Mycobacterium leprae infection in nude mice: bacteriological and histological responses to primary infection and large inocula.

Previous studies have demonstrated that congenitally athymic, nude mice are highly susceptible to infection with Mycobacterium leprae. In this study, we showed that footpad inoculation of nude mice with different inoculum sizes of M. leprae resulted in exponential growth of bacilli until bacillary numbers reached approximately 10(10) bacilli per footpad. There was dissemination of the infection from approximately 10 months after inoculation. When nude mice were compared with thymectomized and irradiated mice and normal intact mice for the ability to detect growth from large inocula of low viability, nude mice were the most sensitive, permitting the detection of 10(2) viable M. leprae among 10(7) irradiation-killed organisms. There was widespread dissemination of the infection throughout the reticuloendothelial system and the tissues of the cooler body sites from approximately 10 months after inoculation. Histologically, the lesions resembled those seen in lepromatous leprosy, although the bacillary load appeared larger and was similar to that seen in heavily infected tissues of the nine-banded armadillo. An unusual feature was the presence of numerous foci of neutrophil polymorphs in the footpads and liver of infected nude mice.