The bacterium Flavobacterium columnare was recovered and identified as the aetiological agent causing freshwater columnaris infection in farmed striped catfish Pangasianodon hypophthalmus (Sauvage) fingerlings that had suffered high mortality rates within commercial hatchery ponds in Vietnam. The gross clinical signs were typical of columnaris-infected fish. Histological examination found numerous Gram-negative, filamentous bacteria present on the skin, muscle and gill tissues of affected fish. The yellow-pigmented bacteria were isolated and identified as F. columnare using primary, biochemical and PCR methods. An experimental immersionchallenge study with 2 strains was also performed. It fulfilled Koch's postulates and showed a median lethal concentration (LC 50 ) of 4.27 × 10 5 and 1.66 × 10 6 cfu ml −1 for the F. columnare strains FC-HN and FC-CT, respectively. To the best of our knowledge this is the first report of freshwater columnaris infection in P. hypophthalmus.
KEY WORDS: Flavobacterium columnare · White patch disease · Pangasianodon hypophthalmus
Resale or republication not permitted without written consent of the publisherDis Aquat Org 100: [83][84][85][86][87][88] 2012 with similar signs to those described for columnaris infections in other fish species. Outbreaks of F. colum nare in P. hypophthalmus have not yet been reported from Vietnam or elsewhere, and so the aim of the present study was to investigate the disease and identify the aetiological agent associated with the high fingerling mortalities in Vietnamese P. hypo phthalmus hatcheries. An experimental bacterial challenge study was also completed to confirm Koch's postulates and to determine the median lethal concentration (LC 50 ) for 2 different F. columnare strains on P. hypophthalmus.
MATERIALS AND METHODSA total of 83 Pangasianodon hypophthalmus (weighing 3 to 20 g) were collected during natural disease outbreaks from 5 hatchery and 7 earthen pond farms located in Vietnam. Histology samples of the skin, gill, kidney, liver and spleen were aseptically removed, placed in 10% (v/v) neutral buffered formalin and processed for routine wax sections. Four-micron thick sections were cut and stained with haematoxylin and eosin (H&E) and Giemsa (Roberts 1989). These were examined under light microscopy. Bacterial isolation was performed where samples were taken aseptically from gills, skin, liver, kidney and spleen of the affected animals directly onto peptone, yeast extract, salt and agar (PYES) (Triyanto & Wakabayashi 1999) and tryptone soy agar (TSA; Oxoid) and then incubated at 28°C. Inoculated agar plates were checked daily up to 4 d post-inoculation for the presence of yellow-pigmented colonies, and primary identification tests included Gram stain, oxidase, oxidation/fermentation (O/F) of glucose, and motility, as described in Frerichs & Millar (1993). Biochemical tests on the yellow-pigmented bacteria (YPB) included catalase, temperature tolerance (13, 15 and 36°C), NaCl tolerance (0.5 and 1%), citrate utilisation, productio...