Mycobacterium paratuberculosis monoassociated nude mice as a paratuberculosis model.

In this study, a paratuberculosis (Johne's disease) model was developed by intragastrically dosing gnotobiotic athymic nude mice with Mycobacterium paratuberculosis. The mice infrequently shed bacilli from their intestinal tracts during the first 4 months after inoculation. Following this time, increasing numbers of M. paratuberculosis (greater than 4.0 log,, bacilli per fecal pellet by 40 weeks) were recovered from the feces of the 12 mice that remained in the isolator. A similar pattern of recovery of M. paratuberculosis was obtained from the ileum, cecum, colon, and liver. Histopathologic lesions and acid-fast bacilli were rare during the first 4 months of infection and then, with time, increased in prevalence and severity. Mice maintained for 7 months or longer exhibited severe granulomatous inflammation and large numbers of acid-fast bacilli in the gastroin- testinal tract and liver (up to lo8 log,, colony forming units per gram wet weight). Five mice maintained for 7 months or more developed clinical signs consistent with those seen in paratuberculosis (weight loss, chronic diarrhea); three of these mice eventually died or became moribund and were euthanatized. M. paratuberculosis monoassociated mice released increased levels of tumor necrosis factor activity into their sera, as compared to uninfected control mice, when they were injected with bacterial lipopolysaccharide. The clinical signs, fecal shedding of M. paratuberculosis, granuloma formation, and progressive bacillary multiplication observed with these mice are consistent with naturally occurring M. paratuberculosis infection of ruminants (Johne's disease). This model will be useful for future studies of immunoregulation and antimicrobial therapy of paratuberculosis. Johne's disease (paratuberculosis) is a chronic gran- ulomatous enteritis of domestic and wild ruminants." It has worldwide distribution and causes significant losses in the cattle industry. The etiologic agent of this disease, Mycobacterium paratuberculosis, is a slow growing, mycobactin-dependent m ycobacterial species that infects and multiplies in mononuclear phagocytes. Johne's disease ranges in seventy from inapparent sub- clinical infection to persistant diarrhea that inexorably leads to decreased production, cachexia, and death. It is thought that calves ingest the mycobacteria soon after birth and that the bacilli slowly multiply over a period of several years until clinical disease appears during ad~lthood.~' In clinically ill animals, granulo- matous lesions can be found predominantly in the dis- tal ileum; however, lesions may also occur in other portions of the gastrointestinal tract, the regional lymph nodes, and the liver.6 Macrophages and giant cells, often packed with acid-fast bacilli infiltrate the lamina propria of the intestinal mucosa. Acid-fast bacilli can also be observed within aggregates of macrophages and giant cells in the regional lymph nodes and occasionally in the liver. Although it has been recognized for over 75 years, Johne's disease remains an economically significant chronic infectious disease of the cattle industry.lI De- spite its economic and clinical importance, we lack a detailed understanding of the pathogenesis of paratu- berculosis. A major hindrance has been the lack of a suitable small animal model. In a previous report, we provided preliminary evidence that M. paratubercu- losis can colonize the gastrointestinal tracts of germ- free and flora-defined mice after intragastric inocula- tion.2i Although there were only a few nude mice included in that study, our results suggested that in- testinal multiplication of M. paratuberculosis occurred in nude mice that lack a mature thymus and are, there- fore, congenitally deficient in cell-mediated immunity. In contrast, euthymic mice experienced a self-limiting infection, and there was sporadic recovery of low num- bers of M. paratuberculosis from feces and tissues.*I In the present study we describe in detail the course of