Treatment or Protection Strategies

Management
Once E. ictaluri is endemic to an area, particularly if fish are being produced in earthen ponds, the disease is difficult to prevent through management practices alone. However, the incidence and severity of the disease can often be reduced by avoiding the practice of under-stocking susceptible or naive fingerlings together with larger fish previously exposed to or surviving the disease. Whenever possible, also make sure that all earthen ponds are well drained and thoroughly dried and disinfected before being refilled and stocked with fry or fingerlings.

The withholding of feed at critical times has also been shown to be of value in significantly reducing mortality but this practice also significantly reduces growth rates and lengthens production cycles and the time to harvest. Unless the fish are either protected by vaccination or by acquired immunity from prior exposure to the disease, it is recommended that feeding rates and frequencies be cut in half or even temporarily suspended when water temperatures are between 20 and 28 °C (68 to 82 °F), a condition which typically occurs for several weeks in the spring and autumn, depending on local weather conditions.

Prophylaxis
Vaccines have been shown to provide significant protection against ESC disease. Their use can not only reduce the incidence and severity of the disease but also allow full feeding during the spring and autumn seasons when acute ESC disease is most prominent. 

Intervet now has the only fully licensed and commercially available vaccine against E. ictaluri and ESC disease .The vaccine is the first live attenuated vaccine approved for use in fish. It is based on a patented mutant strain of the organism designated as RE-33, which is a natural Rifampin escape mutant of Edwardsiella ictaluri.

Due to the unique pathogenesis of E. ictaluri in channel catfish, which includes a sequestered infection of macrophages, vaccine antigens based on killed bacterins (which induce primarily a humoral immune response) are not particularly effective in preventing the disease. However, the AQUAVAC-ESC® vaccine stimulates both a humoral as well as a strong cellular response that provides improved long-term protection.

It is typically applied to channel catfish fry as a bath treatment in the transport tank during the process of moving fry out of hatchery buildings and stocking outdoor ponds.

AQUAVAC-ESC® was extensively field tested in the USA in 1998 and 1999, and first introduced for sale in Spring 2000. Based on the performance of the vaccine and the increased survival, growth and productivity experienced by the farmers, use of the product increased significantly this past season (2001).

Treatment
E. ictaluri is susceptible to a number of broad spectrum antibiotics. However, clinical isolates should be routinely tested for in vitro sensitivity before treatment is initiated as plasma-mediated resistance is common, depending on drug use history. In the U.S.A., both the Romet 30®  formulation of sulfadimethoxine potentiated with ormetoprim (Hoffmann-LaRoche, Inc.) and various Terramycin® formulations of oxytetracycline-HCl (Pfizer) are available for use and have been shown to be effective in treating ESC disease. 

Romet 30 is typically incorporated into the diet and fed at the rate of 50 mg active ingredient per kg fish per day for five consecutive days. Increasing the amount of fish meal or fish oil in the diet or feeding on alternative days can help reduce loss of appetite associated with this drug and ensure that all of the medicated feed is consumed. Alternatively, Romet 30 can also be incorporated into the feed at lower levels and the feeding rate increase to achieve the desired dosage.

TM-100 is typically incorporated into the diet and fed at rates of up to 37.5 to 50 mg/kg/day for 12 to 14 days. Since Terramycin is relatively heat sensitive, it should not be used in feeds manufactured with a high-temperature extrusion process, typical of some floating-type feeds.

As always, all drugs should be properly registered and labelled for the use intended and label directions should be strictly adhered to.

Disease Prevention

• Removal and destruction of dead fish.
• For isolated facilities, removal of infected fish populations and maintenance of ESC-free environment.
• Disinfection of premises, materials and ponds.
• Control of water sources.
• Control of newly introduced stocks, in accordance with quarantine measures.