Characterization and experimental infection of Vibrio harveyi isolated from diseased Asian seabass (Lates calcarifer)

Aims: Vibrio harveyi causes vibriosis to Asian seabass (Lates calcarifer). The disease spreads rapidly among fish stocked in the same cage. It causes high mortality especially in weak and small sized fish stocked at high density in poorly managed net cage. Study to determine the virulence levels of the bacterial pathogen in various aquaculture animals is a key to prevent vibriosis in marine aquaculture. Methodology and Result: Isolation of bacteria from diseased Asian seabass was done using tryptic soy agar (TSA) and thiosulphate citrate bile sucrose agar (TCBS) plates. Virulence of two strains of Vibrio harveyi (VHJR4 and VHJR7) was tested against clinically healthy aquaculture animals. The analysis revealed that the two bacterial strains differ in pathogenicity. The V. harveyi strain VHJR7 was virulent to Asian seabass at 1.40 x 104 c.f.u. g-1, humpback grouper (Cromileptis altivelis) at LD50 8.33 x 103 c.f.u. g-1 and black tiger shrimp (Penaeus monodon) at LD50 3.26 x 104 c.f.u. g-1 , respectively. The V. harveyi strain VHJR4 was not virulent to Asian seabass and humpback grouper but it caused mortality to black tiger shrimp at LD50 1.32 x 106 c.f.u. g-1. Phenotypically, the two strains shared most of the biochemical features except that the V. harveyi strain VHJR7 was a urease positive and grew at 8.5 % NaCl, and at 10 °C. The percentage similarity of nucleotide sequences of 16S rDNA in V. harveyi VHJR4 and V. harveyi VHJR7 was higher (99%) but reduced at 95 % in hemolysin gene. Conclusion, significance and impact of study: Pathogenic strain of V. harveyi causes mortality and affects aquaculture production of Asian seabass. Hence, vaccine development against the bacterial pathogen is urgently needed for sustainability of Asian seabass aquaculture in Malaysia.

Distribution of red-spotted grouper nervous necrosis virus (RGNNV) antigens in nervous and non-nervous organs of European seabass (Dicentrarchus labrax) during the course of an experimental challenge

The distribution of red-spotted grouper nervous necrosis virus (RGNNV) antigens was examined by immunohistochemistry in the nervous and non-nervous organs of juvenile European seabass (Dicentrarchus labrax) during the course of an intramuscular infection. Histological changes resulting from the infection were evaluated from 3 days to 2 months post-infection. The specific antibody response was also studied 2 months post-challenge. Viral proteins were present throughout the experimental period in the retina (inner nuclear layer, ganglion layer, outer limiting membrane, and outer plexiform layer), brain (cerebellum and tectum opticum), and liver (hepatocytes and endothelial cells). These proteins were also observed in the renal tubular cells, white pulp of spleen, and in fibroblasts and cartilage of caudal fin. This is the first report of RGNNV proteins appearing in these organs, where the immunostaining was only detected at certain sampling times after the onset of mortality. Brain and retina of virus-exposed fish showed high levels of vacuolation, while accumulation of fat vacuoles was observed in the liver. RGNNV infection also induced a specific antibody response as measured by an ELISA. In summary, this is the first study demonstrating the presence of viral proteins in cells of caudal fin, kidney and spleen of European seabass.

Infection and immunization trials of Asian seabass (Lates calcarifer) against fish pathogen Vibrio anguillarum.

Vibriosis is one of the most prevalent fish diseases caused by bacteria belonging to the genus Vibrio sp. Fish disease will be controlled by proper vaccination trials and maintenance of fish form. Pathogenicity for Asian seabass (Lates calcarifer) against V. anguillarum results in necrosis and haemorrhagic areas near the base of fins, exopthalmia and ulcers on the skin surface. Around 50, 100, 200 )l of formalin killed bacterial cells were injected (Intraperitoneal) to three different size (5-10, 20-30, 35-50g) of seabass fishes respectively and control sere as saline were maintained separately. The Relative Percentage Survival (RPS) for vaccinated fishes was 60, 75, and 62.5 respectively and the vaccinations for 20-30 g fishes stay alive. These results stated that the vaccination for fishes with 20- 30 g size may fabricate good immune response.