Sensitivity of a standard host resistance assay using Streptococcus agalactiae for assessing exposure to immunotoxicants in wild cotton rats (Sigmodon hispidus)

Resident small mammals have been used for in situ biomonitoring of contamin ated waste sites containing suspected immunotoxicants. Host resistance assa ys, which involve challenging animals with an actual pathogen, allow for te sting of overall immune system function in animals. Because such assays hav e not been evaluated for use with wild rodent species, it was our objective to assess the efficacy of Streptococcus agalactiae as a pathogenic model f or use in a host resistance assay for detecting alterations in immune syste m function in wild cotton rats (Sigmodon hispidus). The ability of the assa y to detect immunosuppression was evaluated by inducing immunosuppression c hemically (cyclophosphamide or dexamethasone) and by protein malnutrition. The estimated lethal dose of S. agalactiae that killed 50% of challenged an imals (LD50) was 5.76 x 10(7) colony-funning units (CFUs). Although bacteri al agglutination titers indicated that animals developed an antibody respon se when immunized, immunization was not sufficient to adequately protect an imals from a subsequent pathogenic challenge. Sensitivity of the host resis tance assay was only suitable for detecting substantial immunosuppression, such as that induced by protein malnutrition or dexamethasone administratio n.