H. T. DongS. SenapinB. LafrentzC. RodkhumChulalongkorn UniversityThailand National Center for Genetic Engineering and BiotechnologyMahidol UniversityUSDA ARS Aquatic Animal Health Research Laboratory2018-12-112019-03-142018-12-112019-03-142016-06-01Journal of Fish Diseases. Vol.39, No.6 (2016), 649-65513652761014077752-s2.0-84929094618https://repository.li.mahidol.ac.th/handle/20.500.14594/40892© 2016 John Wiley & Sons Ltd. Numerous isolates of Flavobacterium columnare were previously recovered from red tilapia, Oreochromis sp., exhibiting columnaris-like disease in Thai farms, and the phenotypic and genetic characteristics were described. The objective of this study was to determine the virulence of two morphotypes (rhizoid and non-rhizoid colonies) of F. columnare and to determine their ability to adhere to and persist in red tilapia fry. The results showed that the typical rhizoid isolate (CUVET1214) was a highly virulent isolate and caused 100% mortality within 24 h following bath challenge of red tilapia with three different doses. The non-rhizoid isolate (CUVET1201) was avirulent to red tilapia fry. Both morphotypes adhered to and persisted in tilapia similarly at 0.5 and 6 h post-challenge as determined by whole fish bacterial loads. At 24 and 48 h post-challenge, fry challenged with the rhizoid morphotype exhibited significantly higher bacterial loads than the non-rhizoid morphotype. The results suggested that an inability of the non-rhizoid morphotype to persist in tilapia fry may explain lack of virulence.Mahidol UniversityAgricultural and Biological SciencesArticleSCOPUS10.1111/jfd.12385