Duration of protection against Aeromonas salmonicida brook trout immunostimulated with glucan or chitosan by injection or immersion

Brook trout (Salvelinus fontinalis) were given the immunostimulants glucan or chitosan by single injections or by a 30‐min immersion to induce protection against the fish disease furunculosis. At 1, 2, 3, 7, 14, 21, and 28 d after immunostimulation, sublots from these groups were challenged by a 1‐min bath exposure to a 48‐h culture of Aeromonas salmonicida. All four groups showed high levels of protection against challenges given 1, 2, and 3 d after immunostimulants were administered. The groups injected with the immunostimulants showed a higher degree of protection than the immersed fish. They also retained a higher degree of protection against the pathogen challenge. However, by 14 d after immunostimulation, protection in both groups was greatly reduced. No significant differences in protection levels were noted between the glucan and chitosan immunostimulants. The effective short‐term protection induced by these immunostimulants demonstrates the importance of nonspecific defenses in fish and how, under certain conditions, these immunostimulants could be used for the prevention of diseases in fish culture.