Leprosy is cause
by infection with an intercellular pathogen known as Mycobacterium
leprae. M. leprae is a strongly acid-fast, rod-shaped bacterium.
It has parallel sides and rounded ends, measuring 1-8 microns in length
and 0.2-0.5 micron in diameter, and closely resembles the tubercle
bacillus. M. leprae has the longest doubling time of all known
bacteria (13 days) which makes doing laboratory research (in vitro)
on this organism quite difficult. Therefore, innoculation of the foot
pad of the mouse and the armadillo are the main mechanisms for research.
The organism infects the skin because it thrives at temperatures slightly
lower than that found inside the human body. It also has an affinity
for nerve cells, which is why leprosy is characterized by loss of
feeling on the skin surface. M. leprae is the only mycobacterium
known to infect nervous tissue.
photomicrograph of M. leprae from a leprosy skin lesion (CDC)
the only known resevoir is the armadillo. It is thought that they
are a good host for Mycobacterium leprae because of their low
become the main source of M. leprae for genetic, biochemical,
and immunological research including development of a vaccine.
5% of armadillos in Louisiana have naturally occurring clinical disease.
About 20% have serologic evidence of infection with organisms indistinguishable
from M. leprae. However, only occasional cases are reported
among individuals handling armadillos.
infection also has been reported in non-human primates including the
African chimpanzee, sooty mangabey, and cynomolgus macaque.
It is uncertain
whether or not insects can act as a vector for M. leprae. Acid-fast
bacilli, like M. leprae, have been demonstrated in biting insects.
transmission of M. leprae by intracutaneous inoculation in
the mouse footpad model has been reported. However,
the question whether insects actually transmit the infection remains
remains some uncertainty about the mode of transmission of leprosy,
most researchers agree that it is spread from person to person in
respiratory droplets or nasal discharge. M. leprae may survive
outside a human host for a period of hours or even days. Only the
lepromatous form of the disease is thought to be infectious.
respiratory tramsmission is thought to be the likely cause of most
infections, exposure to insect vectors, infected soil, and animal
reservoirs may also be possible modes of transmission.
Most people are
immune to leprosy. In endemic areas, subclinical levels of the disease
are common, but only in a select few cases will the infection progress
to clinical disease levels.
People that live
in close contact with patients who have untreated, active, predominately
multibacillary leprosy and people living in countries with endemic
leprosy are at an increased risk of infection.