Dissertation
Identifying the etiologic agent of strawberry disease in rainbow trout
Washington State University
Doctor of Philosophy (PhD), Washington State University
12/2009
DOI:
https://doi.org/10.7273/000005932
Abstract
Strawberry disease (SD) is an inflammatory skin disorder of unknown etiology that leads to downgrading or rejection of farmed rainbow trout (Oncorhynchus mykiss) at processing and thus has an economic impact on trout producers. An infectious cause for SD is suggested by studies of limited scope that showed transmission and by the apparent response of SD to antibiotic treatment. I used culture-independent methods (16S rDNA libraries) to identify candidate bacterial pathogens from seven SD lesions and two healthy skin samples from SD-affected fish. A 16S rDNA sequence highly similar to members of the order Rickettsiales was present in three lesion libraries and absent in healthy tissue libraries. I developed and applied a nested-PCR assay to screen 25 SD-affected fish for 16S rDNA from this Rickettsia-like organism (RLO). Sixteen of 25 lesion samples and four of the 25 healthy samples were positive for the RLO sequence indicating a significant positive association between SD lesions and the presence of RLO DNA (P < 0.001). Efforts to culture the RLO organism were unsuccessful. Instead, I developed a quantitative-PCR assay (qPCR) to enumerate RLO in lesions of varying severity under the working hypothesis that if RLO is the etiologic agent of SD, then there should be a positive correlation between lesion severity and RLO copy number. The assay targeted the RLO 16S rDNA sequence and had an analytical detection sensitivity of 100 copies. I then tested 18 lesions from 13 fish representing high or low lesion severity as judged by gross examination. QPCR detected higher numbers of RLO (mean of 11,015 copies) in high severity lesions compared to lower numbers (mean of 3,160 copies) in low severity lesions (P < 0.001). Samples of unaffected skin from SD-affected fish were all negative except two samples (121 and 139 copies). My results demonstrate a positive correlation between copy number and lesion severity. This information combined with the association between SD lesions and the presence of RLO supports the hypothesis that the RLO is the etiologic agent of SD.
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Details
- Title
- Identifying the etiologic agent of strawberry disease in rainbow trout
- Creators
- Sonja Jane Lloyd
- Contributors
- Douglas R. Call (Chair)Thomas Eugene Besser (Committee Member) - Washington State University, Department of Veterinary Microbiology and PathologyJ. Lindsay Oaks (Committee Member)Guy Hughes Palmer (Committee Member) - Washington State University, Paul G. Allen School for Global Animal HealthKevin Roy Snekvik (Committee Member) - Washington State University, Department of Veterinary Microbiology and Pathology
- Awarding Institution
- Washington State University
- Academic Unit
- College of Veterinary Medicine
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 87
- Identifiers
- 99901055030401842
- Language
- English
- Resource Type
- Dissertation