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Leprosy is a specific chronic bacterial infection caused by a bacterium called Mycobacterium leprae. It primarily affects the skin, mucous membranes, peripheral nervous system, eyes and testes.

The severity of the symptoms depends on the person's immune response to the infection. Leprosy is also known as Hansen's disease and is one of the oldest known diseases of mankind. Dr. Armauer Hansen of Norway was the first to see the leprosy germ under a microscope. This was in 1873, and Hansen's discovery was revolutionary. This disease is fortunately curable but if it is left untreated it can lead to severe deformities. Destruction of the nerve endings causes the affected areas to lose sensation causing the fingers and toes to become mutilated and fall off.

Incidence of the condition

It is estimated that about 6000 patients with leprosy live in the United States and about 95% of these patients acquired the disease in some of the developing countries. In the US, 200-300 cases are reported each year. The worldwide prevalence of leprosy is reported to be just less than 1 case per 10,000 population. Leprosy can affect people of all races anywhere in the world. However, it is most common in warm, wet areas of the tropics and subtropics. The worldwide prevalence is reported to be around 5.5 million. Leprosy most often occurs in children between the ages of 10 and 14 and in adults between the ages 35 and 44. 

Types of leprosy

There are several forms of leprosy that range from the mildest to the most severe. The more severe forms arise with the less effective immune response. 

Depending on clinical features, leprosy is classified as:

  • Indeterminate leprosy (IL)
  • Tuberculoid leprosy (TT)
  • Borderline tuberculoid leprosy (BT)
  • Borderline leprosy (BB)
  • Borderline lepromatous leprosy (BL)
  • Lepromatous leprosy (LL)

The prognosis can vary from patient to patient. Those with the indeterminate leprosy, a very early form of leprosy, may either be cured or progress to one of the other forms of leprosy depending on their immune status.

Within each type of leprosy, a patient may remain in that stage, improve to a less debilitating form or worsen to a more debilitating form depending on their immune state.

Causes and symptoms

Mycobacterium leprae

The exact organism that causes leprosy is a rod-shaped bacterium called Mycobacterium leprae. It is very similar to Mycobacterium tuberculosis, which causes tuberculosis. 

Body reactions

When Mycobacterium leprae invades the body, one of the two possible scenarios can take place:

  1. The first possible scenario happens when the body's immune cells attempt to cut off the infection from the rest of the body. Since this response by the immune system occurs in the deeper layers of the skin, the hair follicles, sweat glands, and nerves can be destroyed. As a result, the skin becomes dry and discolored and loses its sensitivity. 
  2. The alternative scenario is that the body's immune system is unable to provide a strong response to the invading organism. That’s why the organisms multiply freely in the skin. The characteristic feature of this condition is the appearance of large nodules or lesions all over the body and face. Sometimes the mucous membranes of the eyes, nose, and throat may be involved.  This type of leprosy can lead to blindness, drastic changes in voice, or mutilation of the nose. 

The incubation period varies anywhere from six months to ten years. On average, it takes four years for the symptoms of tuberculoid leprosy to develop.

Symptoms in different types of leprosy

Leprosy type

Most common symptoms


  • One large red patch with raised borders or a large asymmetrical spot

  • Lesions dry and hairless

  • Loss of sensation occur at site of some lesions

  • Tender, thickened nerves with loss of function

  • Spontaneous resolution may occur in a few years or it may progress to borderline or rarely lepromatous types

Borderline tuberculoid

  • Lesions are smaller and more numerous

  • Disease may stay in this stage or convert back to tuberculoid form, or progress


  • Numerous, red, irregularly shaped plaques

  • Sensory loss is moderate

  • Disease may stay in this stage, improve or worsen

Borderline lepromatous

  • Numerous lesions of all kinds, plaques, papules and nodules.  

