Whole body net ion fluxes, plasma electrolyte concentrations and haematology during a Loma salmonae infection in juvenile rainbow trout, Oncorhynchus mykiss (Walbaum)

Loma salmonae infections of salmonids culminate in the development of branchial xenomas and subsequent focal hyperplasia of the lamellar or filament epithelium following xenoma rupture and spore release. The effects of this acute branchial disruption upon net ionic flux rates and plasma electrolyte concentrations were determined in juvenile rainbow trout given an experimental oral exposure to L. salmonae. Mean numbers of branchial xenomas peaked at week 5 post-exposure (PE), which coincided with a reduction in the specific growth rate, although there were no significant differences in mass, length or condition of Loma-exposed fish compared with unexposed controls. Following exposure, negative net whole body Na(+) and K(+) fluxes decreased, whereas net Cl(-) fluxes remained unchanged compared with non-exposed control fish. At week 3 PE during the initial branchial xenoma formation stage, there was a significant negative whole body net K(+) flux in Loma-exposed trout compared with other points during the exposure and subsequent infection. Additionally, Loma-exposed fish had marginally elevated plasma Na(+) and Cl(-) concentrations, whilst K(+) levels remained unchanged, compared with control fish. Although there was a progressive decrease in leucocrit, haematocrit remained unchanged over the course of the Loma exposure and subsequent infection. These results suggest that ionic compensation can occur at the gills during the development of xenomas during exposure to L. salmonae and the resultant infection, therefore allowing defence of plasma electrolyte concentrations, unlike the acute ionic disturbances seen with some other parasitic diseases.