EPIDEMIOLOGY

Leprosy is an ancient disease that has been the cause of great morbidity and mortality for centuries. The causative agent, Mycobacterium leprae, is an unculturable, obligate intracellular, gram-positive, acid-fast bacillus. It multiplies very slowly in the host and grows best at 33°C (91.4°F), which accounts for its predilection for cooler parts of the body such as the skin, testis, anterior segment of eye, mucous membranes of nasal passages, and ear lobes and extremities.

Leprosy is endemic in a number of regions, mainly in Asia, Africa, South America, and the Pacific. It is especially prevalent in India and Brazil. Isolated pockets of disease are found in many parts of the world, and as a consequence of international travel, affected individuals may be encountered in any location. In the United States, infected patients may be found in any state, but most are in California, Hawaii, Florida, Texas, and Louisiana. Most cases are seen in immigrants born in endemic regions.

Worldwide, the overall number of registered cases (prevalence) of leprosy has fallen as a result of multidrug therapy (MDT), but the number of newly registered cases (incidence) remains relatively constant, suggesting that current therapy strategies have not had an appreciable impact on transmission. The primary mechanism of transmission is thought to be via nasal inhalation of aerosolized organisms. Mycobacterium leprae cannot breach intact skin. Armadillos are known to harbor M. leprae, and a number of cases have been traced to exposure to these animals, although direct transmission of the organism to humans has not been definitively demonstrated.