Efficacy of Vaccination against β-hemolytic Streptococcal Infection in Cultured Yellowtail, Seriola quinqueradiata

Abstract

The purpose of this investigation was to evaluate the efficacy of vaccination against β-hemolytic streptococcal infection in cultured yellowtail, Seriola quinqueradiata. Formalin killed cells ofβ-hemolyticStreptococcussp. isolated from diseased yellowtail were administered to yellowtail fingerling weighing 10.1 g by immersion, oral, and intraperitoneal injection methods. Twenty and 35 days after vaccination, each group was challenged by intraperitoneal inoculation of the bacteria. At this time, agglutinating antibody titers were undetectable (<1: 2) in the sera of fishes vaccinated by immersion and oral methods, whereas in those vaccinated by injection were barely detectable (< 1: 2 - 1: 8) . After the challenge, fishes vaccinated by injection showed neither clinical signs of the disease nor they died. On the other hand immersion and oral vaccination were ineffective, showing typical clinical signs of the disease. Changes in viable cell numbers of inoculated bacteria in the organs of fish vaccinated by injection were determined. According to the data, bacterial cell numbers decreased rapidly in the blood, spleen, and kidney whereas remained static in the brain. These results indicate that vaccination by injection method is very effective in preventing β-hemolytic streptococcal infection in cultured yellowtail.