DAO 58:17-26 (2004)  -  doi:10.3354/dao058017

ABSTRACT: At 10°C, rainbow trout Oncorhynchus mykiss (n = 13 per group) infected with Cryptobia salmositica Katz, 1951 became anorexic at 3 wk post-infection (w.p.i.), with feed-intake decreasing significantly from 1.33 to 0.94% body weight (b.w.). Anorexia was most severe at 4 w.p.i. (0.80% b.w.), coinciding with peak parasitemia (9.2 × 10⁶ parasites ml blood^-1) and anemia. At 8 w.p.i., fish had recovered their appetite although they still had contained detectable parasites (6.8 × 10⁵ parasites ml^-1) and were anemic (pack cell volume, PCV, of 24.4%). However at 5°C, anorexia occurred at 5 w.p.i. (0.81% b.w.), and was most severe at 7 w.p.i. (0.40% b.w.). At 8 w.p.i. (0.43% b.w.), fish displayed high parasitemia (4.6 × 10⁶ parasites ml^-1) and low PCV (10.8%). Fish at 5°C had lower gastric evacuation (GE) rates (GE_48h) than 10°C fish, however there were no differences between infected and naïve fish at both temperatures. Before anorexia, there was no significant correlation between mean share of meal (MSM, a measure of how food was partitioned within a group) and coefficient of variation in feeding but this became significant during anorexia (p = 0.02 and p = 0.0002 at 10 and 5°C respectively). Significant correlations were detected between b.w. and MSM before onset of anorexia at 10°C (p = 0.005) and 5°C (p = 0.02); this was maintained at 10°C (p = 0.001) but not at 5°C (p = 0.98). Fish on an anorexic diet (0.93% b.w.) responded well at 10°C to a live C. salmositica vaccine; this could partly be due to constant antigenic stimulation by the live vaccine.

KEY WORDS: Anorexia · Cryptobia salmositica · Temperature · Gastric evacuation · Feeding hierarchies · Immune response · X-radiography · Vaccine