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

Abstract

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 leucocytes. Extrasporogonic stages were last detected in early September in cohorts. Sporogonie stages first became detectable in the kidney tubules in late August. The earliest stage comprised an enveloping pseudoplasmodial cell containing two sporoblast cells. This appeared homologous to the secondary cells containing two tertiary cells formed in, and released by, extrasporogonic stages. These stages appeared to reach the kidney tubules via two routes; penetrating between adjacent cells of the basal lamina of the kidney tubule, and by the disruption of the glomerular capillaries followed by entrance to the tubule via the Bowman’s space. Glomerular means of entry was more common and caused histopathological changes. Although some tubules were completely occluded by sporogonie stages, they caused little pathology. The two sporoblast cells of each pseudoplasmodium subsequently divided until twelve were present; the twelve then differentiated to form two spores. Sporogenesis was described from both light and electron microscopical observations. The parasite was identified as a member of the genus Sphaerospora by virtue of its spore morphology and dimensions, and presence of an extrasporogonic stage, but showed close affinities with Leptotheca. Spores were considerably more broad and deep when immature. Experimental transmissions via the IP-injection of kidney suspensions containing extrasporogonic stages resulted in infections in naive Atlantic salmon and brown trout but not in rainbow trout. Spores and sporogonie stages were not transmissible by IP-injection or orally. The Sphaerospora sp. from Atlantic salmon was compared with related species in the literature, especially PKX and S. truttae. Despite a close similarity to S. truttae, the parasite was not assigned to this species due to a lack of information on S. truttae ultrastructure and extrasporogonic stages, and the presence of intracellular sporogonie development in S. truttae. Studies of infection intensity showed distinct variations between individual farms. A number of aspects of the epidemiology of the salmon Sphaerospora pointed to a limited potential for proliferation for individual extrasporogonic stages. Studies of salmon cohorts with different infection histories indicated that previously infected fish were not susceptible in their second year. Fish in their first year, and year old fish not previously exposed, were susceptible. Mature spores could be found in fish held on freshwater away from any source of reinfection 18 months after they had first been detected. However spores were lost from the kidney tubules of fish within three months of their transfer to sea-cages as smolts. A study of the habitats at and around infected farms for actinosporean life cycle stages in alternate hosts resulted in the discovery of five species of actinosporean of four distinct genera. All species were detected from the oligochaete population of a settlement pond at one farm. Two were identified to species level, Synactinomyxon longicauda and Triactinomyxon mrazeki, but the remainder appeared to be new species of the genera Aurantiactinomyxon, Raabeia and Synactinomyxon. Polar filaments of Aurantiactinomyxon, Raabeia, Synactinomyxon sp. 1 and Synactinomyxon longicauda everted in response to mucus from Atlantic salmon, brown trout and bream. In the case of S. longicauda, the sporoplasm of spores hatched and was motile. The spore release patterns of worms infected with Aurantiactinomyxon sp. were studied and showed that peak release occurred during the night. The ultrastructure of Aurantiactinomyxon was described, polar capsule formation being particularly unusual.