Sunday, 4 December 2016

When Bad Bugs Go Badder

The Beijing lineage of M. tuberculosis is the villain in a movie sequel. Nastier, scarier, harder to kill. You thought tuberculosis (TB) was bad? Think again. The Beijing lineage is that little bit worse, associated with a speedier disease progression and increased antibiotic resistance.

I’ve always had a thing for studies that attempt to pick apart the origins of infectious diseases. So when I spotted a paper on the makings of the Beijing lineage, my excitement levels came close to when I discovered there’s going to be a Guardians of the Galaxy 2.

A while back, a paper came out suggesting that the Beijing branch of the TB family tree emerged some 30,000 years ago in Southeast Asia. Then, as the Han Chinese population embraced farming as a way of life, BOOM! The Beijing lineage exploded in northern China and started its journey towards worldwide misery.

Another study, however, put the approximate age of the lineage at around 6,600 years old. Suffice to say, attempts to date the M. tuberculosis family tree are notoriously unreliable. They all rely on some pretty big—and pretty shaky—assumptions, mainly based around how quickly the bacterium accumulates changes in its DNA.

So on to this new piece of research. It takes advantage of the increasing availability of whole-genome sequencing to look at how the pathogen has evolved in fine detail. The researchers were interested in a specific branch of the Beijing lineage—the central Asian clade, which recently reared its evil little head in Western Europe.

Like previous studies, I doubt that the molecular clock used in this study was calibrated accurately. But what’s clever about work like this is that it’s possible to superimpose expansions in the bacterial population size over big human upheavals. More bacterial diversity=worse living human living conditions. TB basically takes advantage of low points in human history. Anything that results in poor health, overcrowding, decaying healthcare systems, etc etc.

The study suggests that back in the 1950s to 1960s, the central Asian clade first appeared in the former Soviet Union. Then, thanks to the Soviet-Afghan war, it entered Afghanistan 1979-1989. Following the American invasion in 2001, the clade spread further as people were displaced from their homes and former lives. As Afghan refugees found their way to Europe, so did the central Asian clade of the Beijing lineage.

Back in the former Soviet republics, the clade had been similarly busy taking advantage of the fall of the Soviet Union. When it comes to historical events with huge ramifications for TB, the implosion of the Soviet Union is among the big ones. Bye bye, decent public healthcare and TB control; hello, drug resistant TB. Even today, multi-drug resistant TB is a gigantic issue in many ex-Soviet countries.

What studies such as this one demonstrate is how easily M. tuberculosis rides upon the back of political instability, war, population displacements and all the other crap us humans heap upon each other. And based on current events, none of that crap is going anywhere. So where does that leave TB?

Eldholm V, et al. (2016) Armed conflict and population displacement as drivers of the evolution and dispersal of Mycobacterium tuberculosis. PNAS 113(48):13881-13886.

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