Enteric Septicemia of Catfish (ESC), caused by the bacterium Edwardsiella ictaluri, is a source of significant economic loss to the commercial catfish industry in Mississippi. Direct losses are attributed to both fish mortality and to high costs of treating infected fish with medicated feeds. Indirect losses occur in the form of poor feed conversion and decreased rates of gain. Although the pathogenesis of this disease is not fully understood, it is possible that repeat outbreaks may result from the common practice of placing naive fingerlings into previously infected production ponds where the organism may exist in the environment and in surviving fish.

Very large amounts of vitamin C (3000 mg/kg ration) have been shown to enhance the immune response of channel catfish fingerlings under laboratory conditions, resulting in significantly reduced mortality following challenge with E. ictaluri (Li and Lovell, 1985). Consequently, many producers are spending an additional $50 per ton for high levels of vitamin C supplementation in their feeds during those times when environmental conditions favor an ESC outbreak. The objective of this study was to determine if this expense, which amounted to more than $1 million dollars in 1986, was justified.

Materials and Methods

Twelve cages (1 m 3) were placed in a 0.4 ha pond at the Delta Branch Experiment Station and stocked with 100 channel catfish fingerlings each. The fingerlings averaged 40 g each and were obtained from a commercial catfish farm in the Mississippi Delta. They had no known history of exposure to E. ictaluri.

A 5 week acclimation period was needed to train the fish to reliably accept a floating diet offered by hand feeding. When the fish were feeding reliably at a rate of 2% body weight per day a 6 week feeding trial was initiated. The 12 cages were randomly assigned to one of 3 treatment groups for this 6 week period. Fish in group A were fed a regular 32% floating catfish feed which was purchased from a commercial mill and had an ascorbic acid content of 41.7 mg/kg in the finished feed. Fish in group B were fed this control ration for 3 weeks and then put on a high C diet which had 932.0 mg ascorbic acid per kg finished feed for the final 3 weeks of the trial. Fish in group C were fed the high C ration for the entire 6 week period.

After 6 weeks of feeding the diets described above, all 12 cages were removed from the pond and placed in concrete raceways. Fish from each cage were counted, weighed and separated into 2 groups. Sixty of the 100 fingerlings from each cage were injected with E. ictaluri by the intraperitoneal (IP) route and returned to their respective cages. The number of bacteria injected into each fish was 1 x 10. These fish were observed for 3 weeks post-injection and records were kept of daily mortality. Although all carcasses could not be retrieved because of inaccessibility in the cages, bacterial cultures were taken from the brain, liver, and posterior kidney of those fish which were recovered in a condition suitable for this evaluation.

Approximately 40 fish from each cage were not injected with bacteria and these were placed in a separate holding facility for comparison of the ascorbic acid content of the liver tissue of fish from each treatment group. Sampling of these fish was begun 48 hours after the last feeding and continued for 3 weeks.

Results and Discussion

Mortality data from fish in the 3 treatment groups is summarized in Table 1. Analysis of variance indicated that there was no statistically significant difference in mortality between any of the groups. Edwardsiella ictaluri was routinely identified from the bacterial cultures taken from moribund fish and fresh carcasses, which confirmed the Identity and virulence of the infectious agent.

Table 1. Mean mortality following injection of Edwardsiella ictaluri of caged channel catfish fingerlings fed 3 dietary levels of ascorbic acid

Intraperitoneal injection was selected as the means for bacterial challenge because of the consistent results, in terms of resultant mortality, associated with this method. A concentration of I x 10 bacteria per fish was selected because previous work (Francis-Floyd et al., In Press) had shown that mortality of 30 to 50% could be expected with this level of infection. To detect a difference in mortality patterns between treatment groups it was felt that mortality had to be high enough that a large portion of the population was affected, but not so overwhelming that all infected fish died.

The selection of a three week observation period was arbitrary, but in no case did mortalities continue beyond the second week. There was no indication that observations should be continued for more than 3 weeks.

Assays of liver tissue for the concentrations of ascorbic acid (CI) and ascorbate-2-sulfate (C2) have been completed and the analysis of this data is in progress. A very preliminary evaluation of the data indicates that the ascorbic acid (Cl) levels in liver tissue of fish fed the high C diets for 3 and 6 weeks, respectively, were similar at the termination of the feeding trial. Liver tissue from those fish which had been fed the high C diet had demonstrably higher levels of ascorbic acid (Cl) and ascorbate-2-sulfate (C2) than tissue from fish fed the control ration. Information on the exact concentration of these compounds and their depletion from the liver tissue of fish in the 3 groups is not yet available.

This study has documented no benefit, in terms of improved survival, of channel catfish fingerlings which were fed diets containing high levels of vitamin C prior to bacterial challenge. Laboratory studies have documented increased resistance to bacterial infection in fish which were fed high levels of vitamin C (Durve and Lovell, 1982; Li and Lovell, 1985). Stress factors and the many variables which exist in commercial catfish ponds may account for the lack of measurably improved survival in the fish fed the high C diets. This study must be kept in perspective, however, as individual survival is not the only means of measuring the benefits of vitamin C. The metabolic functions of the vitamin C are many (Halver, 1971). It may be appropriate to question whether the more than one million dollars spent by catfish farmers for vitamin C supplementation in 1986 was justified, but the effort to optimize fish health through nutritional manipulation must continue.

References

1.  Durve, V. S. , and R. T. Lovell. 1982. Vitamin C and disease resistance in channel catfish (Ictalurus punctatus). Can. J. Fish. Aquat. Sci. 39:948-951.

2.  Francis-Floyd, R. , M. H. Beleau, P. R. Waterstrat, P. R. Bowser. In Press. Effect of temperature on the clinical outcome of infection with Edwardsiella ictaluri in channel catfish. J. Am. Vet. Med. Assn.

3.  Halver, J. E. 1972. The role of ascorbic acid in fish disease and tissue repair. Bull. Jap. Soc. Sci. Fish. 38(1):79-92.

4.  Li, Y. , and R. T. Lovell. 1985. Elevated levels of dietary ascorbic acid increase immune responses in channel catfish. J. Nutr. 115:123-131.