Studies on the biology of Sphaerospora Sp. (myxozoa: myxosporea) from farmed Atlantic salmon, Salmo salar L. in Scotland

• Main Author: McGeorge, James
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Stirling 1994
• Subjects: Atlantic salmon; Fishes Parasites; Fishes Diseases; Salmo salar
• Online Access: http://hdl.handle.net/1893/29303; http://dspace.stir.ac.uk/bitstream/1893/29303/1/McGeorge.pdf

The life cycle, morphology, development, epidemiology and pathology of a previously unreported myxosporean parasite of Atlantic salmon is described. The parasite infects fish at a number of freshwater smolt-producing hatcheries across Scotland. A three year epidemiological sampling programme indicated that the parasite had two distinct life cycle phases in the fish. The first, an extrasporogonic stage, was first detected in late June/early July, though a retention experiment showed that infections were pre-patent for two to four weeks prior to their becoming detectable. Extrasporogonic stages rapidly rose in prevalence in a single cohort; this was related to a presumed synchronous release of infective agents from alternate hosts in the rivers supplying farms, perhaps as a result of a temperature stimulus. Extrasporogonic stages were found in the circulating blood, liver sinuses and spleen, but were concentrated in the interstitium of the kidney. Such stages measured 10-60/xm in diameter and consisted of a primary cell containing 1-120 secondary cells in its cytoplasm. Secondary cells could contain one, or two, tertiary cells. Ultrastructural observations noted that secondary and tertiary cells were situated in vacuoles in the cytoplasm of primary and secondary cells respectively. Primary cell nuclei showed marked developmental changes in appearance, from early released stages with few secondary cells through to more advanced ones containing many secondary cells. Tertiary cells appeared to be formed by two means; endogenous cleavage of the secondary cell, and the engulfment of one secondary cell by another. The extrasporogonic stages were responsible for morbidity, mortality and histopathological changes. The role of the extrasporogonic stage as a stressor, rendering fish more susceptible to secondary invaders and lowering tolerance to environmental factors and additional stresses was discussed. The host response resulted in the engulfment and/or the attachment and destruction of some extrasporogonic stages by ...