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Improved husbandry to control an outbreak of rainbow trout fry syndrome caused by infection with Flavobacterium psychrophilum

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  • 1 Freshwater Institute, 1098 Turner Rd, Shepherdstown, WV 25443
  • | 2 USDA National Center for Cool and Coldwater Aquaculture, 11861 Leetown Rd, Kearneysville, WV 25430
  • | 3 US Geological Survey National Fish Health Research Laboratory, 11649 Leetown Rd, Kearneysville, WV 25430
  • | 4 Maryland Department of Agriculture, 8077 Greenmead Dr, College Park, MD 20740
  • | 5 Northwest ZooPath, 654 W Main St, Monroe, WA 98272.

Abstract

Case Description—A cohort of 35,200, 13-week-old, female rainbow trout at a fish farm was evaluated because of a 2-week history of anorexia and lethargy and a mortality rate of approximately 100 fish/d.

Clinical Findings—Affected fish were lethargic and thin and had disequilibrium, bilateral exophthalmia, pale red gills and kidneys, red-tinged coelomic fluid, and pale brown livers. Some fish were differentially pigmented bilaterally. The presumptive diagnosis was bacterial or viral septicemia. The definitive diagnosis was rainbow trout fry syndrome caused by infection with Flavobacterium psychrophilum.

Treatment and Outcome—A strategy for controlling the outbreak based on reducing pathogen numbers in affected tanks and reducing pathogen spread among tanks was developed. The option of treating with antimicrobial-medicated feed was discussed with the farmer, but was declined. After changes were made, mortality rate declined quickly, with no more deaths within 10 days after the initial farm visit.

Clinical Relevance—Bacterial coldwater disease is the most common manifestation of infection with F psychrophilum in fingerling and adult rainbow trout. However, the organism can also cause rainbow trout fry syndrome. This condition should be included on a list of differential diagnoses for septicemia in hatchery-reared rainbow trout fry.

Abstract

Case Description—A cohort of 35,200, 13-week-old, female rainbow trout at a fish farm was evaluated because of a 2-week history of anorexia and lethargy and a mortality rate of approximately 100 fish/d.

Clinical Findings—Affected fish were lethargic and thin and had disequilibrium, bilateral exophthalmia, pale red gills and kidneys, red-tinged coelomic fluid, and pale brown livers. Some fish were differentially pigmented bilaterally. The presumptive diagnosis was bacterial or viral septicemia. The definitive diagnosis was rainbow trout fry syndrome caused by infection with Flavobacterium psychrophilum.

Treatment and Outcome—A strategy for controlling the outbreak based on reducing pathogen numbers in affected tanks and reducing pathogen spread among tanks was developed. The option of treating with antimicrobial-medicated feed was discussed with the farmer, but was declined. After changes were made, mortality rate declined quickly, with no more deaths within 10 days after the initial farm visit.

Clinical Relevance—Bacterial coldwater disease is the most common manifestation of infection with F psychrophilum in fingerling and adult rainbow trout. However, the organism can also cause rainbow trout fry syndrome. This condition should be included on a list of differential diagnoses for septicemia in hatchery-reared rainbow trout fry.

Contributor Notes

Dr. Bebak's present address is USDA Agricultural Research Service, Aquatic Animal Health Research Unit, 990 Wire Rd, Auburn, AL 36832.

Supported in part by a grant from the USDA Agricultural Research Service under agreement No. 59-1930-1-130.

Presented in part at the Eastern Fish Health Workshop, Shepherdstown, WV, March 2004.

Address correspondence to Dr. Bebak.