Mycoplasma pulmonis genital disease: Effect of rat strain on pregnancy outcome

Background and Purpose: Mycoplasma pulmonis is a natural pathogen of the re spiratory and genital tracts of rats. Differential susceptibility and sever ity of the respiratory form of the disease, known as murine respiratory myc oplasmosis (MRM), exist between rat strains. We now report that specific ra t strains vary in susceptibility to genital tract infection and pregnancy o utcome. Methods: Specific-pathogen-free (SPF) female F344, LEW, Wister (WIS) and Sp rague Dawley (SD) rats were intravaginally inoculated with 3 x 10(7) colony -forming units (CFU) of M. pulmonis strain X1048 or sterile diluent, and al lowed to breed at 10 days after inoculation. Pregnant darns and pups were n ecropsied within 24 hours of parturition. At necropsy, culture for M. pulmo nis was performed on dam and pups, and adverse effects on pregnancy outcome were assessed by determination of the incidence of infertility, fetal reso rption, stillbirths, changes in litter size, and pup birth weight. Blood fr om darns was collected prior to inoculation and at time of necropsy for mea surement of IgM and IgG antibodies to M. pulmonis. Results: At time of necropsy, WIS (50%) and SD (60%) dams had a higher freq uency of M. pulmonis culture positivity in the genital tract than did LEW ( 22.2%) and F344 (17.6%) dams. Dams that were still infected with M. pulmoni s at time of necropsy had various complications. The SD rats had the greate st degree of adverse effects on pregnancy outcome, which were: infertility, decreased litter size (P less than or equal to 0.01), decreased pup birth weight (P less than or equal to 0.01), increased frequency of resorptions, stillbirths (P less than or equal to 0.05), and the highest rate of pup pul monary infection (23.1%) (P less than or equal to 0.001). Despite a 50% col onization rate, WIS dams were the least adversely affected. The WIS pups bo rn from M. pulmonis-infected dams had slight decrease in birth weight, and only 6% had pulmonary infections. The LEW infected dams developed infertili ty and lower numbers of liveborn pups without evidence of vertical transmis sion. The F344 infected darns had lower numbers of liveborn pups that were smaller than their control counterparts, and none had pulmonary infections. None of the animals had detectable IgM and IgG antibodies to M. pulmonis b efore inoculation. At time of necropsy, all animals inoculated with M. pulm onis developed significantly (P less than or equal to 0.001) higher amounts of M. pulmonis IgG and IgM antibodies, with SD rats developing the highest amounts (P less than or equal to 0.005), Conclusions: Both F344 and LEW rats are more resistant to vaginal inoculati on with M. pulmonis than are WIS and SD rats. However, only SD darns suffer ed severe adverse effects on pregnancy outcome. The SD dame also had the gr eatest IgM and IgG antibody response to M. pulmonis. Our studies clearly in dicate differences among rat strains in their susceptibility to vaginal ino culation with M. pulmonis and in secondary complications associated with in fection. This system may be a useful model for determining host-specific fa ctors that influence the outcome of natural mycoplasmal infections of the g enital tract.