4. Diagnostic methods
4.1. Field diagnostic methods
4.1.1. Clinical signs
Affected fish become lethargic, exhibit severe anaemia, petechiae in the gills, and enlargement of the
spleen.
4.1.2. Behavioural changes
Diseased fish swim inactively and show abnormal and conspicuous respiratory exercises caused by
anaemia.
4.2. Clinical methods
4.2.1. Gross pathology
Pale gills and enlarged spleen.
4.2.2. Clinical chemistry
Low haematocrit value.
4.2.3. Microscopic pathology
See methods for smears (Section 4.2.5) and electron microscopy/cytopathology (Section 4.2.6).
Microscopic pathology should confirm the presence of abnormally enlarged cells in tissues such as the
spleen, heart, kidney, intestine or gill.
4.2.4. Wet mounts
None.
4.2.5. Smears
Confirm presence of abnormally enlarged cells in Giemsa-stained stamp-smear of the spleen.
4.2.6. Electron microscopy/cytopathology
Confirm presence of virions (200–240 nm in diameter) in the enlarged cells.
4.3. Agent detection and identification methods
4.3.1. Direct detection methods
4.3.1.1. Microscopic methods
4.3.1.1.1. Wet mounts
None.
4.3.1.1.2. Smears
Examination of acetone-fixed stamp-smears from diseased fish may reveal abnormally enlarged cells
from spleen, heart or kidney. These enlarged cells react to an anti-RSIV MAb (M10) by the antibodybased
antigen detection (IFAT) test (26).
4.3.1.1.3. Fixed sections
Examination of histological sections from diseased fish may reveal abnormally enlarged cells from the
spleen, heart, kidney, liver, intestine or gill. These enlarged cells may react to anti-RSIV MAb by the
immunohistochemistry test. However, this method is not yet fully validated.