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What Is Leprosy?

Leprosy (Hansen’s Disease), is a chronic infectious disease that primarily affects the peripheral nerves, skin, upper respiratory tract, eyes, and nasal mucosa. The disease is caused by a bacillus (rod-shaped) bacterium known as Mycobacterium leprae.

The Bacterium: Mycobacterium leprae

M. leprae, discovered by G.A. Hansen in Norway in 1873, is a slow-growing, intracellular pathogen that cannot live outside its host. Because it can only be grown in animals, not in a laboratory, it is more difficult to study than other bacteria. Armadillos and immunocompromised mice are the two primary sources for growing the bacteria for research purposes.

Another factor complicating studies of leprosy is that M. leprae multiplies slowly, and symptoms can take as long as 20 years to appear. Armadillos are the only animals other than humans that have been found to become naturally infected with M. leprae.

The Disease

Researchers are still unclear about the mode of transmission of leprosy. Though widely assumed to be spread via the respiratory system through nasal droplets, broken skin is also a possibility. Because M. leprae survive best at low temperatures, it primarily affects the superficial sites of the skin and peripheral nerves.

Susceptibility to infection with M. leprae appears to be genetic, and the form of disease that a person develops depends on the immunity of infected people. Some people in a family may have the infection, but other close family members will not develop it, depending on their personal ability to fight off the bacteria.

Leprosy usually affects the skin, peripheral nerves, and upper airways but has a wide range of clinical manifestations. Clinical forms of leprosy represent a spectrum reflecting the cellular immune response to M. leprae. Patients with good T-cell immunity (Th1 type) towards M. leprae exhibit tuberculoid (TT) leprosy which is also known as pauci-bacillary leprosy, a milder form of the disease, characterized by skin discoloration. Those with poor T-cell immunity towards M. leprae typically exhibit lepromatous (LL) leprosy or multi-bacillary leprosy, which is associated with symmetric skin lesions, nodules, plaques, thickened dermis, and frequent involvement of the nasal mucosa resulting in congestion and nose bleeds. In between these forms of leprosy are the borderline tuberculoid (BT), borderline-borderline (BB) and borderline lepromatous (BL) forms.

LL leprosy is also characterized by large numbers of organisms in the skin, many skin lesions with slight hypopigmentation, and less sensory loss in the lesions. While people with LL have high titer antibodies to M. leprae, they also have an impaired cellular immune response to the bacillus. Changes in immunity of the host as well as treatment can result in worsening of the clinical course of the disease.

All forms of leprosy may cause some degree of peripheral neurological damage (nerve damage in the arms and legs) that causes sensory loss in the skin as well as muscle weakness. People with long-term leprosy may lose the use of their hands or feet due to repeated traumatic injury resulting from lack of sensation. If left untreated, it can cause progressive and permanent damage to the skin, nerves, eyes, and limbs.

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Last Updated April 25, 2011