Studies on the development of a vaccine against Mycobacterium sp

Mycobacteriosis, caused by Mycobacterium sp., results in severe loss of fish production in Japan’s aquaculture industry. Since the bacterium can survive and grow in side host cells, chemotherapeutic agents are not effective and potent vaccines are required to control the disease. In this study, the effects of two vaccine candidates, Bacillus Calmette and Guèrin (BCG, an attenuated strain of Mycobacterium bovis) and formalin-killed cells (FKC) of Mycobacterium sp. strain 012971, on the immediate and acquired immune response were evaluated in Japanese flounder, Paralichthys olivaceus. On 1, 3 and 7 days post treatments with these candidates, gene expression levels of inflammatory cytokines encoding interleukin-1β (IL-1β), IL-6 and tumor necrosis factor α (TNFα) increased. This suggests that the vaccine candidates stimulate immediate immune responses. To determine if specific defenses against mycobacteriosis were acquired, purified protein derivative (PPD) from Mycobacterium sp. strain 012971 was injected intramuscularly into fish at 4 weeks after vaccine treatments and the inflammatory cytokine gene expression levels were analyzed. Gene expression levels of the inflammatory cytokines were increased only for the fish treated with BCG, but not those with FKC. Subsequently, the vaccinated fishes were challenged with Mycobacterium sp. The relative percentage survival (RPS) of BCG and FKC vaccinated groups at 12 days post challenge were 37.5 and 18.1, respectively. These data indicate that BCG might be useful as a vaccine against fish mycobacteriosis.