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Diseases of Aquatic Organisms

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DAO 63:119-127 (2005)  -  doi:10.3354/dao063119

Application of surveillance data in evaluation of diagnostic tests for infectious salmon anemia

Carol A. McClure1, K. Larry Hammell1,*, Henrik Stryhn1, Ian R. Dohoo1, Leighanne J. Hawkins2,3

1Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island C1A 4P3, Canada
2New Brunswick Department of Agriculture, Fisheries and Aquaculture, 107 Mount Pleasant Road, St. George, New Brunswick E5C 3K5, Canada
3Present address: Maritime Veterinary Services, 246 Main Street, PO Box 1105, St. George, New Brunswick E5C 3S9, Canada
*Corresponding author. Email:

ABSTRACT: Infectious salmon anemia (ISA) is a viral disease in farmed Atlantic salmon Salmo salar, characterized by lethargy, anorexia, anemia and death. Test methods used for regulatory decisions to remove infected cages include the indirect fluorescent antibody test (IFAT), the reverse transcription–polymerase chain reaction test (RT–PCR), and virus isolation (VI). However, no thorough evaluation of these diagnostic tests has been carried out on field samples. The objective of this study was to evaluate the sensitivity and specificity of ISA diagnostic tests as individual tests and in combinations, using data collected by the provincial government surveillance program. Because a ‘gold standard’ reference test for ISA was not available, cage status was based on clinical disease records. A pool of fish from negative farms that had never had clinical ISA and a pool of fish from positive cages that were experiencing an outbreak of clinical ISA were obtained and assumed to be uninfected and infected respectively. A total of 1071 fish were used in this study. Depending on the test’s cut-off value, the sensitivity and specificity for histopathology ranged from 30 to 73 and 72 to 99% respectively. IFAT had sensitivities and specificities in the range of 64 to 83 and 96 to 100% respectively. For the RT-PCR, sensitivity and specificity were 93 and 98% respectively. Test performances were also evaluated in series and in parallel combinations. Sensitivities are maximized when tests are evaluated in parallel, and ranged from 75 to 98%. Specificities are maximized when the tests are evaluated in series, and ranged from 99 to 100%. Current surveillance testing protocols should be reviewed to capitalize on this newly available information on test characteristics.

KEY WORDS: ISA · Infectious salmon anemia · Diagnostic test · Surveillance · Sensitivity · Specificity · Parallel · Series

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