European colonizers could be to blame for bringing tuberculosis to other parts of the world. tuberculosis is one of the most serious health issues worldwide killing 1.6 million people per year.
In a new study published in Science Advances on October 17, researches looked at how the most common strain of tuberculosis developed, and how antibiotic resistance spread around the world.
Study researcher Vegard Eldholm, from the Norwegian Institute of Public Health, and colleagues, focused on only one of tuberculosis' seven lineages.
They chose the so-called "lineage 4," the most widely dispersed type of TB around the globe. They also investigated how the lineage originally evolved and how its antibiotic-resistant forms have spread.
Eldholm and colleagues studied 1,669 DNA samples of lineage 4 that were previously collected in South America, Africa, Europe, North America, and Vietnam. They also looked at older samples of TB, which include those that were collected from three 18th-century mummies.
The genetic variations across these different TB strains allow researchers to estimate how closely related the lineage 4 strains were to one another. This could help determine when the lineage arrived on different continents and how old these different TB strains are.
Analysis revealed that the arrival of lineage 4 in the Americas, Africa, and Southeast Asia coincided with the arrival of the European colonizers, suggesting that Europeans may have carried the disease across continents.
"We find that dispersal of L4 has been completely dominated by historical migrations out of Europe," the researchers wrote in their study.
"We demonstrate an intimate temporal relationship between European colonial expansion into Africa and the Americas and the spread of L4 tuberculosis (TB)."
DNA analysis also showed that the lineage originated relatively recently, which is about 1,000 years ago.
The disease's resistance to antibiotics, on the other hand, appears to have evolved at the local level.
The researchers investigated if the resistant lineage 4 strains have spread around the world, but they did not find any antibiotic-resistant strains in the study that had crossed national borders. This means that if a resistant strain developed in the Netherlands, this strain is not likely to have reached Portugal or another country.
"We saw antibiotic resistance has developed all over the world multiple times," Eldholm told NPR. "That wasn't surprising. But the fact that we didn't find any [resistant] strains jumping from one country to another — that was a really big surprise."
The relative newness of antibiotics may be attributed for the regional boundaries of resistance. Antibiotics emerged in the mid-20th century and since then, some TB strains have evolved to become resistant to antibiotic treatment. The antibiotic-resistant strains of TB may not have had enough time to spread around the globe.