Epidemiology of infectious salmon anemia in New Brunswick Atlantic salmon farms

Infectious Salmon Anemia (ISA) is a recently described disease in farmed Atlantic Salmon, Salmo salar. A member of the newly identified Orthomyxovirus genus Isavirus, Infectious Salmon Anemia Virus (ISAV), has been identified as the causative agent. Virally infected Atlantic salmon experience lethargy, anorexia, severe anemia, exopthalmia, pale gills, ascites, and internal organ damage. Financial losses to the New Brunswick industry have been estimated to be $45M since 1997. The epidemiology of ISA in New Brunswick was investigated through five different studies.

Sensitivity and specificity of ISA diagnostic tests were evaluated as individual tests and in combinations, using data collected by the provincial government surveillance program. 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 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%.

The prevalence of ISAV in cages experiencing an outbreak was compared with healthy cages from the same farm, neighboring farms, and distant farms. Salmon from five different groups were tested using an RT-PCR test. Groups included moribund fish from a cage experiencing an outbreak (A), healthy fish from an outbreak cage (B), healthy fish from a negative cage from a farm experiencing an outbreak in a different cage (C), healthy fish from a negative farm near an outbreak farm (D), and healthy fish sampled at a negative farm located in an area with only negative farms (E). Apparent prevalences (standard error) for the different groups (A–E) were 0.94 (.026), 0.41 (0.062), 0.29 (0.040), 0.08 (0.037), and 0.08 (0.037) respectively. All groups were significantly different (p < 0.002) from each other except between groups B and C and between groups D and E. (Abstract shortened by UMI.).