LEUKOCYTES FROM ATLANTIC SALMON, SALMO-SALAR L, EXPERIMENTALLY INFECTED WITH INFECTIOUS SALMON ANEMIA (ISA) EXHIBIT AN IMPAIRED RESPONSE TOMITOGENS

The proliferative response to mitogens of head kidney leucocytes from Atlantic salmon, Salmo salar L., experimentally infected with infectio us salmon anaemia (ISA) was examined. The mean haematocrits of ISA-ino culated fish were significantly lower than the mean haematocrits of co ntrol-inoculated fish at day 14. Mortality in ISA-inoculated fish appe ared at day 16 after inoculation. Seven days after inoculation, leucoc ytes from ISA-inoculated fish showed an increased response to phytohae magglutinin (PHA) compared to control-inoculated fish, while no change in the response to lipopolysaccharide from Escherichia coli (LPS) cou ld be observed. At days 14 and 22 after inoculation, the responses to both LPS and PHA of leucocytes from ISA-inoculated fish were severely impaired. These suppressions of the immune response of leucocytes from ISA-inoculated fish were found in fish with low haematocrits (<15) as well as in fish with haematocrits higher than 30, suggesting that sup pression of the immune system and the development of anaemia are indep endent events in the pathogenesis of ISA.