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Atlantic salmon Salmo salar L. (Salmonidae) were experimentally infected with Spironucleus barkhanus (Diplomonadida: Hexamitidae). Parasites were found in the blood 1 to 8 wk after infection, after which they disappeared from the blood and were found mainly in the internal organs (e.g. spleen and liver), eye socket or muscles. Mortality (38 out of 40 infected fish) occurred when fish had lesions in internal organs and/or on the body surface. Uninfected fish cohabiting with infected fish became infected after 4 wk, indicating direct transmission. There was no difference in susceptibility to spironucleosis between 3 different families of Atlantic salmon. All families developed the disease with a similar pattern of parasitaemia in the blood, similar clinical signs and gross pathology, and with very high mortality (29 out of 30). Clinical signs of systemic spironucleosis may include anemia, skin blisters, muscle ulcerations or unilateral exophthalmia. Gross pathologies include hemorrhaging of internal organs, splenomegaly or deformed (globulated) spleen, or granulomatous lesions in the spleen and liver.

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.

The course of Spironucleus barkhanus (Diplomonadida: Hexamitidae) infection in Atlantic salmon Salmo salar L. (Salmonidae) has 2 distinct phases, a blood phase and a tissue phase. To detect and quantify an infection, 3 parasitological techniques, namely Wet Mount Examination (WME), Hematocrit Centrifuge Technique (HCT) and the Hemocytometer (HCM) were used. In addition, 1 immunological technique, enzyme-linked immunosorbent assay (ELISA), was developed to detect specific antibodies against S. barkhanus. This technique would be particularly useful for epidemiological studies where large numbers of fish had to be examined. It would also be a good technique to detect infection during the tissue phase of the disease when there were no or a low number of parasites in the blood.