discovered a fairly close correlation between SNP type and geographic location of the leprosy patient. Type 2, predominant in a small region of East Africa and Central Asia, is the rarest and oldest, the scientists believe. Type 1, present in Asia and the Pacific region, represents the eastward migration of leprosy, while type 3, seen in Europe, North Africa and the Americas, is the form that migrated west. The most recently evolved, type 4, is predominant in West Africa. Because type 4 leprosy is more closely related to type 3 than it is to either type 1 or 2, the researchers concluded that North Africans or Europeans probably brought the disease to West Africa.
Compared with other disease-causing organisms, M. leprae has very few SNPs ?only one in every 28,400 letter pairs. The rarity of SNPs is an indication of extreme genetic stability: all the strains of leprosy throughout the world are essentially identical.
The discovery of the four SNP types could help health officials better understand leprosy in present day human populations, says Christine Sizemore, Ph.D., of NIAID's Division of Microbiology and Infectious Diseases. Aggressive therapy with multiple drugs has helped drive down the number of registered leprosy cases around the world, notes Dr. Sizemore. However, despite drug treatment, the number of new cases of leprosy detected each year has stayed the same or risen. The new understanding of the genetic makeup of the leprosy bacterium will allow clinicians to characterize at a molecular level the M. leprae strain infecting a leprosy patient, which will show whether the patient has a new infection or if the previous infection was incompletely treated and has returned. This, in turn, will aid efforts to fully treat patients so that the bacteria are completely eliminated.
Reference: M Monet et al. On the origin of leprosy. Science 308 (2005) DOI: 10.1126/science.1109759.
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