Ten isolates of bacteria were performed from diseased black tiger shrimp collected from farms in Nakhon Si Thammarat, Patthalung, Songkhla, Satun and Patthani. All isolates belonged to Vibrio harveyi, forming rounded colonies with smooth periphery and hemolysis of red blood cells. The colonies appeared green and yellow on TCBS selective media and indeeded both luminescent and non-luminescent groups. To support good growth as cell proliferation, an addition of 2-8% salt to the culture media was required. Sensitivity tests against antibiotics showed that all ten isolates were sensitive to chloramphenicol, norfloxacin, trimethoprim-sulfamethoxazole, oxolinic acid, oxytetracyclin and sarafloxacin at 80, 60, 50, 40, 40 and 30%, respectively. When 12-15 g juvenile shrimp were tested for the virulence by injection, the isolates were highly virulent with OD at 640 nm in the range 0.007-0.139. The bacteria concentration of 1.60×106 - 7.27×107 CFU/ ml caused 50% juvenile mortality in ten days. The physiological changes after susceptibility included a decline in the blood cells (p<0.05), higher plasma pH (p<0.05), decrease in serum protein within 24 h (p<0.05), and a return to normal levels within 48 h. There were no differences in blood sugar levels and phenoloxidase activity between the healthy and the infected individuals. Histological study showed that at an early period of susceptibility, there were swollen tubular lumen, minor cell degeneration of the hepatic tubules and lymphoid organs and aggregation of blood cells around the degenerating cells. During 7-day susceptibility period, there was large scale cell degeneration of the hepatopancreas, lymphoid organs, gills and blood forming organs with a great extent of blood cell aggregation and eventually mortalities occurred. During 14 day susceptibility period, there was almost entire degeneration of cells in hepatic tubules and lymphoid organs causing hepatopancreatic tubular necrosis. During this period, juveniles did not accept feed and total mortality occurred.
