How Leprosy Works

In advanced cases of leprosy, gangrene can occur, resulting in parts of the body becoming deformed, as with these bones.
In advanced cases of leprosy, gangrene can occur, resulting in parts of the body becoming deformed, as with these bones.
Science & Society Picture Librar; /Getty Images

Riddled with sores, maybe missing toes, definitely unclean. Lepers spark some pretty hideous things in our imaginations, don't they? That's imagination and NOT reality. Yet there's a long-standing stigma when it comes to this disease; think about what you're really saying when you call someone a leper. It was even once believed the victims of the diseases were, actually, victims of sin (mentioned — through possible dubious translation — in Leviticus 13:14 in the Old Testament). Although leprosy has a history of being thought of as a highly-contagious (it's not) and deadly, it's actually totally treatable. And there's such a low risk of transmission there's no reason to isolate or ostracize people with leprosy.

Leprosy has been with us since roughly 1500 B.C.E. (that's when it's first mentioned in the Egyptian Ebers Papyrus, but it's also mentioned in other ancient writings, including in prehistoric Asian writings dating to 600 B.C.E.). But it's probably been plaguing humans for much longer, at least since the ancient civilizations of China, Egypt and India. In 2009, anthropologists discovered evidence of leprosy in a 4,000-year-old skeleton, which dates the infection back to prehistoric India, around 2000 B.C.E. [source: Robbins]. Scientists theorize that the infection spread as empires and trade routes grew, and that it likely arrived in the Americas during European exploration of the New World.

In 1866, after leprosy began infecting Hawaiians, the Kalaupapa peninsula of the island of Molokai was turned into the first leprosy colony — which it remained through 1969. (That's is about a decade after Hawaii became a U.S. state). Eight thousand Hawaiians were sent to live (and die) there, quarantined in an effort to prevent the disease from spreading. Today the only leprosy victims living on Kalaupapa number only about a few more than dozen or so; there are more tourists.

Modern-day medicine has renamed leprosy to Hansen's disease (HD), after Gerhard Henrik Armauer Hansen, the scientist who discovered the cause of the infection — a germ — back in 1873. Although it's not eradicated, not many cases appear these days; in fact, in 2012 there were only 232,857, and those that do pop up are usually contained in 16 countries, mainly in the tropics. The U.S. isn't immune from the disease, but incidents are few. There were only 213 new cases of leprosy reported in the U.S. in 2009 (most occurred in California, Florida, Hawaii, Louisiana, Massachusetts, New York, and Texas, and most occurred in people who had recently immigrated). In total there are about 6,500 people living with leprosy in the U.S. (including some who still live on Molokai), and typically between 150 to 250 newly diagnosed infections every year [sources: National Hansen's Disease (Leprosy) Program , Doerr]. And let this ease your mind: About 95 percent of humans are naturally immune to the infection.

Causes and Symptoms of Leprosy

Modern-day medicine renamed leprosy to Hansen's disease, after Gerhard Henrik Armauer Hansen, the scientist who discovered the cause of the infection back in 1873.
Modern-day medicine renamed leprosy to Hansen's disease, after Gerhard Henrik Armauer Hansen, the scientist who discovered the cause of the infection back in 1873.
Public domain, via Wikimedia Commons

Once considered punishment from a higher authority, we now understand that leprosy is actually an acquired chronic bacterial infection caused by Mycobacterium leprae (M. leprae). It's contagious — between two humans, yes, but it's also transmitted from armadillo to human. Because skin lesions are visible they are the distinguishing characteristic of the disease, but it's the damage to peripheral nerves that is at the core.

No one is 100 percent sure how you become infected with M. Laprae, but it's widely accepted that among humans it's transmitted through the small droplets that are released with a cough and sneeze — a sneeze, for instance, can disperse those droplets as fast as 10 miles (16 kilometers) per hour [source: Engber]. That might not be fast for a car, but it's definitely a good clip for something flying from a respiratory tract.

M. Leprae is a parasitic organism, which means it relies on those host cells for its survival, and it's kind of tricky once it gets into your body. This class of bacteria target your Schwann cells — those are nerve cells that the body uses to fix its peripheral nervous system — and changes the way they work. First, the organism hides in these cells, which means your immune system doesn't see them, and the immune system won't fight what it doesn't know is there. It may take years for the disease to appear after you've been infected, but inside your body the infected cells remain under attack. As the infection takes hold, the cells begin to break down, and once they lose their protection M. Leprae exploits them. Bacilli-infected Schwann cells are converted into cells that behave like stem cells, and here's what makes that so deadly. Because stem cells have the ability to convert into another type of cell in your body, imagine the power this gives to the organism. If an infected Schwann cell is converted to a mycocyte (a muscle cell), for instance, M. Leprae is now infecting the body's muscle tissue. And as nerve fibers are affected, symptoms of the disease begin to appear.

