Susceptibility and antibody production against pathogenic and vaccine strains of the haemoflagellate, Cryptobia salmositica were investigated in five full-sib families (A-E) of Atlantic salmon, Salmo salar. Humoral response and susceptibility of families were compared within three treatments: infection, vaccination and vaccination followed by challenge. Parasitaemias caused by the vaccine strain of C. salmositica were considerably lower than those caused by the pathogenic strain. All vaccinated families were protected when challenged with the pathogenic strain. Family B had significantly lower parasitaemias (with both strains) than the other families. When naïve fish were infected with the pathogenic strain, this family had a significantly lower and earlier peak parasitaemia (4.3 +/-1.3 x 10(6) parasites mL(-1) blood at 3 weeks post-infection; w.p.i.) than the other families. Family C had the highest peak (11.1 +/- 1.2 x 10(6) parasites mL(-1) blood), which occurred at 4 w.p.i. Antibodies against C. salmositica were detected earlier in Family B (3 w.p.i.) than in Family C (5 w.p.i.). This demonstrates an association of increased susceptibility with a delayed antibody response. Western immunoblot identified antibodies against 112, 181 and 200 kDa antigens earlier in more resistant fish (Family B). Antigenic stimulation leading to a stronger antibody response was shown with the vaccine strain and in the later stages of infection.

The effects of a live Cryptobia salmositica (Kinetoplastida) vaccine on the humoral and cellular immune response, and changes in the peripheral leukocyte populations of Salmo salar were investigated. The vaccine produced detectable parasitemia in the blood which peaked at 5 weeks post-vaccination (w.p.v). Antibodies were detectable at 4 w.p.v. and the antibody titer increased as parasitemia declined. Respiratory burst activity in vaccinated fish was significantly higher than in control fish; the highest activity occurred with rising parasitemia and lower activity with declining parasitemia. There was a significant increase in the proportion of granulocytes (to total leukocytes) at 4 w.p.v. At 6 w.p.v., the proportion of lymphocytes and monocytes increased significantly and remained elevated. These results demonstrate innate (respiratory burst activity and an increase in the proportion of granulocytes corresponding to rising parasitemia) and adaptive (antibody production and increases in the proportion of monocytes and lymphocytes corresponding to declining parasitemia) immune responses to the live vaccine.

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 × 10 6 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 × 10 5 parasites ml ) 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.