Leprosy

Leprosy, also known as Hansen’s disease, is a chronic infectious condition caused primarily by Mycobacterium leprae or, more rarely, Mycobacterium lepromatosis. It has affected human populations for thousands of years and remains a significant public health concern in several tropical and subtropical regions. Although it can lead to severe disability if untreated, leprosy today is entirely curable with modern multidrug therapy provided globally.

Nature of the Disease and Clinical Features

Leprosy mainly targets the peripheral nervous system, skin, eyes, and upper respiratory tract. One of its hallmark effects is progressive nerve damage, which can diminish the ability to feel pain. This loss of sensation increases vulnerability to wounds, burns and infections that may result in deformities, particularly of the hands and feet. Additional symptoms include muscle weakness, numbness of extremities, dry scalp, nasal congestion, alterations of facial appearance, thickening of skin, and changes in speech.
Symptoms typically develop gradually. The average incubation period is around five years, but onset may occur within a year or, in some cases, only after two decades. Early signs often include pale or pink patches of skin with reduced sensation. As nerve damage progresses, secondary infections may lead to tissue loss and, eventually, physical deformities if not treated in time.
Two principal clinical types are recognised:

• Paucibacillary leprosy, marked by five or fewer skin lesions and minimal bacterial presence.
• Multibacillary leprosy, involving more than five lesions and high bacterial load.

Diagnosis is confirmed through skin biopsy demonstrating acid-fast bacilli.

Causative Organisms

The disease is caused by M. leprae or M. lepromatosis, both acid-fast, rod-shaped bacteria belonging to the genus Mycobacterium. These organisms have a thick waxy cell wall and are obligate intracellular parasites, meaning they cannot be grown in artificial culture. They can, however, be propagated in certain animals, including mice and armadillos.
Natural infections have been documented in armadillos, several primate species and red squirrels. Genetic studies show that the strains affecting armadillos likely originated from historical human infections before spreading within wildlife. The discovery of M. lepromatosis in 2008 expanded the known diversity of leprosy-causing organisms, although clinical presentation is similar to that caused by M. leprae.
Leprosy may therefore follow zoonotic transmission routes in rare circumstances, with evidence suggesting wildlife-to-human spread in some regions.

Transmission and Risk Factors

Leprosy spreads through prolonged close contact, most commonly via droplets from the nose or mouth of an infectious person. It possesses low pathogenicity, and about 95 per cent of people exposed never develop the disease due to innate or acquired immunity. The disease does not spread through sexual contact or from pregnant individuals to their unborn children.
Risk factors include:

• Long-term exposure to an untreated patient.
• Living in conditions of poverty or overcrowding.
• Poor nutrition or weakened immune function.
• Certain genetic predispositions influencing immune response.

Unlike several other infections, co-infection with HIV does not appear to increase the risk of developing leprosy.

Global Distribution and Epidemiology

Although leprosy has been greatly reduced worldwide, it remains endemic in parts of Asia, Africa and Latin America. In the 1980s, global incidence exceeded five million cases, but sustained public health efforts and widespread access to multidrug therapy have reduced this to fewer than 200,000 cases annually.
India reports the largest share of new cases, accounting for more than half of global prevalence, followed by Brazil, Indonesia and a group of other endemic countries. Between 1994 and 2014, an estimated sixteen million individuals were successfully treated.
In the United States, roughly two hundred cases occur each year. A notable proportion of these are reported in Central Florida, where environmental or zoonotic factors may contribute to transmission.

Treatment and Management

Leprosy is fully curable with multidrug therapy (MDT) consisting of:

• Dapsone
• Rifampicin
• Clofazimine

This combination is provided free of charge worldwide through programmes supported by the World Health Organization. Treatment typically lasts six months for paucibacillary cases and twelve months for multibacillary cases. Other antibiotics, such as macrolides or fluoroquinolones, may be used in certain circumstances.
Early treatment prevents further nerve damage and helps maintain quality of life. Most individuals undergoing therapy can continue their usual activities, live with their families and attend school or work.

Classification as a Neglected Tropical Disease

Leprosy is categorised as a Neglected Tropical Disease (NTD) due to its persistence in disadvantaged populations and the social barriers associated with diagnosis and treatment. Stigma surrounding the disease continues to impede early reporting in many countries. Historically, individuals were isolated in leprosaria or colonies; however, such practices are now discouraged and not medically warranted.
Despite legal reforms, isolated communities of former patients still exist in some countries, including parts of India, Japan, several African states and Thailand.

Historical Background and Cultural Impact

Leprosy has been noted in written records for millennia. Its name derives from the Greek word for “scale”, referencing its dermatological manifestations. Hansen’s disease honours Gerhard Armauer Hansen, the Norwegian physician who identified M. leprae in the nineteenth century.
The condition has long been associated with social stigma, myth and discrimination. Cultural narratives frequently portrayed affected individuals as unclean or ostracised, contributing to enduring negative perceptions. Modern public health campaigns—including World Leprosy Day, established in 1954—aim to dispel these misconceptions, promote early detection and support affected communities.

Immune Response and Complications

The clinical spectrum of leprosy depends heavily on a person’s immune response. Stronger cell-mediated immunity produces tuberculoid or paucibacillary forms, whereas weaker immunity leads to lepromatous or multibacillary disease. Around one-third of affected individuals develop episodes of acute inflammation known as leprosy reactions, which can worsen nerve damage.
When untreated, nerve impairment may result in paralysis, sensory loss and deformities due to repeated injuries. Timely therapy can halt progression and prevent long-term disability, although nerve damage that persists for several months before diagnosis is often irreversible.