Bacterium Flavobacterium columnare is the causative agent of columnaris disease in many wild and farmed fish species.
Immunostimulants are used with success in aquaculture against many pathogens, but the ability to improve innate resistance to columnaris disease has not been studied. Fingerling rainbow trout were treated with two
immunostimulants, yeast
beta-glucan and
beta-hydroxy-beta-methylbutyrate (HMB). Selected innate immune function parameters, the production of
reactive oxygen species (ROS) by whole blood and by isolated head kidney leukocytes, plasma
lysozyme activity and
complement bacteriolytic activity, were determined to assess the immune status of fish. The fish were then bath challenged with virulent F. columnare bacteria, and the mortality of fish was recorded. Given orally both stimulants raised the levels of immune function parameters, but did not improve survival in challenge at any concentration of the stimulants used. Intra peritoneal injection of
beta-glucan increased parameter values several fold, but no beneficial effect of injected
glucan on survival was noted. As a control,
antibiotic medication administered prior to and during the challenge
infection prevented the mortality. Innate immune mechanisms, even when induced to high levels with
immunostimulants, as evidenced here, were not able to increase resistance against F. columnare. This may be connected to the external character of the
infection. The results from the treatments with
beta-glucan and HMB suggest that there is little prospect of preventing columnaris disease by means of
immunostimulants in early life stage of rainbow trout. However, the efficacy of other immune stimulants remains open.