It happened more than 20 years ago, but we’re still feeling some unexpected consequences today. The political collapse of the Soviet Union in the 1990s seems to have spurred on the killer strain of tuberculosis that is now spreading around the world.
A genetic analysis of the TB bacterium’s family tree has linked its global expansion to major historical events over the past few hundred years.
TB spreads from person to person in droplets released when you talk or cough. Strains that resist standard treatments are a major health problem in countries such as Russia, China and South Africa. In Western countries, multidrug-resistant (MDR) cases are less common, but still very contagious and hard to treat – extreme caution is taken with patients, including air-tight hospital rooms.
“Because it’s around, one has to assume every TB patient has it,” says Stephen Spiro of University College London Hospitals. “A small number of cases cause a huge number of problems.”
To uncover the source of the most common strain of MDR TB, called the Beijing lineage, Thierry Wirth of the Ecole Pratique des Hautes Etudes in Paris and colleagues genetically tested samples from nearly 5000 TB patients from 99 countries. They also sequenced the full genome of 110 of these samples, allowing them to draw up a detailed family tree.
By looking at the tree’s branching pattern, and length of these branches, and by factoring in other data about the bacterium, such as its mutation rate, they worked out the microbe’s global population size over the past 6600 years – giving the total number of TB microbes that were alive at any one time during that period.
Analysing this data, the authors found that the population size of the non-drug-resistant form of the Beijing lineage had been relatively stable for several hundred years, before expanding sharply from the early 1820s until the mid-1840s. The researchers blame this on the industrial revolution in the West, when large numbers of people began living together in unsanitary and crowded conditions, which helps to spread TB.
There was another sharp expansion around the time of the first world war. “If people are weak or tired or lack food, this weakens the body, making them more susceptible” says Wirth. The population shrank around the 1960s, when antibiotics for TB were introduced. But shortly after that, the genes that confer drug resistance emerged. Bacteria develop resistance to antibiotics when people take their drugs intermittently or do not finish the course of treatment. Around the 1990s, the downward trend went into reverse, and although numbers have not yet climbed back to pre-antibiotic levels.
As well as the rise in HIV, which makes people more susceptible to TB, the authors attribute the recent surge to the collapse of the Soviet Union, which has a well-known major problem with MDR TB. The social upheaval would have made it hard for people to access long courses of expensive drugs, and the region’s jails are hotspots of drug resistance, says Wirth. “In jails drug treatment has been irregular and not of best quality,” he says.
Russia and Eastern Europe still have high rates of MDR TB today. Spiro says that many of the cases he sees in London, the TB capital of Europe, can be traced back to those countries. “The control over treatment regimes out there is dreadful,” he says. “It’s chaos.” The number of MDR cases has been rising in England from 28 cases per year in 2000 to 81 cases in 2012.
While ordinary TB can be cured in 6 months, people with MDR TB must take different drugs that also have side effects such as liver and nerve damage, for one to two years; even then they have only a 50 per cent chance of a cure. “Patients often die from it,” says Spiro.
They also have to stay in hospital isolation rooms for several months until their sputum tests negative for the bacterium, making it costly to treat.
Journal reference: Nature Genetics, DOI: 10.1038/NG.3195
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