How Leprosy (Hansen's Disease) is Contracted
Leprosy is transmitted primarily through respiratory droplets from prolonged close contact with an untreated infected person, though the disease has low pathogenicity despite being highly infective. 1
Primary Transmission Route
- Respiratory droplet transmission is the main mode of spread, occurring when an infected person coughs or sneezes, releasing Mycobacterium leprae into the air that can be inhaled by others during prolonged close contact. 1, 2
- The upper respiratory mucosa serves as both a portal of entry and a site of bacterial colonization, making respiratory transmission the dominant pathway. 1, 3
Secondary Transmission Routes
- Broken skin-to-skin contact represents an alternative but less common transmission route, particularly when there is direct contact with skin lesions of an infected individual. 2, 4
- The bacteria require entry through compromised skin barriers rather than intact skin. 2
Environmental Exposure
- Exposure to infected nine-banded armadillos has been documented as a zoonotic source of infection, particularly in endemic areas of the United States such as Texas, Louisiana, and California. 5, 2
- This represents a unique non-human reservoir that can transmit the disease to humans through direct contact or handling of infected animals. 5
Important Transmission Characteristics
- Less than 5% of people exposed to M. leprae actually develop clinical disease, indicating that most individuals have natural immunity and the bacterium has relatively low pathogenicity despite high infectivity. 1, 6
- Prolonged close contact is required for transmission—casual or brief contact does not typically result in infection. 1
- The bacteria have a predilection for cooler areas of the body (skin, peripheral nerves, upper respiratory mucosa), which influences both transmission patterns and clinical manifestations. 5, 7
Critical Pitfalls to Avoid
- Do not assume leprosy is highly contagious—historical stigma incorrectly portrayed the disease as extremely transmissible, when in reality it requires prolonged exposure and most people are naturally resistant. 1, 6
- Do not overlook travel and immigration history—leprosy can present in non-endemic areas due to global migration, and clinicians in regions like South Dakota must maintain awareness despite rarity. 4
- Do not dismiss armadillo exposure in endemic U.S. regions—this zoonotic source is a legitimate transmission route that should be explored in patient histories. 5, 2