11 July 2019
This is part of a series of posts from our own Shane Hanlon’s disease ecology class that he’s currently teaching at the University of Pittsburgh Pymatuning Laboratory of Ecology. Students were asked to write popular science posts about (mostly) wildlife diseases. Check out all the posts here.
By Cameron Gill
Repeatedly referenced throughout the Bible, leprosy, also known as Hansen’s Disease, may often be perceived by the general public to be an ancient disease that has ceased to endanger the modern world. Much to the misfortune of people living in Africa, Brazil, India, and the Philippines, where the majority of outbreaks occur, nearly 700,000 people throughout the globe annually contract leprosy. Today the disease is largely endemic to these regions and incidence is extremely low among those living in the West, but the bacterium which causes leprosy, Mycobacterium leprae, still persists in the United States in nine-banded armadillo reservoirs.
Each year approximately 200 Americans develop Hansen’s Disease with most contractions occurring in citizens who have traveled abroad to the aforementioned countries in which leprosy remains disproportionately prevalent compared to other parts of the globe. The cases occurring in Americans who were not exposed to infected individuals overseas are mostly concentrated in southern states such as Louisiana and Texas. Not coincidentally, this region is where the majority of nine-banded armadillos live within the United States. With an inability to hibernate, a lack of adequate fat reserves, and a low metabolic rate, these shell-covered creatures are confined to warm climates such as that of Texas, whose state mammal is the nine-banded armadillo. With a body temperature of about 90 degrees Fahrenheit, the nine-banded armadillo is internally cooler than most mammals and provides the stable environment needed by Mycobacterium leprae to persist. In some southern regions of the United States, up to 20% of armadillo populations harbor Mycobacterium leprae. Most nine-banded armadillos fail to live long enough to exhibit symptoms from the bacterium however, so hunters who target them are most often unaware if they are infected with Mycobacterium leprae.
The precise route of leprosy transmission is not known, but some scientists believe that the disease spreads via direct exposure to infected fluids or indirect exposure via airborne viral droplets. Indirect airborne transmission can result from the cough of an infected individual and may be the primary route by which Hansen’s Disease is spread throughout countries where leprosy outbreaks are most common. Direct transmission of the zoonotic disease may result from handling the carcass of an infected nine-banded armadillo and is the most probable mechanism by which leprosy transmission can domestically occur in the United States.
When established in human hosts, Mycobacterium leprae mainly targets nerves, skin, and mucous membranes. Early symptoms of infection include dermal discoloration, ulcer formation, loss of tactile sensation, and muscle weakness. If untreated, Hansen’s Disease may result in paralysis, disfigurement, permanent ulcers, blindness, organ failure, and death. The CDC estimates that untreated leprosy has resulted in 2 million people becoming permanently disabled. Thankfully for its human hosts, the bacterial infection can be effectively treated with an antibiotic protocol, but antibiotics are often not widely available in the developing countries with the highest rates of leprosy incidence.
When freely living outside a host, Mycobacterium leprae is poorly adapted to survive and cannot reliably be cultured in laboratory conditions. This poor external survival may have resulted from reductive evolution of the bacterium’s genome. Scientists have found that many of the genes necessary to persist outside of a host for any significant duration, particularly those regulating metabolism, have been eliminated from Mycobacterium leprae’s genome. This evolutionary change may have led to decreased transmissibility over time. Humans and nine-banded armadillos are the only hosts in which Mycobacterium leprae is known to persist. Further confining its possible host range is the immunity to the bacterium that is possessed by the vast majority of humans. Research has shown that 95% of the population cannot contract Hansen’s Disease due to innate genetic immunity to Mycobacterium leprae infection.
Unlike nine-banded armadillos, leprosy is not native to the Western Hemisphere. The disease is thought to have been introduced to the Americas by European colonization where it then spread to nine-banded armadillos. While a sizable minority of nine-banded armadillos are presently infected with Mycobacterium leprae, domestic transmission to Americans is extremely rare, and no significant risk of a major leprosy outbreak exists in the United States.