Leprosy Symptoms, Signs, and Cause

What causes leprosy?

Leprosy is caused by Mycobacterium leprae, a rod-shaped bacterium. M. leprae reproduces very slowly and must grow inside a host cell.

What are risk factors for leprosy?

The human genome has significant influence on susceptibility to leprosy. It is thought that up to 95% of people are genetically resistant to leprosy. Risk factors for acquiring leprosy include residing in a home with an infected individual or living in a high-prevalence area. Armadillos are among the few animals that can become infected with M. leprae. Handling infected armadillos is a risk factor for the disease, although the risk is low.

How is leprosy transmitted?

Because of the low risk of contagion, sustained or repeated exposure to infected secretions is probably required to transmit the disease. It is likely that the disease is spread in respiratory droplets from untreated patients during coughing or sneezing. Treatment rapidly eliminates the risk of transmission, and people on effective treatment are not considered contagious.

What are leprosy symptoms and signs?

Symptoms do not appear until approximately two to 10 years after exposure, reflecting the very slow-growing nature of the bacteria. As peripheral nerves become infected, the skin becomes numb. This may lead to unrecognized trauma and secondary infection, resulting in the loss of digits or tissue. Pale patches (macules) can appear on the skin. Nodules and thickened skin may occur. In advanced disease, infection of the nasal mucosa may cause tissue thickening, congestion, and nosebleeds. As nerves thicken, muscles may become weak or even paralyzed. Infection of the eyes may lead to blindness. Infection of the testes may lead to low testosterone levels, infertility, or impotence.

The type and severity of symptoms depend on the extent of infection and on the body's immune response. There are two general categories of disease based on the bacterial load: paucibacillary and multibacillary disease. As the name implies, paucibacillary disease is associated with lower numbers of organisms in the tissue and is generally milder, sometimes limited to a few numb macules on the skin. Multibacillary disease is associated with higher numbers of bacteria in the tissue and causes more extensive disease, including more macules, symmetrically distributed nodules, plaques, thickening of the skin, and involvement of nasal mucous membranes, testes, or eyes.

The body's immune response also affects clinical manifestations. An effective response can confine the disease to a few macules or thickened nerves and may be referred to as tuberculoid disease. Disease that is unchecked by the immune system may be called lepromatous disease and can lead to numerous macules or nodules, large areas of thickened skin, and other advanced manifestations. The delineation between tuberculoid and lepromatous forms of leprosy is not always clear, leading to diagnoses of intermediate and borderline stages of disease.

How do physicians diagnose leprosy?

Leprosy is diagnosed by a skin biopsy. Special stains show the bacteria under a microscope. The skin tissue may show increased levels of blood cells known as lymphocytes that may be arranged in clumps known as granulomas. These laboratory results coupled with an appropriate clinical picture confirm the diagnosis. Serological (blood) tests for antibodies exist but have variable results in different forms of the disease and thus are not definitive.

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Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease


Longo, D.L., et al. Harrison's Principles of Internal Medicine, 18th ed. New York: McGraw-Hill Professional, 2011.