Sensitivity of a Standard Host Resistance Assay Using Streptococcus agalactiae for Assessing Exposure to Immunotoxicants in Wild Cotton Rats ( Sigmodon hispidus )

Abstract

Resident small mammals have been used for in situ biomonitoring of contaminated waste sites containing suspected immunotoxicants. Host resistance assays, which involve challenging animals with an actual pathogen, allow for testing of overall immune system function in animals. Because such assays have not been evaluated for use with wild rodent species, it was our objective to assess the efficacy of Streptococcus agalactiae as a pathogenic model for use in a host resistance assay for detecting alterations in immune system function in wild cotton rats (Sigmodon hispidus). The ability of the assay to detect immunosuppression was evaluated by inducing immunosuppression chemically (cyclophosphamide or dexamethasone) and by protein malnutrition. The estimated lethal dose of S. agalactiae that killed 50% of challenged animals (LD50) was 5.76x10(7) colony-forming units (CFUs). Although bacterial agglutination titers indicated that animals developed an antibody response when immunized, immunization was not sufficient to adequately protect animals from a subsequent pathogenic challenge. Sensitivity of the host resistance assay was only suitable for detecting substantial immunosuppression, such as that induced by protein malnutrition or dexamethasone administration.