Tuberculosis (TB) is one of the world’s deadliest infectious diseases; around one third of the world’s population is estimated to be infected with TB and more than a million people die of the disease every year. While we know that animals play a role in the spread of this disease, how it entered the human populations has been a matter of debate for some years. Now, new findings are finally shedding light on this mystery, suggesting that seals were responsible for spreading TB to natives in South America before Europeans rocked up on the continent. The study has been published in Nature.
Tuberculosis in humans is caused by a slow growing bacterium called Mycobacterium tuberculosis. Circulating strains show remarkable diversity around the world, in particular those isolated from Africa. By comparing the genomes of these modern isolates, scientists gathered evidence that suggested this bacterium spread throughout the world after humans began to migrate out of Africa during the Pleistocene epoch.
How TB first reached humans in South America, however, has been unclear. Modern strains sequenced from the Americas appeared to be closely related to European strains, suggesting that the disease was introduced when Europeans arrived here. However, several skeletons that predate Columbus’s arrival had telltale signs of TB infection, suggesting the disease was already present in the New World when Europeans came onto the scene.
To gain a better understanding of the history of this disease, researchers sequenced the DNA of 68 ancient Peruvian human remains that all showed signs of TB. They found 1,000-year-old mycobacterial genomes in three of the skeletons, confirming that TB was present in the Americas before European contact.
To find out where TB might have come from pre-contact, the researchers compared the three bacterial sequences obtained with those of modern strains in both humans and animals. They found that the ancient strains were distinct from any strains currently circulating in the human population, but they were very similar to those found in seals and sea lions. It’s therefore likely that these marine mammals contracted TB from infected animals in Africa and then brought it to South America. Humans then probably picked it up by hunting and consuming these animals.
Of course, Europeans were not entirely blameless in the situation as when they arrived on the continent they brought different, more virulent strains along with them that rapidly replaced the marine mammal strains.
Interestingly, by looking at mutation rates in the bacterial genomes, the researchers were able to ascertain that the most recent common ancestor for M. tuberculosis lived less than 6,000 years ago. This is quite a different figure to previous estimated that suggested it may have been infecting humans for some 100,000 years, which has attracted some criticism.
“It was a surprise for all of us to find that tuberculosis, formerly believed to have spread around the world with ancient human migration events, is in fact a relatively young disease,” study author Kelly Harris said in a news-release.
The researchers believe that the results may help to further our knowledge of the speed and process of adaptation when a disease enters a new host. Furthermore, future studies may increase our understanding of TB transmission, which is important given the fact that the disease is once again on the rise.
[Header image "Hi," by Mark Gunn, via Flickr, used in accordance with CC BY 2.0]