International Team Determines Geographic
Origin of Leprosy
Leprosy likely originated in East Africa and spread
to Asia and Europe before being imported into West Africa
by explorers, report scientists in this week's issue
of Science. The findings enrich the historical
understanding of leprosy's global migration and contradict
a commonly held view that leprosy spread to West Africa
directly from East Africa, say the researchers.
The international team of investigators, who identified
rare genetic variations among strains of the bacterium
that causes leprosy, included Patrick J. Brennan, Ph.D.,
of Colorado State University in Fort Collins. Dr. Brennan
is a grantee of the National Institute of Allergy and
Infectious Diseases (NIAID), part of the National Institutes
of Health.
"Using modern genetic techniques, these researchers
uncovered clues to the origin of a disease that, since
ancient times, has been one of the most stigmatizing," notes
NIAID Director Anthony S. Fauci, M.D. "Their findings
may help public health officials better track and treat
leprosy, which remains a significant problem in some
parts of the world today."
The research team included scientists from institutions
in the United States, France and seven other countries.
Led by Stewart T. Cole, Ph.D., of the Institut Pasteur
in Paris, the investigators scanned the genetic material
of Mycobacterium leprae, the bacterium that
causes leprosy, for tiny variations known as single
nucleotide polymorphisms (SNPs). SNPs, pronounced "snips," are
variations in a single "letter" of DNA's four-letter
code. Scientists can use SNPs to trace the lineage of
an organism, in this case M. leprae, and to
develop a picture of how leprosy spread from its point
of origin. The team looked for SNPs in 171 clinical
specimens of M. leprae taken from people infected
with the bacterium. The specimens came from 21 countries
representing five continents.
Four types of SNP appeared in the samples, but their
distribution was not random. Instead, the investigators
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.
NIAID is a component of the National Institutes
of Health, an agency of the U.S. Department of Health
and Human Services. NIAID supports basic and applied
research to prevent, diagnose and treat infectious
diseases such as HIV/AIDS and other sexually transmitted
infections, influenza, tuberculosis, malaria and illness
from potential agents of bioterrorism. NIAID also
supports research on transplantation and immune-related
illnesses, including autoimmune disorders, asthma
and allergies. |