Leprosy is caused by Mycobacterium leprae. This bacterium shares many common characteristics with M. tuberculosis. Both are acid-fast bacilli, containing large amounts of lipids in their cell walls. Both induce hypersensitivity, and both multiply slowly. The incubation period of M. leprae is several years to more than twenty. Under optimal conditions, it can take fourteen days to double and a year to multiply sufficiently to cause disease. Historically, leprosy and tuberculosis rank among the most devastating of all human diseases, with an estimated figure of beween ten and twelve million people suffering from leprosy alone.
It is speculated that leprosy is transmitted from person to person living under poor sanitary conditions. The bacterium may gain entry through the respiratory tract or skin lesions since M. leprae is found in enormous numbers of certain lesions of infected persons. It was this feature that allowed Gerhard Hansen in 1873 to make the first reliable association between a bacterial agent and a human disease. Since then, leprosy has often been referred to as “Hansen’s disease,” partly to honor the scientist’s discovery, and partly to soften the stigma of the name ‘leprosy.’ Although the Bible mentions leoprosy, archaeologists have found no skeletal remains before 500 CE to indicate it is the same disease.
There are two forms of leprosy – lepromatous and tuberculoid.
The lepromatous form is more severe, characterized by large nodular lesions. Since the immune response is impaired in this form, the formation of granulation tissue or scarring is limited.
The tuberculoid form is less severe, and is associated with a normal immune response that causes granulation-type lesions. Where the bacilli in the lesions are few, tissue damage is less, and response to therapy is better. Forms between the two are also seen, and all cause crippling disfigurement.
The major growth of the organism occurs in the low-temperature areas of the body as in the nose, ears, and skin of the extremities. The leprosy bacilli are easily phagocytized, but not destroyed, and large numbers grow inside macrophages. Nerves are uniquely susceptible to infection, and early symptoms are often associated with a lack of sensation (anesthesia) in areas of the body. Even though the disease is an ancient one, its development is still not totally understood. It is thought that the tissue destruction results from a combination of massive accumulation of bacteria, neurological damage, and immune reactions.
Although the leprosy bacterium is often found in great numbers in infected tissue, it has not been possible to cultivate it artificially. Such bacteria as Mycobacterium leprae can grow only within certain types of eukaryotic host cells. Some growth has occurred when injected into the foot pad of a mouse. It is now known that armadillos may be a natural non-human host to the bacterium.
The disease is still a major problem worldwide. It is estimated that only 10% of those afflicted are under any form of treatment. Treatment involves antimicrobial agents called sulfones, which are related to the sulfonamides. They are fairly effective in arresting the progression of the lesions, allowing them to heal. They also render the patients non-infectious, which allows them to return to society sooner as outpatients. Treatment can last for months or years, and it is doubtful that a complete cure is ever obtained. The use and effectiveness of the antibiotic rifampin is still under investigation. Other antituberculosis drugs are not effective against leprosy. There are also reports of increasing bacterial resistance to the leading antileprosy drug, dapsone. It is thought that the lipids in the cell walls tend to render the bacteria highly resistant to methods of inactivation like dyhydration, disinfectants, and other environmental factors – not to mention many antimicrobials.
An unusual vaccine trial took place in South America. The vaccine was given to patients who already had the disease. It is a vaccine where BCG is mixed with killed cells of M. leprae, which is apparently able to increase the patients’ immune response to the active disease. Because the disease progresses slowly, it is possible to give this vaccine after onset of the disease and still have time for to stimulate the immune system.
Preventative measures include avoiding poor sanitary conditions. This is rarely an option for those who must exist in such conditions, especially young children who are more susceptible than adults and, therefore, more prone to the disease. Tatoo parlors in endemic areas have been known to transmit the disease on contaminated needles. Diet is another significant factor. A poor or inadequate diet depletes the immune system, allowing for pathogen growth.