Strategies for the diagnosis of Ceratomyxa shasta using the polymerase chain reaction: Comparison of lethal and nonlethal sampling with microscopic examination

A single-round and a nested polymerase chain reaction (PCR) assay were comp ared with microscopic examination for their ability to detect infections ca used by the myxozoan parasite Ceratomyxa shasta. Tissues for the comparison were periodically sampled from susceptible rainbow trout Oncorhynchus myki ss naturally exposed to the parasite. Spores of the parasite were first det ected in the intestine by microscopic examination at 41 d postexposure. Par asite DNA was detected in the intestine at 20 d postexposure by single-roun d PCR and at 7 d using the nested PCR assay. Using a nonlethal sampling met hod, parasite DNA was detected from intestinal swabs at 27 d postexposure b y single-round PCR and at 13 d using the nested reaction. Although PCR diag nosis using the nonlethal sampling method was less effective in detecting C . shasta than diagnosis using lethally collected tissue, it was a more reli able assay than microscopic examination.