Leprosy is cause by infection with an intercellular pathogen known asMycobacterium 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.
A photomicrograph of M. leprae from a leprosy skin lesion (CDC)
Besides humans, the only known resevoir is the armadillo. It is thought that they are a good host for Mycobacterium leprae because of their low body temperature.
Armadillos have become the main source of M. leprae for genetic, biochemical, and immunological research including development of a vaccine.
Approximately 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.
Naturally occurring 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.Successful 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 unanswered.
Although there 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.
While human-to-human 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.
