Quantitative PCR demonstrates a positive correlation between a Rickettsia-like organism and severity of strawberry disease lesions in rainbow trout, Oncorhynchus mykiss (Walbaum)

Strawberry disease (SD) is an inflammatory skin disorder in rainbow trout, Oncorhynchus mykiss (Walbaum). The aetiology of SD is unknown although the 16S rDNA sequence of a Rickettsia-like organism (RLO) has been associated with SD lesions using a nested PCR assay. In this study, we developed a Taqman quantitative PCR assay (qPCR) that targeted the RLO 16S rDNA sequence to examine the distribution of RLO relative to lesion status. We compared 18 lesion samples from 13 fish representing high or low lesion severity as judged by gross examination. QPCR results showed that there was a higher number of RLO sequences in high severity lesions (mean of 12,068 copies) compared with fewer copies of RLO sequence in low severity lesions (mean of 3287 copies, P = 0.012). Grossly normal skin samples (n = 13) from SD-affected fish were all negative by qPCR except two samples (121 and 139 copies). The qPCR assay described herein is a useful tool to investigate the role of RLO in SD in the absence of a culture system for RLO. Our results demonstrate a positive correlation between copy number and lesion severity consistent with the hypothesis that the RLO is the aetiologic agent of SD.