Temperature effect on the immune defense functions of Arctic charr Salvelinus alpinus.

The Arctic charr Salvelinus alpinus is an endangered fish species in Finland, and thus farming is carried out mainly for stocking purposes. Farmed charr are susceptible to infection with atypical Aeromonas salmonicida (aAS). Losses of valuable brood stock will severely reduce the genetic diversity of stocked charr. No commercial vaccines are available to prevent aAS infection, and vaccines against furunculosis (caused by typical A. salmonicida, tAS) do not protect the charr against aAS infection. The effects of a metabolizable oil-adjuvanted, bivalent vaccine (containing killed aAS and A. salmonicida salmonicida bacteria) on the immune system of 1 yr old hatchery-reared charr originating from Lake Inari in Northern Finland were examined. Fish vaccination in Finland generally takes place either from October to November or from February to April, when the water temperature is low (1 to 3degrees C). The water temperature starts to increase in mid-May. Therefore, we also investigated whether post-vaccination (p.v.) temperature had an influence on the immune system of this cold-water fish species. The fish were immunized intraperitoneally at 2.9 degrees C at the end of April. After 52 d, during which the water temperature increased from 2.9 to 10.0 degrees C, the charr were exposed to 1 of 3 test temperatures: 10.3, 14.1 or 18.1 degrees C. Prior to vaccination, and 49, 75 and 103 d p.v., several immune parameters were measured in both unvaccinated and vaccinated charr. Vaccination induced a significant anti-aAS-specific antibody response, and increased plasma lysozyme activity at all p.v. temperatures. The haemolytic activity of the complement system was unaffected either by vaccination or p.v. temperatures. There was a slight positive correlation between p.v. temperature and lysozyme activity of the charr. The significant increase in lysozyme activity took place in vaccinated charr in the first 49 d p.v. as water temperatures increased from 2.9 to 10 degrees C. Furthermore, the highest activity of lysozyme in the plasma was observed 49 d p.v. Our results indicate that a rise in water temperature above 10 degrees C does not significantly enhance the vaccination response of charr. This could be one reason why farmed Arctic charr, which are well adapted to a cold climate, are highly susceptible to aAS infection in the summer.