Hansen’s disease, commonly known as leprosy, is a chronic bacterial infection caused by the bacterium Mycobacterium leprae. It primarily affects the skin, nerves, and upper respiratory tract in humans. For centuries, transmission was understood almost exclusively as human-to-human. However, research established that armadillos represent the only significant non-human reservoir for the bacteria in the world.
Identifying the Carrier Species
The armadillo species overwhelmingly associated with carrying and transmitting Mycobacterium leprae is the Nine-banded Armadillo, scientifically named Dasypus novemcinctus. This species is the most widespread armadillo in the Americas, ranging from the southern United States down through Central and South America. While there are approximately twenty different armadillo species globally, the Nine-banded Armadillo is the one implicated in most documented cases of zoonotic transmission to humans.
The species has been studied extensively since the 1970s, when researchers first successfully infected the animals in a laboratory setting. While other armadillo types, such as the Six-banded Armadillo (Euphractus sexcinctus), have shown some susceptibility, the Nine-banded Armadillo develops a disseminated, systemic infection comparable to the severe form seen in humans. Its role as a reservoir is so well-established that it is the primary animal used for propagating the bacteria for scientific research.
The Unique Biological Link
The reason the Nine-banded Armadillo is uniquely susceptible to M. leprae lies in a specific physiological trait: its unusually low core body temperature. The bacterium is highly fastidious and thrives best at temperatures slightly below normal mammalian core temperatures. The average core body temperature of the Nine-banded Armadillo ranges from approximately 86 to 93 degrees Fahrenheit (30–34 degrees Celsius), a range that aligns perfectly with the optimal growth temperature for M. leprae.
This natural characteristic allows the bacteria to proliferate successfully throughout the armadillo’s tissues, including the liver, spleen, and lymph nodes, leading to a high bacterial load. In contrast, most mammals have a core temperature that is too high for the bacterium to establish and multiply widely. The armadillo’s cool body temperature essentially provides an ideal, living incubator for the pathogen, turning the animal into an effective reservoir for the disease.
Geographic Distribution and Human Cases
Infected armadillos are predominantly found in the southern United States, specifically across the Gulf Coast states and the Southern Central US. States including Texas, Louisiana, Mississippi, Alabama, Georgia, and Florida have documented the presence of M. leprae in wild armadillo populations. The geographic range appears to be expanding northward, following the armadillo’s natural migration pattern, with recent evidence of infected animals found as far north as Missouri.
Within these endemic regions, a significant percentage of the wild armadillo population can be infected, with prevalence estimates exceeding 20 percent in some local areas. The United States sees a low number of leprosy cases annually, typically around 150 to 250 new diagnoses. A notable portion of these cases are considered autochthonous (acquired domestically). Genetic sequencing has confirmed that the strain of M. leprae found in human patients in these states is identical to the strain circulating in the local armadillo population, establishing a clear link between the animal and human health.
Minimizing Transmission Risk
While the risk of contracting Hansen’s disease from an armadillo is low for the general public—since up to 95 percent of people are naturally immune—simple preventative measures are advised in endemic areas. The primary advice is to avoid all direct contact with armadillos, whether the animal is alive or dead. Transmission most often occurs through close, prolonged interaction, such as handling the animals or consuming their meat.
People living in or visiting areas with known infected armadillo populations should refrain from hunting, cleaning, or eating armadillo meat. For outdoor activities like gardening or landscaping where armadillos may burrow, wearing gloves can help reduce the risk of contact with contaminated soil or tissues. Since the bacteria can be present in the armadillo’s skin and tissues, the safest approach is to observe these animals from a distance and prevent pets from interacting with them.