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Mycobacterium leprae (leprosy)



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Environment: human, mice, nine-banded armadillo
Gram + rod
Spore former: NO
Motile: NO
Susceptibility: anyone
Communicability: infectious
Exposure: exposure to skin
Incubation: 2-20 years
Primary Treatment: antibiotics; multi-drug treatment
Prognosis: antibiotics keep disfigurement to a minimum
Quarantine recommended: occasionally
Use as a biological weapon: NO 


Mycobacterium leprae is the causative agent of the disease, leprosy, also known as Hanson's Disease. The bacterium was discovered in 1873 by a Norwegian physician named Gerhard Armauer Hansen. M. leprae is a gram-positive, aerobic rod surrounded by the characteristic waxy coating unique to Mycobacteria. In size and shape, it closes resembles M. tuberculosis. Due to its thick, waxy coating, M. leprae stains with carbol-fuchsin rather than with the traditional Gram staining method.

M. leprae has never been grown in artificial culture, but will grow in the footpads of mice and in armadillos. The culture can take several weeks to mature.

Laboratory indicators:

  • acid-fast stain

  • growth in armadillo

  • growth in mouse footpad

  • dihydroxyphenylalanine (DOPA) oxidase


There are three forms of the disease, with lepromatous being the most severe, tuberculoid, and borderline leprosy. The precise mode of transmission is not fully understood. The bacteria are likely spread by direct contact and through air dispersement, from coughing or sneezing.

Intracellular and extracellular masses, called globi, are often found in victims of lepromatous leprosy. The bacilli never form chains. Virulence factors include a waxy exterior coating, formed by the production of mycolic acid unique to Mycobacteria. 

The incubation period can be long, perhaps as much as 20 years, in unusual cases, or as little as two.


Infected individuals will notice skin lesions in early stages, leader to paralysis or loss of sensation in those areas, and eventually loss of extremities. Blindness can occur as the disease advances.


Though the bacterium was discovered in 1873, a treatment was available until the 1940s. Dapsone (diamino diphenyl sulfone) has been successfully used for over fifty years to treat leprosy, but recently, the bacilli are becoming more resistant to treatment. Multidrug therapy (MDT), usually consisting of Rifampicin, Clofazamine, and Dapsone, is use to treat all cases of leprosy to diminish the development of resistance genes. In patients receiving MDT, a high proportion of bacilli are dead within days, suggesting that many symptoms of leprosy must be due in part to dead cells. Thus, development of a drug to rid the body of dead organisms is essential. Administration of Dapsone or any other drug as monotherapy should be considered unethical.


Though rare in the United States, the disease is still a problem in parts of Texas and Louisiana possibly due to the presence of the nine-banded armadillo. In 1997, there were approximately 1.2 million cases worldwide, with Africa and Asia reporting the highest numbers. Additionally, about 600,000 new cases are reported annually. Despite famous historical precautions, the disease is not highly contagious.

Crippling effects still occur, but the availability of MDT in recent years has lessened the worst symptoms of leprosy. As previously mentioned, the disease is not as contagious as historical confinement of lepers would have people believe. Leprosy can be avoided by covering the face and hands when in the presence of infected individuals. Modern antibiotics and new treatment regimens are helping to keep the number of severely disfigured individuals to a minimum.

Bacteria Profiles

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Islands of Samoa
Small island off the Upolu coast once designated as a leper colony

M. leprae cells
Photomicrograph of the causative agent of leprosy

Teheran Clinic
A boy infected with
multibacillary leprosy


A particularly crippling case of Leprosy







World Health Organisation

Leprosy Fund

TB and Leprosy Control



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