How Leprosy is Transmitted
Primary Transmission Route
Leprosy is transmitted primarily through prolonged close contact with untreated multibacillary patients via inhalation of respiratory droplets containing Mycobacterium leprae from the upper airways, with the nasal mucosa serving as both the main entry and exit route for the bacilli. 1, 2
Human-to-Human Transmission
Respiratory droplet transmission occurs when susceptible and genetically predisposed individuals have prolonged close contact with untreated multibacillary leprosy patients who release bacilli through coughing and sneezing 1, 2
The nasal mucosa is the primary portal of entry and exit for M. leprae, with the bacillus invading the mucosa through the nasal inferior turbinate to reach peripheral blood 2, 3
Direct contact with broken skin may also serve as a transmission route, though this is less common than respiratory transmission 4
Person-to-person transmission requires prolonged and close contact—casual or brief contact does not typically result in transmission 2
Zoonotic Transmission
In the Americas, the nine-banded armadillo (Dasypus novemcinctus) serves as a natural host and reservoir of M. leprae, with identical strains shared between humans and armadillos 5
Contact with armadillos is a recognized risk factor for leprosy, and in the USA and Brazil, leprosy is classified as a zoonosis 6, 5
In the British Isles, red squirrels (Sciurus vulgaris) have been found to harbor both M. leprae and M. lepromatosis DNA, suggesting additional potential animal reservoirs 5
Key Transmission Characteristics
M. leprae has low pathogenicity and low virulence despite being highly infective, meaning most exposed individuals do not develop clinical disease due to natural immunity 1
The disease requires genetic susceptibility in the host—not everyone exposed will develop leprosy even with prolonged contact 2
Untreated multibacillary patients pose the highest transmission risk, as they harbor large numbers of bacilli in their nasal secretions 2, 3
Critical Clinical Pitfalls
Unknown leprosy contact has been reported in many leprosy-affected individuals, suggesting that transmission pathways are more complex than previously understood and may involve environmental or animal reservoirs 6, 5
The steady global incidence of leprosy despite widespread multidrug therapy suggests that human-to-human transmission alone does not explain all cases—zoonotic and possibly environmental reservoirs play a role 5
Healthcare workers should inquire about both human contact with untreated leprosy patients AND occupational or recreational exposure to armadillos (in the Americas) or other potential animal reservoirs 6, 5