What causes leprosy?

Leprosy is caused mainly by Mycobacterium leprae, a rod-shaped bacillus that is an obligate intracellular (only grows inside of certain human and animal cells) bacterium. M. leprae is termed an "acid fast" bacterium because of its chemical characteristics. When special stains are used for microscopic analysis, it stains red on a blue background due to mycolic acid content in its cell walls. The Ziehl-Neelsen stain is an example of the special staining techniques used to view the acid-fast organisms under the microscope.

Currently, the organisms cannot be cultured on artificial media. The bacteria take an extremely long time to reproduce inside of cells (about 12-14 days as compared to minutes to hours for most bacteria). The bacteria grow best at 80.9 F-86 F, so cooler areas of the body tend to develop the infection. The bacteria grow very well in the body's macrophages (a type of immune system cell) and Schwann cells (cells that cover and protect nerve axons). M. leprae is genetically related to M. tuberculosis (the type of bacteria that cause tuberculosis) and other mycobacteria that infect humans. As with malaria, patients with leprosy produce anti-endothelial antibodies (antibodies against the lining tissues of blood vessels), but the role of these antibodies in these diseases is still under investigation.

What are the risk factors for leprosy?

  • People at highest risk are those who live in the areas where leprosy is endemic (parts of India, China, Japan, Nepal, Egypt, and other areas) and especially those people in constant physical contact with infected people
  • In addition, there is some evidence that genetic defects in the immune system may cause certain people to be more likely to become infected (region q25 on chromosome 6)
  • Additionally, people who handle certain animals that are known to carry the bacteria (for example, armadillos, African chimpanzee, sooty mangabey, and cynomolgus macaque) are at risk of getting the bacteria from the animals, especially if they do not wear gloves while handling the animals.

What are leprosy symptoms and signs?

  • Unfortunately, the early signs and symptoms of leprosy are very subtle and occur slowly (usually over years).
  • The symptoms are similar to those that may occur with syphilis, tetanus, and leptospirosis.
  • Numbness and loss of temperature sensation are some of the first symptoms that patients experience.
  • As the disease progresses, the sensations of touch, then pain, and eventually deep pressure are decreased or lost.
  • Signs that occur, such as relatively painless ulcers, skin lesions of hypopigmented macules (flat, pale areas of skin), and eye damage (dryness, reduced blinking) are experienced before the large ulcerations, loss of digits, and facial disfigurement develop.
  • This long-term developing sequence of events begins and continues on the cooler areas of the body (for example, hands, feet, face, and knees).

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