28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 Phenotypic Correlations Between Susceptibility To Enteric Septicemia Of Catfish And Pathogen Clearance Among Channel Catfish Families A. Lelania Bilodeau1, Brian C. Small1, David J. Wise2, and William R. Wolters1 1USDA-ARS, Catfish Genetics Research Unit, Thad Cochran National Warmwater Aquaculture Center, 141 Experiment Station Rd. P.O. Box 38 Stoneville, MS 38776; 2Mississippi State University, MAFES, Thad Cochran National Warmwater Aquaculture Center, 127 Experiment Station Road, Stoneville, MS 38776 Enteric septicemia of catfish (ESC) is the most prevalent disease affecting commercial catfish farms. The USDA-ARS Catfish Genetic Research Unit has an ongoing program for genetic improvement of disease resistance amongst other economically important traits. Susceptibility to the causative agent of ESC, the bacterium Edwardsiella ictaluri, appears to be consistent within each family/spawn of NWAC103 channel catfish.  Real-time PCR technology was utilized to measure differences in bacterial loads and clearance rates for ESC-susceptible and resistant families of channel catfish during immersion challenge. Only surviving fish at each time point were included. The quantity of E. ictaluri DNA present in blood and spleen samples from individual fish was compared for 6 families of catfish that exhibited strong susceptibility or resistance to ESC in previous challenges. Fish from resistant families had increased survivorship during the challenge when compared to susceptible families. Significant differences (p < 0.05) in the quantity of bacterial DNA between resistant and susceptible families were evident for both blood and spleen tissue 5 days following exposure to E. ictaluri. Mean quantities of bacterial cell equivalents per 100 uL of blood at 5 days post-exposure were 2.84 x 105 ± 1.4 x 105 for the susceptible families and 496 ± 455 for the resistant families. Significant differences in spleen tissue also occurred on day 12, with fish from the resistant families having higher (p < 0.05) levels of bacteria than fish from the susceptible families. Overall, families that are susceptible to ESC carried higher levels of bacterial DNA in their blood than resistant families. Pathogen clearance differed between the two sets of families. Clearance was evident on day 12 in fish from the susceptible families and bacterial levels continued to decrease throughout the remainder of the trial. However, no significant clearance was evident in fish from the resistant families. This may be due to chronic low levels of infection that did not trigger a clearance response from the immune system. The innate immune system may suppress infection only in resistant families, preventing an acute host-response whereas in susceptible families, acute infection occurs and may secondarily be cleared. Clearance rate did not appear to significantly affect mortality. Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page