We have previously investigated the resistance or the susceptibility of carp (Cyprinus carpio) toMyxobolus cerebralis-spores, the causative agent of whirling disease in salmonids, through histological and molecular methods. To further our study we conducted the same studies on the rainbow trout (Oncorhynchus mykiss), which is known to be susceptible to the causative agent. Polymerase chain reaction (PCR), nested-PCR and histological examination Hamatoxylin-Eosin (H&E)methods were compared for the detection of M. cerebralis. The histological investigations of 20 fish between 3 to 5 days post-exposure (p.e) revealed only 3 parasite stages (single, degenerate,agglomerated) in the tail fin of carp. The nested-PCR results showed that only M. cerebralis exists in 15% of the caudal fin samples in the rainbow trout 60 days p.e, although in the single-round PCR test, all the samples were tested negative. Our results indicated that the nested-PCR-method is more sensitive to detect early stages after 12 h in the rainbow trout and carps in comparison to the single-round PCR test and histological examination. The infection of rainbow trout with M. cerebralis-spores was confirmed by clinical symptoms and histological lesions in the cartilaginous areas of the skull, while in all the infected carps, it was not possible to divide the triactinomyxon-spores (TAM) in theepidermis at 12 h p.e; thus, it penetrates deep into the dermis in order to reach the brain.

Key words: Polymerase chain reaction, Myxobolus cerebralis, carps, salmonids.