INFECTIVITY OF INTERNAL TISSUES OF ATLANTIC SALMON, SALMO-SALAR L, EXPERIMENTALLY INFECTED WITH THE ETIOLOGIC AGENT OF INFECTIOUS SALMON ANEMIA (ISA)

Infectivity of internal organs and cells of Atlantic salmon, Salmo sal ar L., was studied at various times after infection with the aetiologi cal agent of infectious salmon anaemia (ISA). Experimentally infected salmon smelts developed anaemia just prior to the onset of ISA-mortali ty. ISA-infectivity of preparations made from liver, kidney, spleen, p lasma, red blood cells (RBC) and head kidney leucocytes was determined by inoculating Atlantic salmon parr with the respective preparations. ISA-mortality was observed after inoculation of salmon parr with prep arations of kidney and liver, and to a minor degree, with spleen and a fraction of head kidney leucocytes (WBC1) collected 7 days post-infec tion. At 11 days post-infection, the infectivity of these preparations increased and ISA-infectivity was also observed with a second fractio n of head kidney leucocytes (WBC2), red blood cells (RBC) and blood pl asma. At this time, the ISA-infectivity of kidney was not significantl y higher (P > 0.05) than that of liver but was significantly higher th an all other preparations as judged by Cox-regression analysis. At day s 14 and 18 post-infection, the infectivity of kidney was significantl y higher (P < 0.05) than that of liver, but not at 21 and 25 days post -infection. Generally, the ISA-infectivity of kidney was higher than s pleen, head kidney leucocytes (WBC1 and WBC2), RBC and plasma, althoug h the difference was not significant at all time points. For example, at day 25 post-infection, the infectivity of kidney was only significa ntly higher than that of spleen and plasma. On a per gram basis, head kidney leucocytes proved to contain higher amounts of infectious matte r than RBC. ISA-infective leucocytes present in the kidney tissue may have contributed to a major part of the infectivity recognized in the kidney preparations. Thus, head kidney leucocytes and other kidney leu cocytes also may be considered to be among the most important target c ells of the aetiological agent of ISA.