under the American model--at least in the past--there were no real differences in strains. That is, the plague was the plague was the plague. This apparently has been modified to allow for various strains, but which evolve and then die off. The Russian model, on the other hand, believes the type of rodent making up the reservoir has an impact on the strain. That is, the y. pestis in rats and American rodents is a different strain from those found in Middle-Eastern gerbils which is different from central Asian marmots.

As I understand the history, Justinian's plague spread out of Africa to the Byzantine empire, and thence into Europe. The Black Plague is generally understood to have spread from central Asia westward across Asia (devastating the Muslim Middle-East) and into Europe; and eastward into China. (As a side note, an estimate of 100,000,000 dead from the Black Plague is probably on the low side--it appears to have killed nearly half of the populations of China and Middle-East, and slightly less--a third--in Europe). Thus, the variance in strains can just as easily be explained by the Russian model as a theory that certain strains had died off.

Interestingly, the books I've read on the Black Plague indicate that the spread of plague was probably enabled by a cooling trend in the Earth's climate. The same is suggested about Justinian's plague--that dust from Haley's comet may have caused global cooling with resulting famines, making it easier for the plague to spread.