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Leprosy, a chronic granulomatous infection caused by a bacterium which affects various parts of the body, including in particular the skin and nerves. (Granulomatous refers to the formation of granulomas, inflammatory nodules that are usually small, granular, firm, and persistent.) The bacterium responsible for leprosy is called Mycobacterium leprae or, for short, M. leprae.

M. leprae is an obligate parasite that has to live within cells. There it is able to withstand the onslaught of enzymes and other forces by virtue of possessing a peculiarly resistant waxy coat and thanks also to its association with lowered cellular immunity.

For thousands of years, leprosy was one of the world’s most feared communicable diseases, because the skin and nerve damage often led to terrible disfigurement and disability. (In ancient sources such as the Bible, the term “leprosy” was used to describe a number of cutaneous diseases, especially those of a contagious and chronic nature, probably including psoriasis.)

The classic clinical form of leprosy is called anesthetic leprosy. It chiefly affects nerves. The condition is marked initially by hyperesthesia (excess sensation) succeeded by anesthesia (lack of feeling) and by paralysis, ulceration, and various other problems, ending horribly in gangrene and self-mutilation.

India accounts for almost four-fifths (nearly 80%) of all cases of leprosy in the world. The World Health Organization (WHO) recorded 800,000 new leprosy patients around the world during the year 1998- 99. Half of the world’s leprosy cases are now found in the five Indian states of Bihar, West Bengal, Uttar Pradesh, Madhya Pradesh and Orissa.

Today leprosy can be cured, particularly if treatment is begun early. The treatment of choice is a multidrug therapy (MDT) using diaphenylsulfone (Dapsone), rifampicin (Rifadin), and clofazimine (Lamprene). Surgery can reconstruct damaged faces and limbs.

Over millennia the leprosy bacterium has undergone “massive gene decay” — the loss of many genes and therefore it has largely lost the ability to adapt. The gene sequence of M. leprae has been compared with that of its close relative M. tuberculosis, which causes TB. M. tuberculosis has clearly succeeded in holding onto many more of its genes.

M. leprae has listless habits. It divides more slowly than any other known bacterium, just once every two weeks — compared with once every 40 minutes for E. coli. M. leprae also is finicky in the laboratory with its growth requirements still poorly defined.

When M. leprae infect a person, the bacteria are first engulfed by white blood cells but not destroyed. Instead, the white cells conveniently carry the bacteria to their new home inside cells called macrophages, which are also part of the immune system. The bacteria then invade certain types of nerve cells, eventually causing the skin lesions and loss of sensation characteristic of leprosy.

Living inside the macrophages keeps M. leprae safe from the immune system, but it also stops it from regularly exchanging DNA with other M. leprae. As a result the bacterium has lost more genes to random mutation than any organism analysed so far, keeping only the few genes that it needs to survive. Many of the lost genes were involved in metabolism. This explains why M. leprae grows so slowly. It is almost in a state of permanent starvation.

The term Hansen disease instead of leprosy is now preferred by some experts, because of it being less perjorative. Hansen disease was named in honor of the Norwegian physician, Gerhard Armauer Henrik Hansen, who in 1873 discovered the bacillus Mycobacterium leprae, the first microbe found to be the causative agent of a human disease. Hansen’s discovery preceded Robert Koch’s demonstration of the bacterial cause of anthrax by 3 years. Hansen’s research helped to establish fundamental principles in microbiology, immunology, and public health.

The word “leprosy” comes from the French “lepre” from the Greek “lepros” meaning scaly. Only people and the nine-banded armadillo are susceptible to leprosy.