A closer look at the mycobacterium leprae

A mycobacterium

Leprosy is an infectious disease caused by mycobacterium leprae, also known as Hansen's Bacillus. Hansen was a Norwegian doctor who discovered the bacillus in 1873. It has not been possible up to now to cultivate the bacillus in an artificial environment. This slows down the study of the disease. However, it is possible to grow it by inoculation into the sole of mice and wild armadillos (Dacypus Novemcinctus). The armadillo is a small mammal living in the south of the United States and Mexico. With the exception of the armadillo, where the disease is endemic, and some species of monkey, leprosy appears to be an exclusively human disease. The bacillus is resistant to acids and alcohol, 1 to 8 mm long and 0.3 wide. It takes the form of small rods that take the classic red colour after Ziehl-Neelsen staining. It is an obligatorily intra-cellular parasite that mainly infects macrophages. In the space of from 11 to 13 days, it will reproduce itself.

Direct contact

Many aspects of leprosy epidemiology and infection are still not very well known. However, it does seem that the microbe is transmitted by direct contact between a sick person and a healthy individual. The exit and entry points of the disease are believed to be mainly respiratory. However, there are descriptions of transcutaneous transmission, especially after scarification or tattooing. From the point of view of transmission, only the fraction of patients infected with lepromatous or multibacilliary leprosy would appear to be of importance. Once a person has become infected, her or his resistance to the disease actually developing will depend upon the individual's natural defences, more particularly immunity of the cellular type. Dependent upon the level of defence in the face of the leprosy bacillus, what will usually happen is that the infected person will never actually develop the disease. Perhaps the patient will develop a couple of the characteristic marks. These may disappear spontaneously in the course of a few weeks. Otherwise, they may develop into a localised, clinical form described as tuberculoid, i.e. limited bacilli or otherwise generalised and described as multi bacilliary.

Characteristics

The bacillus attacks mainly the skin and peripheral nerves. It is at these two levels that clinical symptoms become apparent. It usually happens after an incubation period of a number of years.. The characteristic signs of leprosy which permit its diagnosis in those regions where it is endemic are as follows,

• Stains on the skin that are clear or reddish in colour with a loss of sensibility.
• Attack of peripheral trunk nerves resulting in hypertrophy and loss of sensibility.
• The presence of leprosy bacilli in a smear test of skin fluid

Discovering more about the consequences of the disease.