Skin lesions, which might be flat or raised, and might appear solo or in groups, are the hallmark of leprosy. In more severe cases, the infection may cause respiratory problems ranging from hoarseness to nasal symptoms (including loss of sense of smell, stuffy nose, bleeding, and even a collapsed nose). Eyes are also at risk for damage, and leprosy may cause eye redness, eyebrow and eyelash hair loss, pain, lagopthalmos (which is what it's called when you can't close your eyelids), and other eye ailments, all leading to blindness.

Additionally, and most significantly, M. leprae causes nerve damage. Trophic ulcers (commonly on the plantar area on your feet) may appear along with related loss of pain and skin sensation. Motor impairment, as well as muscle weakness and paralysis (including such problems as claw-hand deformities, facial nerve palsy) also disable leprosy sufferers.

Diagnosing and Treating Leprosy

The Kalaupapa peninsula of the Hawaiian island of Molokai was turned into the first leprosy colony in 1866.
The Kalaupapa peninsula of the Hawaiian island of Molokai was turned into the first leprosy colony in 1866.
Liysa Liysa/Getty Images

Leprosy infections are slow-moving, and with early diagnosis and appropriate treatment, leprosy is not a death sentence.

Skin lesions and the loss of sensation are telltale early signs of the infection, and are usually enough for a health professional to make a diagnosis, but specific tests will determine for sure. The two primary tests are skin lesion biopsies and skin scraping. All leprosy infections are caused by M. Leprae, but how it appears (its expression) and how severe it is varies from person to person.

There are two types of leprosy, paucibacillary and multibacilly. Paucibacillary (PB) leprosy expresses itself with only one to five skin lesions, and skin and nerve biopsies conducted during diagnosis all come back negative for M. Laprae bacilli. On the other side of the leprosy spectrum is multibacillary (MB) leprosy, characterized by more than five skin lesions and a positive result for M. Laprae bacilli in the skin and nerves.

To makes things a little more complicated, leprosy is also categorized by its level of severity; this is called the Ridley-Jopling classification. The classifications, listed from minor to most severe, include: indeterminate leprosy, tuberculoid leprosy, borderline tuberculoid leprosy, mid-borderline leprosy, borderline lepromatous leprosy and lepromatous leprosy.

All forms of the infection are treatable, and have been since the 1940s. The first treatments against leprosy were injections of promin (a sulfone medication, which means it's made from organic sulfur compounds), discovered effective in 1941. In the 1950s, dapsone (an antibacterial, and also a sulfone medication) became the go-to cure, and is still used today. But as M. Laprae began to develop resistance to these medications, other therapies were considered, and in the 1960s, clofazimine (an antimycobacterial), and rifampicin (an antibiotic) were introduced as leprosy treatments.

The most effective treatment, used with great success against leprosy since 1981, is actually not just one thing, but rather a multidrug therapy (MDT) that combines clofazimine, dapsone and rifampicin. Paucibacillary infections require a six-month combination (dapsone and rifampicin) therapy treatment to clear the infection. Multibacillary leprosy infections, the most severe, require the strongest therapy: a combination of all three medications taken over the course of a year. A mild infection with just a single lesion, may require only a single dose of antibiotics that combines rifampicin, ofloxacin and minocyclin [source: WHO]. The World Health Organization (WHO) provides these treatments free to all leprosy patients, and once treatment has begun the infection is no longer contagious.

Even with treatment, there can be long-term complications of the infection, depending how long the infection was allowed to progress before treatment began as well as the overall severity of the disease. Patients may have permanent disabilities including nerve damage and loss of sensation and pain, specifically in the arms and legs, as well as long-term muscle weakness. Sometimes leprosy causes disfigurement. Victims aren't missing toes because they fell off (give your imagination a rest), but because the body is re-absorbing the cartilage in those toes (and fingers, hands and feet, and nose). Complications resulting from nerve damage may sometimes require surgery to treat the physical effects resulting sensory loss; for example, if you can't feel your foot, you may frequently injure it — this can also sometimes lead to loss of fingers and toes. Surgeries may also relieve and repair a claw hand, and in some cases infected body parts may need to be amputated.

