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


RESULTS
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.


RESULTS
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.


CONCLUSIONS
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.
Bebak
Julie A
JA
Freshwater Institute, 1098 Turner Rd, Shepherdstown, WV 25443, USA.
Welch
Timothy J
TJ
Starliper
Clifford E
CE
Baya
Ana M
AM
Garner
Michael M
MM
D000818:Animals; D017756:Aquaculture / Q000379:methods*; D015331:Cohort Studies; D003937:Diagnosis, Differential; D004196:Disease Outbreaks / Q000662:veterinary; D005260:Female; D005393:Fish Diseases / Q000453:epidemiology* / Q000517:prevention & control; D045826:Flavobacteriaceae Infections / Q000453:epidemiology / Q000517:prevention & control / Q000662:veterinary*; D005417:Flavobacterium / Q000302:isolation & purification*; D017686:Oncorhynchus mykiss / Q000382:microbiology*