Leprosy Research Using Armadillos

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Why are armadillos used for research?

Odd though it may seem, armadillos might someday help cure leprosy. Researchers have found that the core body temperature of the armadillo is low enough to favor the growth of the leprosy-causing bacterium Mycobacterium leprae. While this microorganism has been grown in other types of animal tissue, no animal model had previously been found that regularly contracted the most virulent form of the disease (lepromatous leprosy). Because the bacillus only tends to grow in cooler parts of the body, such as the feet, nose and ears, large amounts of bacteria could not be grown (attempts to grow the microorganism in vitro have not been successful). The armadillo, however, has a lower body temperature than most mammals, resulting in rapid development of the disease following inoculation. Because of the armadillo, scientists have been able to develop a vaccine against leprosy. The nine-banded armadillo has become the principal source of M. leprae in biochemical and immunological research.

although there has been some concern about humans contracting leprosy from wild armadillos, there have been no documented cases of this occurring. Also, the incidence of leprosy in wild armadillos is apparently low; very few wild armadillos have been shown to have contracted leprosy. Tight regulation of experimental animals is the standard procedure for any laboratory working with infectious diseases, so short of a misguided animal rights break-in at an armadillo research facility, no experimental animals infected with leprosy will be able to spread the disease among the wild populations.

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The following is an excerpt from the World Health Organization infectious diseases web page.

Please note that this information is at least two years old. It has been so long since I posted this that I really should attempt to find some updated information. I would like to condense some research information into a format that would make it more accessible to most of the visitors to my page. Scientific writing tends to be a little dry, and not easily understandable unless the reader is working in the same field.

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Mycobacterium leprae

The aetiological agent of leprosy is Mycobacterium leprae. It is a strongly acid-fast rod-shaped organism with parallel sides and rounded ends. In size and shape it closely resembles the tubercle bacillus. It occurs in large numbers in the lesions of lepromatous leprosy, chiefly in masses within the lepra cells, often grouped together like bundles of cigars or arranged in a palisade. Chains are never seen. Most striking are the intracellular and extra-cellular masses, known as globi, which consist of clumps of bacilli in capsular material. Under the electron microscope the bacillus appears to have a great variety of forms. The commonest is a slightly curved filament 3-10 m in length containing irregular arrangements of dense material sometimes in the shape of rods. Short rod-shaped structures can also be seen (identical with the rod-shaped inclusions within the filaments) and also dense spherical forms. Some of the groups of bacilli can be seen to have a limiting membrane.

It is believed that only leprosy bacilli which stain with carbol-fuchsin as solid acid-fast rods are viable and that bacilli which stain irregularly are probably dead and degenerating. The differences are valuable pointers in biopsy specimens to the effects of treatment. In patients receiving standard multidrug therapy (MDT), a very high proportion of bacilli are killed within days, which suggests that many of the manifestations of leprosy, including reactions of the erythema nodosum type, which follow initial treatment, must be due in part to antigens from dead organisms rather than living bacilli. We therefore need drugs which will help the body to dispose of dead but still intact leprosy bacilli.

Culture in vitro

Claims of successful culture have been made in the past but none have been substantiated and M. leprae has not yet been successfully cultured in vitro. There have been many reports of cultivation in artificial media of acid-fast bacilli from the skin or other tissues of leprosy patients and many authors have claimed such bacilli to be true leprosy bacilli, but no satisfactory evidence of this has been produced. Most of the organisms isolated in culture from lepromatous tissues appear to be mycobacteria related to M. avium complex.

Culture in vivo

Normal mice

The mouse foot-pad inoculation method developed by Shepherd is still the chief method of culture in vivo. Inoculation of 104 bacilli into the hind foot-pads yields 106 bacilli after five to six months although no clinical disease develops. During the logarithmic phase the mean generation time is 10-20 days which is consistent with the natural history of disease in man and is responsible for the long time of several months taken to measure multiplication in the foot-pad test. No subsequent local increase in bacterial numbers takes place and the bacilli slowly degenerate.

Immunologically deficient mice

Rees has developed an experimental lepromatous leprosy model in animals by inoculating thymectomized irradiated (TR) mice. The generation time remained unchanged but the bacilli continued to multiply until 108-109 bacilli per foot-pad were obtained after nine to 12 months. The histological picture is that of lepromatous leprosy and numerous bacilli can be found in the liver and spleen although the main spread is to the nose, tail, front paws and ears. The TR mouse has been used to detect small numbers of viable organisms (three to ten viable out of an inoculum of 105) and is used to detect 'persisters' after 12 months.

The mouse foot-pad has been used to test the minimum concentration of drugs necessary and the sensitivity of the bacilli to new drugs. It is a valuable tool to measure drug resistance in patients.

The nine-banded armadillo (Dasypus novemcinctus)

An important development has been the discovery that the nine-banded armadillo can be infected with M. leprae and this animal has become the main source of M. leprae for biochemical and immunological research including development of a vaccine. The armadillo has a primitive immunological system and a lower body temperature. Intravenous inoculation produces widespread disseminated disease with yields from the liver and spleen reaching 1012 organisms per gram of tissue.


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