NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Tuesday, March 22, 2011

Hansen's Disease

(updated with pictures 3/23/11)

I am currently traveling on vacation. Today we visited Kalaupapa, on the island of Molokai, Hawaii, the site of the leper colony established by King Kamehameha V in 1865. The site is separated from the rest of the island by high cliffs, and is only accessible by sea, air and a mule track down the cliffs. It made the perfect site to isolate inhabitants once thought to be infectious by their mere presence among undiseased persons. The colony is the site of the wonderful work by Josef De Veuster, Father Damien of Molokai, the Belgian Priest who ministered to the colony from 1873 to 1889, transforming it from anarchy to order, from despair to hope.

I offer this post to the blog, both in the interest of history, and to present a misunderstood disease in the context of neuroscience. Hansen's Disease – infection by Mycobacterium leprae and/or Mycobacterium lepromatosis is not primarily a skin disease, nor does it result in "limbs falling off" as popular belief would have it. Leprosy is primarily a disease of the nervous system. The mycobacterium damages the sensory nerves of the periphery that lie close under the skin, first affecting tactile sense, then the fibers that transmit pain. The pale, blotchy skin lesions that have been the hallmark of leprosy from ancient times, appear as the "dermatomes" of the skin lose their neuron connections.

Dermatomes are regions of the skin that are served by a common source of "afferent" nerves ascending to the spinal and brain, and "efferent" nerves returning to the skin. Severing a single nerve will result in loss of sensation in a patch of skin from <1 to >10 cm square, depending on location on the body. Dermatomes on the fingertips are quite small, reflecting the many densely packed nerve endings that provide fine touch and sensitivity. Dermatomes over the ribs, hips and thighs are quite large, since tactile sense in those regions does not need to be as precise. Hence dermatomes are a necessary feature of the brain being able to localize *where* a sensation is coming from.

With the most common form of Hansen's Disease, the loss of sensation occurs first, and the skin lesions appear later as the dermatome loses all neural connections. A rarer variety exhibits the pale lesions, raised patches, nodules and bumps, with the numbness and sensory loss occurring much later. Ironically, the loss of neuron is due to the body's own immune system, much the same as other neuron diseases such as myasthenia gravis, multiple sclerosis (MS) and amyotropic lateral sclerosis (ALS – Lou Gehrig's Disease). Mycobacteria infect the neurons and change the outer membrane. Immune cells recognize the neurons as infected and damaged and "remove" them, resulting in the loss of neural connections between skin and brain. As more neural connections are lost, they also include the neurons returning from brain to the skin that regulate blood flow, perspiration, and other factors, resulting in the lesions normally associated with the disease. Severe lesions and loss of limbs results not from the disease itself, but from untreated infections that are (A) undetected due to lack of pain, and (B) impaired healing due to loss of neural control of blood and lymph flow. Untreated infections result in gangrene, cartilage damage and loss, and bone loss, resulting in lost or shortened joints and digits.

We now know that leprosy is *not* very contagious. The amount of contact required to be infected is usually only encountered by family members or caregivers, such as Father Damien. It is likely *not* transferred via the skin or by the lesions, but by nasal secretions and mucus, much the same as influenza. There appears to be a genetic susceptibility, resulting in the disease occurring within families (as well as due to the close, repeated contact) and we now also now that >95% of humans are naturally immune.

Contrary to the Ancient Greeks who first described what we know as Hansen's disease, it is not about being "unclean." Instead, it is all about the neurons, but then, isn't everything?

Update 3/23/11:  For my Facebook friends, pictures are up on my Speaker to Lab Animals page.  I actually wrote this column a week ago, and just returned from the actual tour a few hours ago.  Kalaupapa Peninsula is a study in contrasts, incredible beauty for a site of such despair.  The greatest contribution of Father Damien was that he brought hope to the patients and their families.  It is an incredible experience.


  1. Thanks...I always love to learn something new. Hope youre enjoying your vacation.

  2. I am glad y'all are enjoying the posts. It took a bit of effort to write and schedule two weeks worth of blogs ahead of time, but I think it was worth it, to provide continuity, and to allow me a bit of time off. The trip was a 25th anniversary celebration and very relaxing and enjoyable.

  3. Thank you for this interesting and informative article. The aetiolgy/pathology of the disease was clearly and meaningfully presented and the history, photos and info re the current colony have made me really want to visit.


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