Author's Note: How Leprosy Works

I wasn't really surprised that leprosy really did still exist in our modern world (what can I say, the glass is half empty for me), but what did surprise me were the number of cases that happen every year. There's no vaccine against leprosy — yet — however the bacillus Calmette Guerin (BCG) vaccination, which is used to protect us against tuberculosis, may be helpful in preventing the infection, depending on which study you read. (No one can seem to agree on just how much protection it may provide, marginal or moderate.)

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More Great Links


  • Amundson, Ron. "A Wholesome Horror: The Stigmas of Leprosy in 19th Century Hawaii." Disability Studies. Vol. 30, no. 3-4. 2010. (Sept. 26, 2014)
  • Anderson, Natali. "Study: Leprosy Bacteria Turn Nerve System Cells into Stem Cells." Sci-News. Jan. 18, 2013. (Sept. 26, 2014)
  • Bennett, Brian H. "Leprosy: Steps Along the Journey of Eradication." Public Health Reports. No. 123, no. 2. Pages 198-205. March 2008. (Sept. 26, 2014)
  • Centers for Disease Control and Prevention (CDC). "Hansen's Disease (Leprosy). April 29, 2013. (Sept. 26, 2014)
  • Costandi, Mo. "Leprosy spreads by reprogramming nerve cells into migratory stem cells." The Guardian. Jan. 17, 2013. (Sept. 26, 2014)
  • Degang, Yang. "Leprosy as a Model of Immunity." Future Microbiology. Vol. 9, no. 1. Pages 43-54. 2014. (Sept. 26, 2014)
  • Doerr, Steven. "Leprosy (Hansen's Disease)." Sept. 19, 2014. eMedicineHealth. (Sept. 26, 2014)
  • Engber, Daniel. "FYI: How Forceful Is A Sneeze?" Popular Science. Sept. 17, 2013. (Sept. 26, 2014)
  • Fitness, J. "Genetics of susceptibility to leprosy." Genes & Immunity. Vol. 3. Pages 441-453. 2002. (Sept. 26, 2014)
  • Irgens, LM. "[The discovery of the leprosy bacillus]." Journal of the Norwegian Medical Association. Vol. 122, no. 7. Pages 708-709. March 10, 2002. (Sept. 26, 2014)
  • Jackson, Kristin. "Exploring the tragic beauty of Hawaii's remote Kalaupapa." Seattle Times. Feb. 9, 2013. (Sept. 26, 2014)
  • Lalwani, A.K. "Leprmatous leprosy: nasal manifestations and treatment with minocycline." Annals of Otology, Rhinology & Laryngology. Vol. 101, no. 3. Pages 261-264. March 1992. (Sept. 26, 2014)
  • Lewis, Felisa S. "Dermatologic Manifestations of Leprosy Treatment & Management." Medscape. April 9, 2014. (Sept. 26, 2014)
  • Masaki, Toshihiro. "Reprogramming Adult Schwann cells to Stem Cell-like Cells by Leprosy Bacilli Promotes Dissemination of Infection." Cell. Vol. 152, no. 1-2. Pages 51-67. Jan. 17, 2013. (Sept. 26, 2014)
  • Robbins, Gwen. "Ancient Skeletal Evidence for Leprosy in in India (2000 B.C.). PLOS. May 27, 2009. (Sept. 26, 2014)
  • Truman, Richard W. "Probable Zoonotic Leprosy in the Southern United States." The New England Journal of Medicine. Vol. 364. Pages 1626-1633. April 28, 2011. (Sept. 26, 2014)
  • University College London. "City living helped humans evolve immunity to tuberculosis and leprosy, new research suggests." ScienceDaily. Sept. 24, 2010. (Sept. 26, 2014)
  • U.S. Department of Health and Human Services - Health Resources and Services Administration (HRSA). "National Hansen's Disease (Leprosy) Program." (Sept. 26, 2014)
  • U.S. Department of Health and Human Services - National Institute of Allergy and Infectious Diseases (NIAID). "Leprosy (Hanson's Disease." Feb. 8 2011. (Sept. 26, 2014)
  • Von Klein, Gael H. "Full text of 'The medical features of the Papyrus Ebers." Delivered before the 13th Annual Session of the American Academy of Medicine at Chicago. 1905. (Sept. 26, 2014)
  • Vyas, Jatin M. "Leprosy." U.S. National Library of Medicine. Sept. 1, 2013. (Sept. 24, 2014)
  • World Health Organization (WHO). "Leprosy elimination." (Sept. 26, 2014)
  • World Health Organization (WHO). "WHO Model Prescribing Information: Drugs Used in Leprosy." 1998. (Sept. 26, 2014)