A new leprosy bacterium found

In 1873, G. A. Hansen discovered that leprosy was caused by a bacterium, which was given the name Mycobacterium leprae. Pockets of high prevalence of leprosy can still be found in certain countries, including Brazil, India, Nepal, the western Pacific, Madagascar, Tanzania and Mozambique. Dapsone was the mainstay of treatment from the 1940s until resistance appeared but multidrug therapy is used nowadays.

Human beings are the only known source of the infection, although a disease caused by organisms indistinguishable from M leprae is found in armadillos in parts of the southern US. M leprae cannot be grown in a normal culture medium because it has lost the genes to enable it to survive outside the human body — a process called reductive evolution.

Until recently, M leprae was thought to be the only cause of leprosy, but now a second leprosy bacterium has been identified by a team led by Xiang-Yang Han, associate professor in laboratory medicine at the University of Texas M. D. Anderson Cancer Centre.

In a report in the American Journal of Clinical Pathology (December 2008), the researchers say that the new species, which has been named M lepromatosis, was found in two patients who died of diffuse lepromatous leprosy, a form of the disease occasionally found in Mexico, the Caribbean islands and Central America.

The researchers are attempting to sequence the genome of M lepromatosis. However, as with M leprae, they have not yet managed to grow the organism in the laboratory.

Strains of M leprae have been collected worldwide and found to be practically identical, although the clinical features and severity of the disease can vary. It had been suggested that an individual’s immune factors can determine how the disease progresses.

This new research may show that M lepromatosis could account for at least some of this variation.