Chapter 37 - The Armadillo Leprosy Model with Particular Reference to Lepromatous Neuritis

Abstract

The nine-banded armadillo (Dasypus novemcinctus) is the only immunologically intact animal that exhibits a high frequency of natural susceptibility to infection with Mycobacterium leprae. This animal is now emerging as a model for neurologic aspects of M. leprae infection. A primary focus of interest has been the immunologic nonresponsiveness of the animals that develop disseminated infection, analogous to human lepromatous leprosy. Highly susceptible armadillos develop weak reactions to killed M. leprae, without granuloma formation, similar to those of human lepromatous patients. Experimental leprosy infections are initiated with bacilli derived from human biopsy material, armadillos, or nude mice. The incubation period is shortest when high doses of M. leprae are delivered intravenously, although respiratory routes are also effective. Bacilli rapidly become lodged in reticuloendothelial tissues, where they slowly proliferate and later disseminate predominantly to cooler regions of the body such as ears, nose, tongue, and footpads, as well as to the nasopharynx, bronchi, and lungs. The specific response and time course of experimental leprosy varies between individual animals and incubation periods range from 10 months to four years. The disease produces few visual clues, and the earliest sign of successful infection is typically a sustained progressive evolution of IgM antibody to the major M.-leprae-specific antigen phenolic-glycolipid-1 (PGL-1). In successful lepromatous infections, AFB disseminate throughout the body and are usually detectable in ear biopsies within one year. Most develop heavy infections within 24 months of inoculation, with approximately 1012 recoverable bacilli in the liver and spleen.