Aquaculture America 2020

February 9 - 12, 2020

Honolulu, Hawaii

VIRULENCE OF Photobacterium damselae subsp. damselae IN AUSTRALIAN YELLOWTAIL KINGFISH Seriola lalandi

Nipa Gupta* , Gavin  J. Partridg e,  Nicky Buller, Andrew Currie,  and  Alan  J. Lymbery
 School of Veterinary and Life Sciences, Murdoch University, 90 South St, Murdoch, WA, 6150, Australia
 Hajee Mohammad Danesh Science and Technology University, Dinajpur 5200, Bangladesh
 nipagupta@hstu.ac.bd

Yellowtail kingfish (YTK), Seriola lalandi , is a marine, pelagic, piscivorus species with a circumglobal distribution. The high market value and its adaptability to sea cage culture make YTK a viable candidate for commercial production in Australia. However, one of the major impediments to the growth of Australian YTK industry is infectious diseases.  Photobacterium damselae subsp. damselae  (Pdd)  is a gram-negative, halophilic bacterium which has been isolated from a wide variety of fish species around the globe. In recent years, the bacterium has been reported for the first time in wild and cultured fish species, including YTK, throughout Australia .  The major  virulence factors of Pdd are plasmid (pPHDD1)- encoded phospholipase-D damselysin and pore- forming toxin phobalysin P.  However, strains  of Pdd  without plasmid  have  also  demonstrated pathogenicity  due to  the  presence of chromosome I-encoded virulence factors.  The present study was conducted to test the pathogenicity  of plasmid-positive and plasmid- negative Australian isolates of Pdd towards YTK.

An infection trial was conducted in a flow- through system with healthy, unvaccinated  cultured YTK, weighing an average 152.0±17.7 g . T hree isolates of Pdd (one plasmid-positive, AS-16-0963#3; and two plasmid-negative , AS-15-3942#7 and AS-16-0963#1 ) were selected for experimental infection. Throughout the experiment, w ater temperature was maintained at 20-22 °C and fish were fed twice per day to satiation with commercial feed. Dissolved oxygen was monitored daily and kept with in 80-100%. Fish were randomly allocated to 20 (300 L) tanks , with 10 fish per tank.  There were six experimental treatments, with  intraperitoneal injection of one of the three test isolates of Pdd  at two different concentrations, 104 CFU/fish and 107 CFU/fish, and three replicate tanks per treatment. Control fish in two replicate tanks were sham-injected .  Fish mortality was recorded daily for 10 days post-challenge. Blood samples from infected fish after 4 days of infection were collected to  measure hematology indices.

M ortality rates for  the plasmid-positive isolate, AS-16-0963#3, were  47%  and  100% at 104 CFU/fish and 107 CFU/fish respectively, compared to13- 20% and 3- 13% respectively  for the two plasmid-negative isolates. No mortality was observed in the control fish. There was little difference in hematology parameters among isolates .  These results suggest that, while presence of the pPHDD1 plasmid is not essential for pathogenicity, the plasmid does increase the virulence of Photobacterium damselae subsp. damselae  to Seriola lalandi in Australia .