Immunostimulation of striped snakehead Channa striata against epizootic ulcerative syndrome

Five immunostimulants were injected intraperitoneally into striped snakeheads ( Channa striata). The inhibitory effects of the serum and macrophages collected from the treated fish on the germination and subsequent growth of Aphanomyces invadans (= piscicida), the causative agent of epizootic ulcerative syndrome (EUS), were then assessed. Salar-bec, a vitamin premix, and Ergosan, an alginate, both stimulated the inhibitory effects of serum on the germination and subsequent growth of A. invadans cysts, and the inhibitory effect of macrophages on growth. Betamak C85, a yeast extract containing β glucans and mannans, and Lysoforte, a lysophospholipid biosurfactant, induced little or no improvement in the parameters measured. Oro glo layer dry, a xanthophyll preparation, was rejected because of high mortalities among injected fish. Salar-bec showed the greatest improvement in the inhibition of both germination and growth by serum, and of growth by macrophages. It was selected for a tank trial in which snakeheads were fed on pellets coated with 2 g kg −1 Salar-bec, then injected with A. invadans. Control fish were fed on uncoated feed. Although the incidence of infection was not affected, hyphae appeared later in treated fish and granulomata developed faster at the infection site, suggesting an enhanced ability to contain the infection. Relative percent survival of treated fish was 59.2% higher than the controls over the 40-day trial. Anti- A. invadans antibody concentration was higher in treated fish, which may also have contributed to the containment of the infection.