What is the most notable clinical finding in cases of infectious salmon anemia?

The most notable clinical finding in cases of infectious salmon anemia (ISA) is severe anemia and lethargy. This viral disease is characterized by a significant decrease in red blood cells, which directly leads to anemia. Affected fish often exhibit pale gills and body color, weakness, and reduced activity levels, indicating lethargy. The impairment in oxygen transport due to anemia is critical, causing fish to become less active and more susceptible to stressors.

The presence of severe anemia can also lead to secondary complications, such as infection risks from opportunistic pathogens. This combination of symptoms is diagnostic for ISA, making severe anemia and lethargy the hallmark clinical presentation in infected salmon.