Figure 3. Adult channel catfish displaying ascites, one of the clinical signs associated with the acute form of ESC. Other external lesions include white punctate spots on the skin, petechial hemorrhages, and exophthalmia.
Figure 4. Petechial hemmorrhages on the ventral abdomen of an adult channel catfish with ESC.
The Edwardsiella ictaluri genome sequencing project is being conducted by Mark Lawrence, D.V.M., Ph.D., of the Mississippi State University College of Veterinary Medicine; David Dyer, Ph.D., and Allison Gillaspy, Ph.D., of the Laboratory for Genomics and Bioinformatics; and Ronald Thune, Ph.D., of the Louisiana State University School of Veterinary Medicine. These studies are supported by USDA/CSREES grant #2004-35600-14180.
Edwardsiella ictaluri is a small gram-negative rod in the family Enterobacteriaceae, and it is the causative agent of enteric septicemia of catfish (ESC). ESC is the most economically important disease of farm-raised channel catfish, which is the largest aquaculture industry in the U.S. both in terms of acreage and dollar value.
ESC causes signs typical of bacterial septicemia in its acute form. In this disease form, producers experience economic losses due to rapid mortalities (9). In its chronic form, ESC typically has a slower progression and causes a "hole-in-the-head" lesion that may remain localized or progress to septicemia and death (5, 9). Farmers experience losses from chronic ESC due to decreased production, with fish manifesting signs 3 to 4 weeks after an acute outbreak (7). The only treatments currently available are oxytetracycline and sulfadimethoxine/ormetoprim, which are both delivered orally in feed. However, one of the earliest clinical signs associated with ESC is anorexia; therefore, oral antibiotics are only effective in limiting the spread of an outbreak.
E. ictaluri is specifically adapted to the channel catfish host. Although it has been isolated from other fish species (10), disease has only been described in channel catfish. Its optimal in vitro growth temperature (25-30°C) and disease temperature window (22-28°C) reflect its adaptation to a poikilothermic host. Different isolates of E. ictaluri are serologically and biochemically homogenous (1, 8, 11). Because E. ictaluri isolates from ESC outbreaks are all the same serotype (1, 8), it is considered a good candidate for the development of a vaccine.
The E. ictaluri genome consists of a single circular chromosome. DNA hybridization studies indicated that E. ictaluri is most closely related to Edwardsiella tarda (causative agent of emphysematous putrefactive disease in channel catfish and opportunistic infections in humans), with a relative binding ratio of 56-60% at 60°C. E. ictaluri had a relative binding ratio of 31% to Escherichia coli at 60°C. The G+C content of E. ictaluri was estimated at 53% by buoyant density centrifugation (3). Two plasmids (5.6 and 4.8 kb) are consistently present in channel catfish isolates (4, 6). Both plasmid sequences have been published (2). Strain 93-146, which was originally isolated from an ESC outbreak in a commercial catfish pond in Louisiana, is being used to determine the E. ictaluri genome sequence. This strain has been the most widely used in pathogenesis research.