Pensee Journal Vol 75, No. 10;Oct 2013 287 office@penseejournal.com The LD50 of Asian Catfish (Pangasius bocourti, Sauvage 1870) challenge to pathogen Aeromonas hydrophila FW52 strain Hien Van Doan Department of Aquaculture, Faculty of Agriculture Quangbinh University, Quangbinh 45000, Vietnam Tel: +66-90-029-9995 Email: hienqbuni@gmail.com Sompong DOOLGINDACHABAPORN (Corresponding author) Department of Fisheries, Faculty of Agriculture Khon Kaen University, Khon Kaen 40002, Thailand Tel: +66 -85-004-5090 Email: sompng_d@kku.ac.th Amnuaysilpa SUKSRI Department of Plant sciences and Natural Resources Faculty of Agriculture, Khon Kaen University Khon Kaen 40002, Thailand Tel: +66-81-047-7514 Email: amnuay_s@kku.ac.th The research is financed by Khon Kaen University under a KKU Scholarship for ASEAN and GMS Countries Abstract This study aims to determine the lethal dose (96-h LD50) of the bacteria Aeromonas hydrophila of Asian Catfish (Pangasius bocourti), which will be used in challenge tests. One hundred and sixty two fish (weight of 8.65 ± 0.11g fish -1 ) were divided into nine treatments, with the use of different bacterial concentrations: T 0 - control (0.85% saline solution); T 1 (1x10 5 cfu ml -1 ); T 2 (1x10 6 cfu ml -1 ); T 3 (5x10 6 cfu ml -1 ); T 4 (1x10 7 cfu ml -1 ); T 5 (5x10 7 cfu ml -1 ); T 6 (1x10 8 cfu ml -1 ); T 7 (5x10 8 cfu ml -1 ), and T 8 (1x10 9 cfu ml -1 ). Fish were intraperitoneally injected with 0.1 ml of bacterial suspensions and distributed in 27- aquarium with 100 litres of water in each. The water variables were monitored and fish mortality was observed. A Completely Randomized Design (CRD) with three replications was applied and the 96-h LD50 was estimated according to the trimmed Spearman-Karber method. Fish mortality rate increased with the increase of bacterial concentrations of A. hydrophila (T 0 = 0%; T 1 = 0%; T 2 = 0%; T 3 = 11.1%, T 4 = 33.3%; T 5 = 66.7% T 6 = 88.9%; and T 7 , T 8 = 100%) after 96h of injection and the first mortalities were observed after 24 hours (h), although the signs of the bacterial infection were already observed 12h after the injection. The results indicated that the 96-h LD50 value of A. hydrophila was 2.24 x 10 8 cfu ml -1 . Keywords: Aeromonas hydrophila, 96-h LD50, Pangasius bocourti 1. Introduction Asian catfish (Pangasius bocourti) is one species of the Pangasiidae family (Orban et al., 2008). This fish has been recently interested in culturing along the Mekong river region as a new economic species. The frozen fillets of its not only supply for local consumption but also exporting to international market (Thammapat et al., 2010). The demand of fish products have been rapidly increasing (Anonymous, 2005) because of its desirable qualities such as white flesh and low-fat content (Orban et al., 2008) as well as easily digestible protein (Økland et al., 2005). It was found that this species gave the fastest growth being recorded in many aquaculture sectors (Phuong and Oanh, 2009). This fish is well adapted species, but very sensitive to handle and transport stress condition. As a result of utmost activity, scare and mucus