  • Hair growth and sensation are not impaired over the lesions


  • Numerous lesions of all kinds, plaques, macules, papules and nodules

  • Early symptoms include nasal stuffiness, discharge and bleeding, and swelling of the legs and ankles

  • Left untreated, the following problems may occur:

    • Skin thickens over forehead, eyebrows and eyelashes are lost,

    • Eye involvement causing photophobia, glaucoma and blindness

    • Skin on legs thickens and forms ulcers when nodules break down

    • Internal organ infection causing enlarged liver and lymph nodes

    • Voice becomes hoarse due to involvement of the larynx

Epidemiology of the condition

It is still not totally clear how the leprosy bacillus is transmitted from person to person but inhalation is one of the most common ways. Inhaling bacteria that are present in the dust is thought to be one of the modes of transmission. Fortunately, it is not a highly communicable disease. Environmental factors such as unhygienic living conditions, overpopulation, and malnutrition may also be contributing factors favoring the infection.
Nerve infection in leprosy

The most common feature present in all forms of leprosy, beside the skin lesions is definitely a nerve infection. The nerve damage appears to result from the multiplication of bacilli within the Schwann's cells, the cells that cover up the nerves. Most of the deformities in leprosy are based on trauma to, or secondary infection of the denervated tissues.

The common symptoms are:

  • anesthesia to heat and cold are lost before other modalities
  • vasomotor and trophic changes,
  • infection and ulceration of anesthetic parts,
  • pyogenic osteomyelitis,
  • muscle wasting,
  • resorption of soft tissue and bone

The involved nerves may be firm, tender, and enlarged both visibly and palpably. 

Diagnosis of leprosy

Skin biopsy

The presence of an inflamed nerve in a skin biopsy is considered the criterion standard for diagnosis. The skin biopsy sample should be examined for morphologic features and the presence of M.leprae. Biopsy is useful for determining the morphologic index, which is used in the evaluation and treatment of patients. The morphologic index is the number of viable bacilli per 100 bacilli in the leprous tissue.

Lepromin testing

This test indicates the host’s resistance to the bacterium that causes leprosy. The results do not confirm the diagnosis, but they are useful in determining the type of leprosy. A negative finding suggests a lack of resistance to disease and also indicates a bad prognosis.

PCR testing

Polymerase chain reaction (PCR) testing, though useful in detecting multibacillary disease, is not widely done because it fails to reliably detect early or mild forms of leprosy.

Serology can be used to detect the antibodies to M leprae specific PGL-I. This test is useful primarily in patients with untreated LL, as 90% of patients have those antibodies. PCR analysis can be used to detect and identify M leprae.

Histological Findings

Findings are usually nonspecific. Histiocytes and lymphocytes are scattered, with some concentration around dermal appendages and nerves. At times, an acid-fast bacillus can be observed in a nerve bundle. The number of dermal mast cells may be increased.

Treatment of leprosy

Although years are required for the elimination of M. leprae from the skin, most of the bacilli are dead within 3-6 months after the initiation of therapy. To minimize the possibility of relapse, therapy should be continued until all M. leprae have disappeared from the skin, which is usually longer than 5 years.


The antimicrobic of choice for the treatment of leprosy is dapsone. It is a very effective bacteriostatic drug that interferes with folic acid synthesis. A satisfactory response may be expected in patients treated with 200-500 mg once weekly, because effective concentrations persist in the serum for 7 days.


It represents the first antimicrobic known to be bactericidal against M. leprae, and it acts with greater rapidity than any previously available drug.  The only problem is that it is really expensive. That’s why its principal role is adjunctive in the treatment of lepromatous leprosy. 


This is a secondary drug of choice that is used in conjunction with dapsone or rifampin in a combined chemotherapy regimen. It is bacteriocidal, non-toxic and effective but also expensive. It causes a temporary skin discoloration. It is given orally at 100 mg/day, three days per week.


This is an interchangeable medication in a chemotherapy regimen. It may be used in a combined chemotherapy regimen with either dapsone or rifampin. It is administered orally at 500-1000 mg/day, three days per week.

Oral corticosteroids
are helpful in preventing nerve damage by reducing swelling.