Abstract
Columnaris disease can be problematic in tilapia ( O reochromis spp.) production. An understanding of the pathogenesis and virulence of F lavobacterium columnare is needed to develop prevention strategies. The objective of this study was to determine the virulence of genetically defined isolates of F . columnare in sex‐reversed hybrid tilapia, O reochromis niloticus ( L .) × O . aureus ( S teindachner). A series of immersion challenge trials were performed using isolates of the five established genomovars of F . columnare : I , II , II ‐ B , III and I / II . The mean per cent mortality of fish challenged with genomovar I , II and III isolates ranged from 0 to 100, 3.3–78 and 3.3–75%, respectively. The mean per cent mortality of fish challenged with genomovar II ‐ B ranged from 35 to 96.7%, and the only genomovar I / II isolate tested caused no mortality. Contrary to previous work in other fish species, there did not appear to be an association between F . columnare genomovar and virulence in tilapia. The challenge model used resulted in acute mortality. An alternative challenge model was tested by cohabitating healthy fish with dead fish infected with F . columnare . This method resulted in rapid appearance of clinical signs and mortality, suggesting the potential for F . columnare to increase in virulence upon growth on/in a fish host.