People living with leprosy pray at a hospital in Myanmar Reuters

The discovery of ancient bacteria has revealed leprosy might be the oldest human-specific infection, with roots that likely stem back millions of years.

Research at The University of Texas MD Anderson Cancer Centre has unearthed some of the ancient mysteries behind the ailment, also known as Hanson's Disease.

Studies have revealed two leprosy bacteria came from a last common ancestor around 10 million years ago.

Led by Xiang-Yang Han, a professor in laboratory medicine, researchers discovered a new leprosy-causing species called Mycobacterium lepromatosis in 2008. At that time, only one species called Mycobacterium leprae was known.

In the newly published study, Han, along with Francisco Silva, an evolutionary geneticist, came to the hypothesis that leprosy had existed for millions of years. This theory was deduced by connecting the dots from several known facts and published studies.

Lepers in Tahiti in 1898
Lepers in Tahiti in 1898 Wikimedia Commons

Han said: "Locating the additional leprosy bacterium Mycobacterium lepromatosis significantly adds to our understanding of the ancient disease. In particular, tracing the ultimate origin of leprosy through the parasitic adaptive evolution of the leprosy bacteria is rather insightful, not only for this single disease but also for our better understanding of the mechanism behind other human infections."

One piece of evidence is the fact that leprosy is a sole human disease without other hosts or reservoirs. Once outside of the human body, leprosy bacteria are unable to grow in artificial media.

It has been speculated that Mycobacterium leprae is found in wild armadillos, but only in North America and South America. It is believed the animals most likely acquired the infection from early American explorers a few hundred years ago.

Evidence proving the long history of leprosy lies within the bacterial genome. In every worldwide Mycobacterium laprae strain analysed so far, which amounts to more than 400, all have been found to have essentially identical genomes.

This suggests human beings carried the leprosy bacteria when departing Africa to populate the rest of the world, around 100,000 years ago. It also means that leprosy bacteria are extraordinarily stable within their human hosts.

The study also revealed the oldest age of the leprosy bacteria's pseudogenes suggests that gene inactivation began approximately 20 million years ago. This is likely to be the point when the ancestor of leprosy bacteria jumped to our early human ancestors and evolved from free-living to strictly parasitic.

Han and Silva's hypothesis that leprosy existed for millions of years offers new insights into disease pathogenesis. They discovered the parasite hides by mutating or removing harmful molecules while retaining protective ones. In the end, this leads to evasion from host immunity, a phenomenon commonly seen in leprosy. Finally, the scientists concluded that leprosy can be viewed as a natural consequence of a long parasitism.

There are hundreds of thousands of new cases of leprosy worldwide each year. In the US, there are up to 200 new cases annually. Leprosy is known for attacking a patient's skin and nerves, which can be treated with antimicrobial medicines.

Without treatment, however, the disease can lead to extensive skin lesions, deformities in the patient's face and extremities, disabilities, and even death. Leprosy carries a social stigma and diagnosis is frequently and notoriously delayed.