A nested polymerase chain reaction for the detection of genomic DNA of Myxobolus cerebralis in rainbow trout Oncorhynchus mykiss.

A nested polymerase chain reaction (PCR) test was developed to amplify a segment of the 18S rRNA gene from Myxobolus cerebralis, the agent causing whirling disease in salmonid fish. The PCR amplifies a 415 bp amplicon that was identified by dideoxynucleotide terminated sequencing to be identical to the known 18S rDNA sequence of M. cerebralis. There was no amplification of genomic DNA from 4 other myxosporean parasites of salmonid fish from the genus Myxobolus including M. arcticus, M. insidiosus, M. neurobius, and M. squamalis. The efficacy of the PCR test to detect early infections was demonstrated by amplification of the 415 bp fragment from experimentally exposed rainbow trout Oncorhynchus mykiss at 2 h and at 1, 2, and 3 wk postexposure to actinosporean stages (triactinomyxons) of M. cerebralis. In contrast, standard microscopic examinations of stained tissue sections of the same fish used for PCR were less reliable in detecting the presence of the parasite. Additional examinations of fish 5 mo postexposure, after sporogenesis had occurred, found the PCR to be a more reliable indicator of infection than pepsin-trypsin digest (PTD) method, particularly when trout were experimentally exposed to low levels of the infectious stages of the parasite. The PCR was able to amplify to detectable levels the equivalent of a single sporoplasm of M. cerebralis as found in a tissue sample. This test improves the detection of M. cerebralis because it can detect the presence of the parasite: (1) in both hosts, (2) in all known stages of its life cycle, and (3) at lower thresholds than currently used diagnostic methods. Lastly, the PCR test is less susceptible to morphological misidentifications of the spores that can occur with current microscopic procedures.