In 2008, MD Anderson Professor of Laboratory Medicine Xiang-Yang Han, M.D., Ph.D., and colleagues discovered the second species of bacterium ever identified as a cause of leprosy.
Now, Han and collaborators report that this most recent discovery is the dominant cause of the world's oldest human disease in Mexico.
"Mycobacterium lepromatosis not only causes the majority of cases, it accounts for all cases of the most severe form of the disease, diffuse leprotamosis leprosy (DLL), which has been recognized in Mexico for at least 160 years," Han says. The study is in the August issue of the International Journal of Dermatology.
Leprosy afflicts millions of people worldwide; about 200,000 new cases occur each year. The disease is rare in the United States, with 100-200 new cases annually, mostly among immigrants.There are 200-300 new cases in Mexico each year. Leprosy is successfully treated with antibiotics at early and intermediate stages.
Han and colleagues in Laboratory Medicine diagnose infections in cancer patients. Han was drawn into leprosy research after a physician in Phoenix asked him to analyze samples from a patient who died of an unknown cause.
Han had developed a way to distinguish new bacterial species by analyzing small differences in the 16S ribosomal RNA gene, which is nearly identical across a group of bacteria known as mycobacteria. Tiny differences indicate an organism is a unique species. There are approximately 150 species of mycobacteria.
He used this technique to discover M. lepromatosis, distinguishing it from Mycobacterium . leprae, The discovery appeared in the American Journal of Clinical Pathology in 2008. Until then, all forms of leprosy worldwide were thought to be caused by M.leprae.
In the new study, Han collaborated with Mexican scientists to analyze skin biopsies of 120 patients in Mexico with varied forms of the disease. Of these, the bacterial species causing the disease could be identified in 87.
M. lepromatosis infected 55 patients,18 had M.leprae and 14 had both.
There were 16 cases of DLL, 13 in the patients with M. lepromatosis, three among those who had both bacterial species and none among those with M. leprae.
DLL is characterized by large crops of recurrent skin lesions, usually in the legs, that scientists believe are the result of an unusual reaction to leprosy infection, Han said.
In advanced stages, the lesions may ulcerate or spread further, leading to secondary bacterial infections and possibly death. Uniquely among leprosy types, DLL invades blood vessel cells, causing overproduction of those endothelial cells, which can lead to vascular blockage and inflammation of blood vessels in the skin.
DLL is found mainly in Mexico and the Caribbean, but cases have been reported in India, Iran, Malaysia, Singapore, France, Brazil and Tunisia.
"It's reasonable to suspect that M. lepromatosis might be the cause of diffuse lepromatosis leprosy in other countries," Han notes. So far he and collaborators have found M. lepromatosis in Singapore. He also has studies under way of cases from Brazil, Myanmar, and other countries.
The team also found that M. lepromatosis:
- Infected younger patients with a median age of 51 and 19 of 55 were younger than 46
- Occurred in nine Mexican states
- Caused almost all types of leprosy, including lepromatous leprosy, the most common type, in 34 of the cases.
By contrast, the median age of the patients with M. leprae
was 57, with only one younger than 46. Cases were found in six states and the bacterium caused mostly lepromatous leprosy.
"Our studies so far have been retrospective. Next, we need prospective studies to test fresh biopsy specimens and focus on clinical details and treatment results caused by the different species," Han said. He and his Mexican colleagues are designing such studies.
MD Anderson news release of original discovery