Pathogen loads, clearance and plasma cortisol response in channel catfish, Ictalurus punctatus (Rafinesque), following challenge with Edwardsiella ictaluri.

Enteric septicaemia of catfish (ESC), caused by the Gram-negative bacterium Edwardsiella ictaluri (Hawke 1979), is the most prevalent disease affecting farm-raised catfish in the southeastern United States. The disease occurs primarily in the autumn and spring when water temperatures are between 18 and 28 !C (Francis-Floyd, Beleau, Waterstrat & Bowser 1987). The number of ESC cases investigated by fish diagnostic laboratories can be as high as 40% of total cases per year (USDA 1997). The pathology of ESC is well defined (Shotts, Blazer & Waltman 1986; Newton, Wolfe, Grizzle & Plumb 1989; Baldwin & Newton 1993); however, the exact mechanisms of pathogenesis and disease kinetics within populations and between individual fish are not well understood. Management strategies for combating ESC vary in both application and degree of success and include vaccination (Shoemaker & Klesius 1997; Wise, Klesius, Shoemaker & Wolters 2000), withdrawing feed at appropriate times, and treatment of ponds once an ESC outbreak has become evident (Hawke, Durborow, Thune & Camus 1998). Effectiveness of these strategies depends upon timing of treatment and the level of infection in the ponds. An additional approach is to utilize applied breeding strategies to select for disease resistance (Wolters & Johnson 1994). Particular spawns (or families) of channel catfish are consistent in their susceptibility to E. ictaluri challenge (i.e. they demonstrate resistance or susceptibility) (Wolters & Johnson 1994). Stress is known to lower disease resistance in fish (see Schreck, Maule & Kaattari 1993; Iwama, Pickering, Sumpter & Schreck 1997); however, differences in disease kinetics and plasma cortisol concentrations in response to disease progression between resistant and susceptible fish are not well understood. The purpose of this study was to investigate differences in E. ictaluri-resistant and -susceptible families by examining pathogen loads, pathogen clearance and the cortisol response during an ESC challenge. Two families of NWAC103 strain channel catfish, Ictalurus punctatus (Rafinesque), from the USDA-ARS Catfish Genetics Research Unit in Stoneville, MS, USA, were selected based on mortality rates from three prior experimental E. ictaluri immersion challenges; family 81 (susceptible) mean percentage of mortality 1⁄4 86.67 " 13.06 (SE) and family 110 (resistant) mean percentage of mortality 1⁄4 13.33 " 11.55 (SE). A total of 85 juvenile fish from each family were acclimatized for 7 days in 30 120-L aquaria (17 aquaria per family) supplied with 26 !C well water. Of these, 75 fish were challenged with virulent E. ictaluri with a 30-min static challenge (Wolters & Johnson 1994) on day 7 of acclimatization and sampled four times during the 12-day trial. All fish were harvested from a tank on each sample day. The 10 remaining fish (five per family) were Journal of Fish Diseases 2003, 26, 433–437