Key words: Myxozoa, Malacosporea, Lifecycle

TRANSMISSION OF Tetracapsuloides bryosalmonae FROM BROWN TROUT Salmo truttae TO THE FRESHWATER BRYOZOAN Fredericella sultana

David J. Morris, Alexandra Adams

Institute of Aquaculture, University of Stirling, Stirling, Scotland. UK. FK9 4LA, djm4@stir.ac.uk

Tetracapsuloides bryosalmonae is the Myxozoan parasite that causes proliferative kidney disease (PKD), a commercially important condition affecting salmonid culture in Europe and North America. The life cycle of this parasite is known to involve freshwater bryozoans that release spores into the water that infect the fish. However, the role of salmonids in transmitting the parasite has been conjectured upon since 1985 when spores were first identified within the kidney tubules. Colonies of the bryozoan Fredericella sultana, cultivated in the laboratory, were exposed to the effluent from PKD infected brown trout (Salmo truttae) to investigate the role of fish in the parasite’s life cycle. The prolonged culture of the bryozoan colonies, together with PCR results suggested that they were T. bryosalmonae naïve. These experimental colonies were continuously exposed to the effluent of an aquarium containing brown trout that were known to possess T. bryosalmonae spore stages within their renal tubules. After 6 weeks overt T. bryosalmonae infection was present within the exposed colonies whereas control colonies obtained from subcultures of the original F. sultana remained parasite free, indicating that transmission was successful. The identification of proliferative, migratory, presaccular stages within the bryozoan host demonstrates that when exposed, the whole colony becomes infected with subsequent colony fragments also capable of harbouring the parasite. While this may lead to an increase in the local prevalence of infected bryozoan colonies, observations made on infected F. sultana indicate that the parasite is pathogenic suggesting that salmonids are integral to the maintenance and spread of T. bryosalmonae in the environment.