Chapter 29 Hansen’s disease (leprosy)
1. What causes leprosy?
2. Why is leprosy called Hansen’s disease?
Leprosy is referred to as Hansen’s disease to honor Gerhard Armauer Hansen, a Norwegian physician, who discovered the leprosy bacterium in 1873. Mycobacterium leprae was the first bacillus to be associated with a human disease. In addition, as with acquired immune deficiency syndrome, there are considerable social stigmata associated with having leprosy. Therefore, rather than referring to patients as lepers and the disease as leprosy, it is often preferable to use the term Hansen’s disease.
3. How is leprosy transmitted?
Although the mode of transmission is still uncertain, current evidence favors respiratory transmission. Average incubation times are 2 to 5 years for tuberculoid leprosy and 8 to 12 years for lepromatous cases. Evidence for congenital and percutaneous transmission has been presented, but these are probably rare.
4. Are children and adults equally susceptible to acquiring leprosy?
Children and young adults seem to be most susceptible to acquiring leprosy. Only about 5% of adults at risk, such as marriage partners, develop leprosy. As many as 60% of children who have a parent with leprosy develop the disease. M. leprae may be present in breast milk; there is some evidence to suggest that the infection can be transmitted via the placenta.
5. Are humans the only host for M. leprae?
It was once believed that humans were the only natural reservoir for M. leprae. More recently, four other animals have been shown to carry the infection: the nine-banded armadillo, chimpanzee, sooty mangabey monkey, and cynomolgus macaque. Up to 10% of wild armadillos in Louisiana and eastern Texas have naturally acquired leprosy.
7. How common is leprosy?
There were about 2.7 million persons with leprosy worldwide in 1994, with 600,000 new cases yearly. Leprosy is endemic in 53 countries, including India, Bangladesh, and Brazil. There are approximately 6,500 cases of leprosy in the United States. Around 200 to 250 new cases are reported each year in the United States, with about 175 of these being new cases diagnosed for the first time; 166 new cases were reported in the U.S. in 2005.
9. How is leprosy recognized clinically?
The three most useful features are skin lesions, areas of cutaneous anesthesia, and thickened nerves. Other features are nasal stuffiness, inflammatory eye changes, and loss of eyebrows. A reactional state in lepromatous leprosy causes multiple tender red nodules resembling erythema nodosum (erythema nodosum leprosum).
10. Is there more than one kind of leprosy?
Four major variants of leprosy represent a spectrum of disease: indeterminate leprosy, tuberculoid leprosy, lepromatous leprosy, and dimorphous (or borderline) leprosy (Fig. 29-1). The lepromatous form is seen twice as frequently in men as in women. Although it varies from country to country, about 90% of the leprosy cases in the United States are of the lepromatous type.
11. Does indeterminate leprosy mean that you do not know what type it is?
No. Indeterminate leprosy is believed to be the very first sign of infection with the leprosy bacillus. It usually manifests as a solitary macular skin lesion that is vaguely defined and either erythematous or hypopigmented. Patients with indeterminate leprosy may clear spontaneously or progress to one of the other three types of leprosy.
12. What are the two “polar” forms of leprosy? How do they differ?
Tuberculoid leprosy and lepromatous leprosy are considered the two polar forms, and they tend to remain stable clinically. Patients with tuberculoid leprosy have a high degree of immunity against M. leprae and have few skin lesions and few organisms in their skin. Patients with lepromatous leprosy have low immunity against M. leprae and have many skin lesions and millions of organisms in their skin (Table 29-1).
13. Describe dimorphous leprosy.
Dimorphous leprosy, also known as borderline leprosy, shows features intermediate between tuberculoid and lepromatous leprosy. It is a less stable form of leprosy, and its clinical features and immune status may change over time. If dimorphous leprosy develops more features of lepromatous leprosy, it is referred to as dimorphous-lepromatous. If it develops features of tuberculoid leprosy, it is called dimorphous-tuberculoid.
14. What is the unusual feature of the cell-mediated immunity in lepromatous leprosy?
Patients with lepromatous leprosy have a specific anergy to M. leprae. This is in contrast to diseases such as sarcoidosis and Hodgkin’s lymphoma, in which there is loss of immunity to a wide variety of antigens. The clinical spectrum of leprosy appears to depend mainly on an individual’s ability to develop effective cell-mediated immunity against M. leprae. In endemic areas, most individuals appear to be completely resistant to infection with the leprosy bacillus.
