Detection of Nucleospora salmonis in steelhead trout, Oncorhynchus mykiss (Walbaum), using quantitative polymerase chain reaction (qPCR).

Nucleospora salmonis is an intranuclear microsporidian found in salmonid fish. The parasite principally infects lymphoblast cells resulting in chronic severe lymphoblastosis and leukaemia-like conditions (Elston, Kent & Harrell 1987; Hedrick, Groff, McDowell, Willis & Cox 1990; Morrison, MacConnell, Chapman & Westgard 1990). Nucleospora salmonis generally weakens immune system function allowing for other, opportunistic infections, but it can also be fatal (Hedrick et al. 1990). Nucleospora salmonis occurs naturally in the northwestern United States in Chinook salmon, Oncorhynchus tshawytscha (Walbaum), and steelhead trout, Oncorhynchus mykiss (Walbaum), and has also been reported in Europe and South America (Kent, Groff, Traxler, Zinkl & Bagshaw 1990; Chilmonczyk, Cox & Hedrick 1991; MacConnell, Shaw & Smith 1991). This salmonid pathogen, however, poses its greatest threat within commercial and conservation aquaculture where fish rearing densities are higher than those found in the wild. As with other pathogens, identifying N. salmonis