2012 Annual Report 1a.Objectives (from AD-416): Determine the influence of external parasitic infestation on innate resistance in fish to bacterial infection and disease and the effect on the acquired immune responses to bacterial infection and disease on fish vaccination. 1b.Approach (from AD-416): Tilapia, catfish, pintado, pacu and other species of warmwater fish will be used to determine the effect of external parasitic infestation by Gyrodactylidae and Dactylogyridae monogeneans, trichodinids and/or Ichthyophthirius multifiliis on the innate immune responses to Streptococcus (S.) iniae, S. agalactiae and Aeromonas (A.) hydrophila under experimental infection and disease. The effect of these parasites on the protective acquired immune response to S. iniae, S. agalactiae, and A. hydrophila infection and disease will also be determined on fish vaccination against these bacterial pathogens. The techniques employed include those from the areas of parasitology, bacteriology, immunology, histology and molecular biology to quantitatively measure parasite and bacteria infectivity, pathology, ELISA antibody responses and molecular responses of immune and related genes in parasitized fish subjected to bacterial infections. The data will be subjected to statistical analysis. 3.Progress Report: Water temperature is one of the most important factors affecting fish physiology and immunity. ARS scientists at Auburn, AL, in collaboration with a visiting scientist from the Federal University of Santa Catarina, Brazil, conducted a study that evaluated temperature effects on immune response following vaccination. The study results demonstrated that vaccinated catfish were severely impacted by low temperature, either at 15ºC constant temperature or at 15-25ºC cycling temperature. The fish showed no anti-Ich antibodies and suffered high mortality similar to non-vaccinated control fish. Within or near the optimum growing temperature of channel catfish, good immune response and strong host protection were demonstrated. The results show that low temperature impacts fish immune function and can result in poor vaccine response against fish parasites.