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

Thursday, January 26, 2012

The GUIDE: Hansen's Disease - The surprising neural basis of leprosy [Full link to blog for email clients.][FT:C44]

This post is partially a reprise of a blog from last spring written to coincide to my visit to the former leper colony on the island of Molokai.  It is reposted today in the context of "Neural Diseases and Disorders as Plot Devices" due to prior use in Science Fiction / Fantasy (The Chronicles of Thomas Covenant).

Leprosy – Hansen's Disease – is an infection by Mycobacterium leprae and/or Mycobacterium lepromatosis.  Despite the usual misconception, leprosy 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.

In advanced stages, the mycobacterium attacks the cartilage and tissue around the joints, eventually damaging the bone.  The resulting shortening of bones and joints in the hand and feet gives the appearance of loss of the digits even though amputation would only occur if the digit was gangrenous.  The very severe, nonhealing lesions as well as 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 secondary infections can result in gangrene which in turn requires amputation; thus any limbs lost to leprosy usually result from factors other than the disease itself.

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.  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?


  1. So, to a degree, it's an autoimmune disease. Fascinating.

    Pam Uphoff

  2. Psoriasis is one of the oldest and well known skin diseases in the world, and it is also one of the most underrated diseases too. This article looks at the history of psoriasis.

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