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Armadillos and Their Role in Treating Leprosy
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The nine-banded armadillo (Courtesy of Colorado State University)
| Introduction
Human Health Concern
Reliance on an Animal Model: The Armadillo
Health Benefits of These Studies
References
Glossary
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Introduction
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If it weren’t for an unusual mammal, the nine-banded armadillo (Dasypus novemcinctus), very little progress would have been made in collecting scientific data on leprosy, or Hansen’s disease. The cause of the disease is a bacterium, Mycobacterium leprae (M. leprae), which multiplies very slowly—about once every two weeks--and has never been grown in any acceptable bacteriological medium or in cell culture. Many different animal species were tested to find a suitable model for leprosy research, and armadillos were one of the few that could be infected with M. leprae.
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Human Health Concern
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A scourge since pre-biblical times, leprosy is a chronic disease that produces sores on the skin and mucous membranes and infects nerves, producing loss of sensation in the affected areas. Clinically, the disease appears as two types. In the more virulent form, or lepromatous leprosy, the numerous sores contain many bacteria; in the milder form, or tuberculoid leprosy, fewer sores appear because of the body’s immune response to the infection. Signs of the disease often do not appear for many years. While leprosy is rarely fatal, it can cause permanent disfigurements (1).
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Reliance on an Animal Model: The Armadillo
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The armadillo’s susceptibility to leprosy was first discovered in the laboratory in the late 1960s by Eleanor E. Storrs (2) and developed as a model for disease by W. F. Kirchheimer (3). Prior to that time, the only source of organisms for study was the tissues of patients diagnosed with the disease. Because there was no established in vitro culture method--and this is still true--there was an urgent need to find an animal that could produce enough organisms to permit basic research and to test anti-leprosy drugs for their effectiveness in treating people.
The disease in armadillos inoculated with human tissue is widespread and extensive. Following inoculation of infected tissue, an armadillo may develop a systemic infection that provides tissues and organs with large numbers of bacilli. Since the leprosy bacillus concentrates in the body’s extremities (e.g., fingers, ears) in people, scientists suggested that the armadillo’s susceptibility to leprosy was very likely due to its unusually low body temperature of 90 degrees Fahrenheit (33 degrees Celsius), which is more than 8 degrees lower than the temperature of humans and other mammals (4).
In 1975, scientists detected leprosy in wild armadillos by examining sections of ear tissue for bacilli. They found infected armadillos in wide-ranging areas across the south-central regions of the U.S., especially Texas and Louisiana (5,6). Nonetheless, studies have shown that the risk of human infection from contact with armadillos is unlikely. However, scientists are still not sure how leprosy is spread among the human population.
From the 1970s to the present, the armadillo has provided researchers with large amounts of bacilli that were extracted and purified for further study. Scientists also prepared a substance called lepromin, which is used to predict susceptibility to leprosy and to follow the course of the disease in an individual. The availability of the leprosy bacilli has permitted extensive biochemical studies, attempts to develop a vaccine, and the characterization of genetic information from the bacterial DNA of the organism (7).
In the 1990s, scientists developed an experimental vaccine composed of killed organisms in an attempt to eliminate leprosy as a public health problem; however large-scale trials indicated that it was only partially effective (8). While the armadillo remains an established source for the bacillus, immunologically deficient, or nude mice, have since become the animal of choice for testing new drugs because the organisms are more physiologically active in this animal species (9).
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Health Benefits of These Studies
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Thirty years ago, the number of people worldwide with leprosy was estimated to be between ten to twelve million. Since the introduction of multi-drug treatment, the number of new cases reported in 2004 has been reduced to 410,000. Despite recent efforts to eradicate the disease, full control of leprosy has eluded populations in Southeast Asia, India, South America, and other tropical areas of the world. New drugs may still be needed to control new cases resistant to current drug treatment. (10).
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References
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- “Leprosy” – MSN Encarta Online Encyclopedia 2006 an article contributed by James l. Krahenbuhl, Gillis W. Long Hansen’s Disease Center.
- Storrs, E. E. 1971. The nine-banded armadillo: a model for leprosy and other medical research Int. J. Lepr. 39:703-14.
- Kirchheimer, W. F. and Storrs, E. E. 1971. Attempts to establish the armadillo (Dasypus novemcinctus, Linn.) as a model for study of leprosy Int. J. Lepr. 39:693-702.
- Grice, Gordon 2000. Where Leprosy Lurks Discover Vol. 21 No. 11http://www.discover.com/issues/nov-00/features/featprosy/?page=2
- Walsh, G.P. et al. 1986 Naturally acquired leprosy in the nine-banded armadillo: a decade of experience 1975-1985 J. Leukoc. Biol. 40(5):645-56.
- Truman, Richard W. “Human and Armadillo Leprosy” http://svm369.vetmed.lsu.edu/truman2.htm
- Leprosy – Hansen’s Disease 2005 http://www.niaid.nih.gov/dmid/leprosy/default.htm
- Ishii, Norihisa, 2003 Recent advances in the treatment of leprosy Dermatology Online Journal 9:5.
- Scollard. D. M. et al. 2006 The continuing challenges of leprosy Clin. Microbiol. Rev. 19:338-81.
- Leprosy 2006 World Health Organization Fact Sheet No. 101 http://www.int/mediacentre/factsheets/fs/101/en
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