Science Series #8: Leprosy

What is leprosy?

Leprosy, also known as Hansen’s disease, is a chronic infection caused by bacteria called Mycobacterium leprae. This bacteria multiplies very slowly and the incubation period can take a number of years (5 years on average. Leprosy mainly affects the skin, the peripheral nerves, mucosa of the upper respiratory tract, and the eyes. It destroys the body’s ability to feel pain, hence it causes nerve damage and muscle weakness and can lead to deformities, crippling, blindness and isolation. This disease is not very contagious and can be treated once recognized. If left untreated, nerve damage can occur, leading to paralysis of hands and feet. Leprosy is known to affect all age ranges, however it is curable and early detection can prevent the development of disabilities.

There are two main classifications of leprosy:

A mild, less severe form of leprosy, which allows the disease to appear but limits it to only a few skin patches (5 or less). In these patients, the number of leprosy bacilli in the body is quite small (less than a million)

A more severe form of the disease. A very small minority of people have such a weak immune response to the leprosy bacillus that it can multiply almost without any check and spread to almost all parts of the skin and the peripheral nerves. This results in a positive skin smear.

 (Source: World Health Organization, International Federation of Anti-Leprosy Associations)

How is leprosy transmitted?

Leprosy is most likely transmitted by air through droplets from the nose and mouth, during close and frequent contacts with people who have not yet been treated. While it is spread in a similar way to the common cold, it is much less infectious. It multiplies slowly and the vast majority of people have adequate natural immunity and do not contract the disease if exposed. Even when diagnosed, many cases are not considered infectious, and once treatment begins, those infectious cases become non-infectious within the first week of treatment.

(Source: Pan American Health Organization, International Federation of Anti-Leprosy Associations)


Although many people believe that leprosy no longer exists, approximately 200,000 people get infected every year globally with a prevalence rate of 22.7 per million, in more than a 100 countries. Unfortunately, four million people have disabilities as a result of leprosy. In 2019, countries such as India, Indonesia and Brazil accounted for 79% of new cases. The countries that reported more than 100 new cases per year are Argentina, Bolivia, Brazil, Colombia, Cuba, Dominican Republic, Ecuador, Mexico, Paraguay, and Venezuela.

(Source: American Leprosy Missions, World Health Organization, Pan American Health Organization)


Symptoms may occur within 1 year but can also take as long as 20 years or even more to appear. The disease mainly affects the skin, nerves and mucus membranes. The first signs of leprosy are pale patches of skin or numbness in the fingers and toes. Skin lesion has usually a different pigmentation than the surrounding normal skin (less pigmented, reddish or copper-colored) and may have various aspects (flat, raised or nodules). Skin lesion can be single or multiple and may show a loss of sensation in the skin.

If left untreated, the development of leprosy can cause progressive and permanent sequelae, including deformities and mutilations, reduction of the mobility of the limbs and even blindness.

(Source: World Health Organization, Pan American Health Organization)


The main signs that lead to the diagnosis of leprosy are the appearance of one of the three cardinal signs:

  • A definite loss of sensation in one or more pale (hypopigmented) or reddish skin patches.
  • One or more thickened or enlarged peripheral nerves with a loss of sensation and/or weakness in the muscles supplied by the nerve.
  • The presence of acid-fast bacilli in a slit skin smear.

(Source: International Federation of Anti-Leprosy Associations)


Leprosy is conventionally treated with a combination of multiple antibiotics, known as multidrug therapy (MDT). This is necessary to kill the pathogen, halt transmission, and prevent drug resistance. The antibiotics used are dapsone with rifampicin; and clofazimine is added for some types of the disease. The World Health Organization (WHO) Leprosy Guidelines recommend a six-month course of drugs for PBL and a twelve-month or longer course for MBL. These treatments are highly effective and cure 98% of patients with leprosy infection. Unfortunately, there may be side effects to MDT as around 1% experience daspone hypersensitivity syndrome and clofazamine may cause skin discoloration.

(Source: Pan American Health Organization, International Federation of Anti-Leprosy Associations)

Our background in leprosy

In 1938 Dr. Convit began studies on leprosy in the main leprosarium of Venezuela, Cabo Blanco. For years, with a team of doctors and pharmacists he worked to understand the clinical, pathological, social and human aspects of the disease aiming to alleviate the long time suffering and social exclusion of leprosy patients. His aim was to find a drug that could prove that leprosy was curable and with this put an end to the compulsory isolation of patients.

In the 1940s, after various studies, Dr. Convit found that dapsone and clofazimine were effective in treating leprosy. Being able to demonstrate that he was able to control the disease, in 1950, Venezuela became the first country to close the leprosariums and treat patients in their own communities, a model that became a world reference in the treatment of the disease. For decades, his tested the use of Bacillus Calmette-Guérin (BCG) to treat leprosy. In 1980, he published the final description of the first vaccine against leprosy, an effective immunotherapy using the isolated bacteria and BCG.

Dr. Convit’s discoveries became the basis of the MDT that is provided by the WHO since 1995 as a conventional program for leprosy.

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