The effect of bacterial bath by Vibrio harveyi and V.alginolyticus on mortality of white leg shrimp (L.vannamei) and microscopic scrutiny of hepatopancreas, intestine and haemocytes in diseased shrimp

Vibrio spp are the most important bacterial pathogens of shrimp. Vibrio spp are aquatic bacteria that are widely distributed in fresh water, estuarine and marine environments. In this paper the effects of Vibrio harveyi and Vibrio alginolyticus bacteria on mortality rate of 75-day old Litopenaeus vannamei post larvaes were studied. Pathogenicity experiments were done by 30-minute bath method and with 108 and 109 cell/ml concentrations of the two strains of bacteria under optimal circumstance. No significant mortality was seen in the studied concentrations of V.alginolyticus in shrimp population. In V.harveyi challenge at 109 cell/ml concentration significant mortality (61.9%) of shrimp population was seen and the bacterium was isolated from 76.8% of the cases. Histopathological lesions in diseased shrimps in challenge with V. harveyi were dominantly necrosis and sloughing of hepatopancratic epithelial cells, deterioration of acinar structure of tubules, haemocyte infiltration in sinusoidal spaces of hepatopancreas. Tissue debris in tubules and in rare cases melanin aggregation around the necrotic cells was also seen. Focal necrosis and sloughing of epithelial cells in intestine tissue were seen in 28% of diseased shrimps. Population percentage of large granular haemocytes (LGC) and semi granular haemocytes (SGC) in diseased shrimps were decreased significantly in comparison with the control group and population percentage of hyaline cells were increased significantly (p≤0.05). Results of this surevey can help histopathological recognition of vibriosis disease in shrimp.