DAO 37:185-193 (1999)  -  doi:10.3354/dao037185

Development of Kudoa thyrsites (Myxozoa: Myxosporea) in netpen-reared Atlantic salmon determined by light microscopy and a polymerase chain reaction test

J. D. W. Moran1,2, L. Margolis2, J. M. Webster1, M. L. Kent2,*

1Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
2Pacific Biological Station, Department of Fisheries and Oceans, 3190 Hammond Bay Road, Nanaimo, British Columbia V9R 5K6, Canada
*Addressee for correspondence. E-mail:

ABSTRACT: The intramuscular phase of development of Kudoa thyrsites, the myxosporean associated with post-mortem myoliquefaction, or 'soft flesh syndrome', is described using histological preparations of the musculature of seawater netpen-reared Atlantic salmon Salmo salar. Hatchery-reared Atlantic salmon were naturally exposed to the infective stage while held in the experimental seawater netpens of the Pacific Biological Station in Nanaimo, British Columbia, Canada. In fish exposed during the summer of 1995, K. thyrsites infections were first detected in the somatic musculature at 13 wk post-exposure (p.e.) using only light microscopy. In the 1997 exposure, infections were first detected at 6 wk p.e. using a PCR test and at 9 wk p.e. using light microscopy. The earliest stage detected by histology was a small plasmodium containing 4 nuclei. No host response was observed that was directly related to the presence of intact plasmodia within muscle fibers. However, a response was associated with ruptured plasmodia, which was characterized by chronic, multifocal inflammation between the muscle fibers.


KEY WORDS: Atlantic salmon · Kudoa thyrsites · Multivalvulida · Marine myxosporean · Salmo salar · Soft flesh syndrome


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