Rainbow trout (Oncorhynchus mykiss), immunised with horseradish peroxidase, were given horseradish peroxidase intravenously, and the trapping of antigen in the spleen was followed 1, 24, and 48 h after injection. After 1 h, the localisation of horseradish peroxidase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colocalization of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and suggested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very little horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in association with large cells with prominent cytoplasmic extensions. In non-immunized fish given horseradish peroxidase intravenously, antigen was not detected in ellipsoids. Thus, the observed difference between immunised and non-immunized trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary immune response.

ABSTRACT. Thirteen week old Atlantic salmon Salmo salar fry and 15 wk old brook trout Salvelinus fontinalis with average weights of 0.19 and 0.17 g, respectively, were bath challenged with infectious pancreatic necrosis virus (IPNV), serotype Sp, NI strain. The virus had been isolated from a clinical outbreak of infectious pancreatic necrosis (IPN) in Atlantic salmon parr. Cell culture supernatant after a first passage of virus was used as the infective material. Approximately 400 fry of each species were divided into 2 parallel tanks with challenged fish and 2 parallel tanks with non-challenged fish. At 69 d after challenge, cumulative mortalities of Atlantic salmon were 76 and 82 % for the challenged groups and 6 and 15% for the non-challenged groups. The corresponding values for brook trout were 42 and 4 3 % for the challenged groups and 8 and 10% for the non-challenged groups. The onset of mortality in challenged Atlantic salmon was delayed compared with the onset in brook trout. IPNV was reisolated from the fry of all challenged groups. Histological examination of sampled moribund fish showed hepatic degeneration or necrosis and exocrine pancreatic necrosis in fry from all challenged groups. The presence of IPNV in the lesions was demonstrated immunohistochemically. Virological, histological and immunohistochemical examinations of fry sampled prior to challenge and of non-challenged fry gave negative results. This study represents, to our knowledge, the first report of an experimental induction of clinical IPN in Atlantic salmon fry.