DAO 26:33-41 (1996)  -  doi:10.3354/dao026033

Juvenile Atlantic salmon Salmo salar L. at a number of Scottish smolt units become infected each year by the myxosporean parasite Sphaerospora truttae. The first detectable signs of infection occurred in late June/early July in fish in their first year on site, when extrasporogonic stages were found in the blood and kidney interstitium. These stages proliferated in the fish until early September. Sporogonic stages occurred in the kidney tubules from early August, with mature spores first present in September. The prevalence and intensity of each of these developmental stages was recorded over a 3 yr period at 2 hatcheries. Marked temperature differences existed between the 2 farms which appeared to be responsible for differences in the intensity of the parasite at each location. Temperature also appeared to be important in the short term synchronous release of the infective stage of the parasite. A pre-patent period of 2 to 4 wk was identified between the point at which fish became infected and when infections became detectable. Mature spores remained in fish held away from sources of reinfection for at least 18 mo but, on transfer to sea, spores were lost within 4 mo. A comparative study of fish aged 1+ which had been exposed to infection in their first year with those which were naive indicated that previously exposed fish were subsequently refractory to reinfection, but that susceptibility was not age dependent.

Sphaerospora . Epizootiology . Atlantic salmon . Extrasporogonic stages . Reinfection studies