The effect of vaccination and sea water entry on immunocompetence and susceptibility to Kudoa thyrsites in Atlantic salmon (Salmo salar L.)

The effect of intraperitoneal (IP) vaccination and sea water entry (SWE) on the immumocompetence of Cascade Atlantic salmon was investigated. Smolts were IP injected with Aqua Health's Forte(TM) vaccine (Listonella (Vibrio) anguillarum, Listonella ordalii, Vibrio sabnonicida and Aeromonas salmonicida) at four times (42, 238, 433 and 630 degree days, DD) prior to SWE and were examined for immunocompetence. Immune response measurements included mitogen-driven proliferation of peripheral blood leukocytes (PBL), head kidney leukocyte respiratory burst activity and alternative complement hemolytic titres and were measured 24 h prior to SWE, 72 h post-SWE and again 3.5 weeks post-SWE. A 50% reduction in the number of PBL was observed 3 days post-vaccination. At this time LPS-driven proliferation was low (stimulation index, SI, 1.5-2.9) in all groups prior to SWE compared with that of PBL from freshwater-reared Atlantic salmon parr (6.7). By 72 h and 3.5 weeks post-SWE, the LPS-driven SI from unvaccinated salmon and those vaccinated 630 and 433 DD prior to SWE increased 3-fold. In contrast, SI from salmon vaccinated 42 and 238 DD prior to SWE remained low. A similar pattern was observed for cultured PBL stimulated with PHA, although unlike LPS-stimulated PBL, the SI of cells from parr and unvaccinated control smolts remained low following SWE but increased in fish vaccinated 433 and 630 DID prior to SWE. The respiratory burst activity of head kidney leukocytes was not affected by SWE but showed a transient 50% depression 3 days post-vaccination. The alternative complement activity (ACH50) was similar for all treatment groups prior to and at 72 h post-SWE. By 3.5 weeks post-SWE, ACH50 values in salmon vaccinated 42 and 238 DD prior to SWE doubled to 874 and 860 U/ml, respectively. The prevalence and severity of Kudoa thyrsites infections, detected in all treatment groups approximately 2400 DD following SWE, were not significantly different among groups. Atlantic salmon parr should be IP vaccinated no earlier.