Experimental leprosy in the armadillo and nude mice: comparative histobacteriology and ultrastructure

In last 14 years, armadillo has proved an ideal animal model for studying experimental leprosy and mass production of Mycobacterium leprae. However recently a number of groups working with nude mice have claimed its ability as a better experimental model as far as leprosy research and production of leprosy bacilli is concerned. We therefore decided to compare experimental M. leprae infection of both armadillo and nude mice. We compared the degree of infection as well as the physiological and morphological state of proliferating bacteria by histobacteriological and electron microscopic studies. Histobacteriological studies of the infected nude mice showed that the highest number of acid-fast bacilli (AFB) was present in the leprome formed at the site of inoculation, the foot-pad, but often the morphological index (MI) was low. A bacillary diffusion was observed, however the internal organs did not show extensive lesions and contained few AFB with a still lower MI. Similarly the AFB present in the nodes were few in number, formed smaller globi than those formed in the foot-pad, and were essentially non-solid staining bacilli with a lower MI. In the armadillo on the other hand, lesions were extensive and apparent in the internal organs, a much higher AFB count was found and the MI was higher than in nude mice. Ultrastructural studies showed that a much higher proportion of M. leprae cells inside armadillo tissues existed as intact bacteria than in nude mice. Essentially damaged bacteria were found in the nodes from nude mice. The number of M. leprae bacilli in lung, liver, spleen and kidney from all the nude mice was too little to be studied under the electron microscope. Intact bacilli were observed only in mice foot-pads, which however contained two distinct populations of M. leprae; the intact and the damaged bacilli, which were arranged together in separate globi.