Isolation of a pathogenic strain of Vibrio alginolyticus from necrotic larvae of Macrobrachium rosenbergii (de Man).

Giant freshwater prawn, Macrobrachium rosenbergii (de Man), is an important commercial species with considerable export value, ideal for cultivation under low saline conditions and in freshwater zones (Kurup 1994). However, despite more than a decade of research on its larval production systems, vibriosis still hampers seed production resulting in high mortality rates. Among the different species of vibrios, Vibrio alginolyticus has been isolated frequently from diseased shrimp as the aetiological agent of vibriosis and has been described as a principal pathogen of both penaeids and nonpenaeids (Lightner 1988; Baticados, Cruz-Lacierda, de la Cruz, Duremdez-Fernandez, Gacutan, LavillaPitogo & Lio-Po 1990; Mohney, Lightner & Bell 1994; Lee, Yu, Chen, Yang & Liu 1996). Vibrio fluvialis, V. alginolyticus, V. cholerae non-O1 (Fujioka & Greco 1984), Aeromonas liquifaciens and V. anguillarum (Colorni 1985) have been isolated from the larvae of M. rosenbergii. A profound relationship between the abundance of members of the family Vibrionaceae and larval mortality (Singh 1990) and the predominance of Vibrio in eggs, larvae and post-larvae of M. rosenbergii (Hameed, Rahaman, Alagan & Yoganandhan 2003) was reported. The present paper reports the isolation, characterization, pathogenicity and antibiotic sensitivity of V. alginolyticus associated with M. rosenbergii larvae during an occurrence of severe mass mortality at the ninth larval stage. Moribund larvae (about 500) were collected from M/s Rosen Fisheries (Trichur, Kerala). The larvae were reared in 15& sea water, fed on freshly hatched Artemia nauplii until stage 6, and supplemented with egg custard subsequently. Water quality parameters monitored were pH (7.5–8.0), temperature (25–28 C), total ammonia (<0.1 ppm) and nitrite (<1.0 ppm). Partial water exchange (30%) was provided daily. The diseased samples were obtained from a tank (5-tonne capacity with 500 000 mysis as the initial stocking density) where the majority of the larvae at stage 9 had displayed anorexia, inactivity, poor growth, morbidity and necrotic appendages. Collection of moribund larvae and the rearing water was made in sterile polypropylene bottles (autoclaved at 121 C for 15 min) with sterile sea water (15&) as the transport medium. The samples were transported in an ice-chest at 4 C and analysed in the laboratory within 2 h of collection. Larvae (n 1⁄4 30) were washed in a 20-mL volume of sterile (autoclaved at 121 C for 15 min) sea water (15&) three times for a duration of 1 min each, and macerated to a fine paste in a sterile glass Journal of Fish Diseases 2006, 29, 187–191