The
route of transmission of leprosy remains uncertain and may be multiple; nasal
droplet infection, contact with infected soil, and even insect vectors have
been considered the prime candidates. Aerosolized M. leprae can cause infection
in immunosuppressed mice, and a sneeze from an untreated lepromatous patient
may contain >1010 AFB. Furthermore, both IgA antibody to M. leprae and genes of M. leprae—demonstrable by
polymerase chain reaction (PCR)—have been found in the nose of individuals
without signs of leprosy from endemic areas and in 19% of occupational contacts
of lepromatous patients.
Several
lines of evidence implicate soil transmission of leprosy: (1) in endemic
countries such as India, leprosy is primarily a rural and not an urban disease;
(2) M.
leprae products
have been demonstrated to be resident in soil in endemic locales; and (3)
direct dermal inoculation (e.g., during tattooing) may transmit M. leprae, and common sites of
leprosy in children are the buttocks and thighs, suggesting that microinoculation
of infected soil may transmit the disease.
Evidence
for insect vectors of leprosy includes the demonstration that bedbugs and mosquitoes in the vicinity of
leprosaria regularly harbor M. leprae and that experimentally infected mosquitoes can transmit infection
to mice. Skin-to-skin contact is generally not considered an important route of
transmission.
In
endemic countries, ±50%
of leprosy patients have a history of intimate contact with an infected person
(often a household member), while, for unknown reasons, leprosy patients in
nonendemic locales can identify such contact only 10% of the time. Moreover,
household contact with an infected lepromatous case carries an eventual risk of
disease acquisition of ±10%
in endemic areas as opposed to only 1% in nonendemic locales. Contact with a
tuberculoid case carries a very low risk. Physicians and nurses caring for
leprosy patients and the coworkers of these patients are not at risk for leprosy.
M.
leprae causes
disease primarily in humans. However, in Texas and Louisiana, 15% of
nine-banded armadillos are infected, and armadillo contact occasionally results
in human disease; armadillos develop
a disseminated
infection following intravenous inoculation of live M. leprae.
Source:
Harrison_s_Principles_of_Internal_Medicine_16th_Edition
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