ABSTRACT: Rainbow trout Oncorhynchus mykiss injected intramuscularly with isometamidium chloride (0.01 or 0.1 mg kg^-1) at 3 wk post-infection and given a booster 2 wk later had significantly lower parasitaemias than infected controls. Packed cell volume increased after treatment and remained higher than in infected controls. The concentration of isometamidium in plasma was highest at 2 wk after injection and then declined. An intramuscular dose of 1.0 mg kg^-1 of isometamidium chloride at 1, 2 and 3 wk postinfection (preclinical) significantly reduced the parasitaemia in rainbow trout 2 wk after treatment. A booster at 9 wk postinfection (chronic disease phase) reduced the parasitaemia further in all fish. The packed cell volume in these fish was higher than in infected controls. Treatment at 5, 6, and 7 wk postinfection (acute disease) had no effects and parasitaemias in treated fish were higher than in infected controls; also, anti-Cryptobia salmositica antibodies and titres of complement-fixing antibody were higher in these than in infected controls. Incubation of immune plasma or complement with isometamidium for 3 h did not affect the lytic titres of complement-fixing antibodies nor rainbow trout complement.

KEY WORDS: Cryptobia salmositica · Isometamidium chloride · Oncorhynchus mykiss

Published in DAO Vol. 37, No. 3 (1999) on September 14
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1999