Abstract
The interactions of Flavobacterium columnare isolates of different virulence with the gills of carp (Cyprinus carpio L.) and rainbow trout (Oncorhynchus mykiss Walbaum) were investigated. Both fish species were exposed to different high (HV) or low virulence (LV) isolates and sacrificed at seven predetermined times post-challenge. Histopathological and ultrastructural examination of carp and rainbow trout inoculated with the HV-isolate disclosed bacterial invasion and concomitant destruction of the gill tissue, gradually spreading from the filament tips towards the base, with outer membrane vesicles surrounding most bacterial cells. In carp, 5-10% of the fish inoculated with the LV-isolate became moribund and their gill tissue displayed the same features as described for the HV-isolate, albeit to a lesser degree. The bacterial numbers retrieved from the gill tissue were significantly higher for HV- compared to LV-isolate challenged carp and rainbow trout. TUNEL-stained and caspase-3-immunostained gill sections demonstrated significantly higher apoptotic cell counts in carp and rainbow trout challenged with the HV-isolate compared to control animals. Periodic acid-Schiff/alcian blue staining demonstrated a significantly higher total gill goblet cell count for HV- and LV-isolate challenged compared to control carp. Moreover, bacterial clusters were embedded in a neutral matrix while being encased by acid mucins, resembling biofilm formation. Eosinophilic granular cell counts were significantly higher in the HV-isolate compared to LV-isolate inoculated and control carp. The present data indicate a high colonization capacity, and the destructive and apoptotic-promoting features of the HV-isolate, and point towards important dynamic host mucin–F. columnare interactions warranting further research.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-015-0164-5) contains supplementary material, which is available to authorized users.
Highlights
Columnaris disease, caused by the Gram-negative bacterium Flavobacterium columnare, is notorious in freshwater aquaculture, amongst others of carp (Cyprinus carpio L.) and rainbow trout (Oncorhynchus mykiss Walbaum), in which it induces severe economic losses due to gill, skin and fin lesions often resulting in high mortality [1,2,3,4,5,6,7,8,9,10]
As the disease progressed markedly faster in trout challenged with the highly virulent (HV)-isolate from SP5 onwards, the remaining trout of all groups were sacrificed 3.5 h earlier compared to the carp advancing the last SP to 12 h instead of 15.5 h pi
The purpose of this study was to track the evolution and discern conspicuous features of the gill lesions in experimentally induced columnaris disease. This is the first description of the sequence of events taking place at the level of the gill tissue before the fish succumb to columnaris disease
Summary
Columnaris disease, caused by the Gram-negative bacterium Flavobacterium columnare, is notorious in freshwater aquaculture, amongst others of carp (Cyprinus carpio L.) and rainbow trout (Oncorhynchus mykiss Walbaum), in which it induces severe economic losses due to gill, skin and fin lesions often resulting in high mortality [1,2,3,4,5,6,7,8,9,10]. Variation in virulence between different F. columnare strains isolated from carp and rainbow trout was shown and the highly virulent isolates induced severe gill lesions in experimentally infected carp and rainbow trout [16]. The carp showed a diffuse lesion pattern, affecting all gill arches bilaterally and the animals died within 12 h after inoculation. Mortality started 15 to 18 h after inoculation, reaching 100% within 72 h
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