Pathophysiology
The mechanism of transmission of leprosy is prolonged close contact and transmission by nasal droplet. In addition to humans, leprosy has been observed in nine-banded armadillo, (which, it has recently been confirmed, are among the primary sources of new cases of leprosy in Americans), and three species of primates. The bacterium can also be grown in the laboratory by injection into the footpads of mice. There is evidence that not all people who are infected with M. leprae develop leprosy, and genetic factors have long been thought to play a role, due to the observation of clustering of leprosy around certain families, and the failure to understand why certain individuals develop lepromatous leprosy while others develop other types of leprosy. It is estimated that due to genetic factors, only 5% of the population is susceptible to leprosy. This is mostly because the body is naturally immune to the bacteria, and those persons that do become infected experience severe allergic reactions to the disease. However, the role of genetic factors is not entirely clear in determining this clinical expression. In addition, malnutrition and prolonged exposure to infected persons may play a role in development of the overt disease.
The most widely held belief is that the disease is transmitted by contact between infected persons and healthy persons. In general, closeness of contact is related to the dose of infection, which in turn is related to the occurrence of disease. Of the various situations that promote close contact, contact within the household is the only one that is easily identified, although the incidence among contacts and the relative risk for them appear to vary considerably in different studies. In incidence studies, infection rates for contacts of lepromatous leprosy have varied from 6.2 per 1000 per year in Cebu, Philippines to 53 per 1000 per year in part of Western India to 55.8 per 1000 per year in a part of Southern India.
Two exit routes of M. leprae from the human body often described are the skin and the nasal mucosa, although their relative importance is not clear. Lepromatous cases show large numbers of organisms deep in the dermis, but whether they reach the skin surface in sufficient numbers is doubtful. Although there are reports of acid-fast bacilli being found in the desquamating epithelium (sloughing of superficial layer of skin) of the skin, Weddell et al. had reported in 1963 that they could not find any acid-fast bacilli in the epidermis, even after examining a very large number of specimens from patients and contacts. In a recent study, Job et al. found fairly large numbers of M. leprae in the superficial keratin layer of the skin of lepromatous leprosy patients, suggesting that the organism could exit along with the sebaceous secretions.
The importance of the nasal mucosa was recognized as early as 1898 by Schäffer, in particular that of the ulcerated mucosa. The quantity of bacilli from nasal mucosal lesions in lepromatous leprosy was demonstrated by Shepard as large, with counts ranging from 10,000 to 10,000,000. Pedley reported that the majority of lepromatous patients showed leprosy bacilli in their nasal secretions as collected through blowing the nose. Davey and Rees indicated that nasal secretions from lepromatous patients could yield as much as 10 million viable organisms per day.
The entry route of M. leprae into the human body is also not definitively known: The skin and the upper respiratory tract are most likely. While older research dealt with the skin route, recent research has increasingly favored the respiratory route. Rees and McDougall succeeded in the experimental transmission of leprosy through aerosols containing M. leprae in immune-suppressed mice, suggesting a similar possibility in humans. Successful results have also been reported on experiments with nude mice when M. leprae were introduced into the nasal cavity by topical application. In summary, entry through the respiratory route appears the most probable route, although other routes, particularly broken skin, cannot be ruled out. The CDC notes the following assertion about the transmission of the disease: "Although the mode of transmission of Hansen's disease remains uncertain, most investigators think that M. leprae is usually spread from person to person in respiratory droplets."
In leprosy, both the reference points for measuring the incubation period and the times of infection and onset of disease are difficult to define, the former because of the lack of adequate immunological tools and the latter because of the disease's slow onset. Even so, several investigators have attempted to measure the incubation period for leprosy. The minimum incubation period reported is as short as a few weeks and this is based on the very occasional occurrence of leprosy among young infants. The maximum incubation period reported is as long as 30 years, or over, as observed among war veterans known to have been exposed for short periods in endemic areas but otherwise living in non-endemic areas. It is generally agreed that the average incubation period is between three and five years.
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