A rickettsia-hke organism was isolated from diseased Atlantic salmon S a h o salar in Norway. Because of morphological and serological similarities to the type strain the suggested name of the organism is Plscinckettsia salmonis. The bacteilum is considered the most probable cause of a systemic disease diagnosed in 51 farms along the west coast of Norway. Most of the cases occurred in the autumn of 1988. The disease was only recorded in smolts after exposure to sea water and cumulative mortality has been low. In 63% of fish with gross lesions examined d u n n g outbreak of disease in 14 of the affected farms, the typical macroscopic finding was a normal coloured liver with white, circular, sometimes haemorrhagic foci. Of fish with gross lesions, 35 % showed pale gllls, a yellow, mottled liver, and haemorrhages scattered throughout the skeletal muscles, perivisceral fat, the stomach wall and the swimbladder Histomorphological changes were most often observed as necrosis and granulomatous inflammation in the 11ver. Intracellular, intravacuolar bacteria-like inclusions with a n affinity for phagocytic host cells were observed Transmission electron rnicroscopy revealed individual or paired organisms enclosed in membrane-bound vacuoles.

Six monoclonal antibodies (MAbs) produced against the infectious pancreatic necrosis virus (IPNV) Nl strain were used in a dot-blot assay to examine reference strains of the nine proposed serotypes and a representative selection of 81 Norwegian aquatic birnavirus isolates. These isolates had earlier been serotyped by use of a panel of 11 MAbs produced against other strains of IPNV. Correlations between the reaction patterns of the two panels of MAbs were found. All reference strains and field isolates shared two epitopes, one on VP2 and one on VP3. Seventy-seven of the field isolates reacted identically with the Nl strain (positive with all sLx MAbs). The Sp type strain was positive with five of the MAbs and was different from all the field strains. The other reference strains (WB, VR299, Ab, Ja, Te, He, Cl, C2 and C3) were positive with two to four of the MAbs. Together with previously published data, these findings indicate that most Norwegian isolates are closely related to, or identieal with, the Nl strain and belong to the Sp serotype. No correlation between the health status of Atlantic salmon and antigenieity of the isolates was found. Testing of the referenee strains in ELISA revealed some diserepaneies with the dot-blot results.

A panel of 11 monoclonal antibodies (MAbs) was used in an immunodot assay to isolated in Norvvay from different speeies of both earrier and diseased fish. All the 81 isolates shared a serogroup A-speeifie epitopc, and 67 of them had a reaetion pattern identieal to the Sp-type strain. Of the other 14 isolates, nine resembled the Sp-type strain, two resembled both the Sp-and He-type strains, two were elosest to the Ab-type strain and one resembled the Te-type strain. Sp-rclated strains were predominant from all nine host groups, including from both Atlantic salmon, Salmo salar L., with infectious pancreatie necrosis (IPN) and healthy Atlantic salmon. The Nl strain reaeted identically to the Sp-type strain.Correspondence: Hans Peter Melby, P.O.B. 8156 Dep, N-()(I33 Oslo, Norway.

. An infectious pancreatic necrosis virus (IPNV) carrier stock of Atlantic salmon parr (100 g) was divided between two tanks and inoculated experimentally with tissue homogenate containing the aetiologic agent of infectious salmon anaemia (ISA) and non‐ISA tissue homogenate (control), respectively. Plasma and kidney samples from ISA‐infected and control fish were taken twice weekly for 25 days. In the kidney samples, IPNV was quantified by a plaque assay. In plasma, anti‐IPNV antibodies were measured using an indirect ELISA. The ISA‐infection did not seem to activate the IPNV‐infection. Neither the proportion of fish with IPNV or anti‐IPNV antibodies, nor the IPNV titre or level of anti‐IPNV antibodies showed any specific trend during the study. Independently of ISA, IPNV was detected in 54 out of 132 fish (41%), while 71 out of 195 fish (36%) had plasma antibodies against IPNV. No association was found between detection of IPNV, and presence or level of anti‐IPNV antibodies in individual fish.

Twelve isolates of aquatic birnaviruses (9 field isolates from marine fish and shellfish and the Ab/A3, Sp/A2 and NI strains) were serotyped using an immunodot assay. The assay with 17 monoclonal antibodies revealed some serological variation among the Norwegian isolates, but the N1 strain and the Sp/A, type strain reacted identically. A genotyping assay based upon restriction fragment analysis of a polymerase chain reaction-amplified fragment from the VP2 coding region was run parallel to the serotyping. All the 9 serogroup A serotypes, except He/A, and Can. 3/A8, were assayed in addition to the Norwegian isolates mentioned above. This method differentiated among all serotypes tested, but only small differences were observed among the Norwegian isolates. The NI and Sp/A2 strains reacted identically. Our results indicate homogeneity between Norwegian birnavirus isolates and support earlier studies that concluded that the NI strain belongs to the Sp/A, serotype.