Five cases of scrapie with unusual features have been diagnosed in Norway since 1998. The affected sheep showed neurological signs dominated by ataxia, and had the PrP genotypes homozygous A136 H154 Q171/ A136H154Q171 or heterozygous A136H154Q171/A136R154Q171, which are rarely associated with scrapie. Brain histopathology revealed neuropil vacuolisation essentially in the cerebellar and cerebral cortices; vacuolation was less prominent in the brainstem, and no lesions were observed at the level of the obex. The deposits of PrPSc were mainly in the cortex of the cerebellum and cerebrum, and no PrPSC was detectable by immunohistochemistry and ELISA in the lymphoid tissues investigated. Western blot analysis showed that the glycotype was different from other known scrapie strains and from the BSE strain. From a diagnostic point of view, these features indicate that this type of scrapie, designated Nor98, could have been overlooked and may be of significance for sampling in scrapie surveillance programmes.

In modern bony fishes, or teleost fish, the general lack of leucocyte markers has greatly hampered investigations of the anatomy of the immune system and its reactions involved in inflammatory responses. We have previously reported the cloning and sequencing of the salmon CD3 complex, molecules that are specifically expressed in T cells. Here, we generate and validate sera recognizing a peptide sequence of the CD3e chain. Flow cytometry analysis revealed high numbers of CD3e + or T cells in the thymus, gill and intestine, whereas lower numbers were detected in the head kidney, spleen and peripheral blood leucocytes. Subsequent morphological analysis showed accumulations of T cells in the thymus and spleen and in the newly discovered gilllocated interbranchial lymphoid tissue. In the latter, the T cells are embedded in a meshwork of epithelial cells and in the spleen, they cluster in the white pulp surrounding ellipsoids. The anatomical organization of the salmonid thymic cortex and medulla seems to be composed of three layers consisting of a sub-epithelial medulla-like zone, an intermediate cortex-like zone and finally another cortex-like basal zone. Our study in the salmonid thymus reports a previously non-described tissue organization. In the intestinal tract, abundant T cells were found embedded in the epithelium. In non-lymphoid organs, the presence of T cells was limited. The results show that the interbranchial lymphoid tissue is quantitatively a very important site of T cell aggregation, strategically located to facilitate antigen encounter. The interbranchial lymphoid tissue has no resemblance to previously described lymphoid tissues.

A sensitive immunohistochemical procedure was used to investigate the presence of prion protein (PrP) in the ileal Peyer's patch of PrP-genotyped lambs, including scrapie-free lambs and lambs naturally and experimentally exposed to the scrapie agent. The tyramide signal amplification system was used to enhance the sensitivity of conventional immunohistochemical procedures to show that PrP was widely distributed in the enteric nervous plexus supplying the gut wall. In scrapie-free lambs, PrP was also detected in scattered cells in the lamina propria and in the dome and interfollicular areas of the Peyer's patch. In the follicles, staining for PrP was mainly confined to the capsule and cells associated with vascular structures in the light central zone. In lambs naturally exposed to the scrapie agent, staining was prominent in the dome and neck region of the follicles and was also found to be associated with the follicle-associated epithelium. Similar observations were made in lambs that had received a single oral dose of scrapie-infected brain material from sheep with a homologous and heterologous PrP genotype 1 and 5 weeks previously. These studies show that the ileal Peyer's patch in young sheep may be an important site of uptake of the scrapie agent and that the biology of this major gut-associated lymphoid tissue may influence the susceptibility to oral infection in sheep. Furthermore, these studies suggest that homology or heterology between PrP genotypes or the presence of PrP genotypes seldom associated with disease does not impede uptake of PrP.

Two-hundred and forty healthy sheep and 32 cases of natural scrapie in Norway were analysed for disease-linked polymorphisms in the prion protein (PrP) gene. Scrapie was strongly associated with the presence of a valine polymorphism at codon 136 (V 136 ), as 68n8 % of the cases were homozygous (VV 136 ) and 15n6 % were valine/alanine heterozygous (VA 136 ). All cases were homozygous arginine/arginine at codon 154 (RR 154 ), except two which were homozygous histidine/histidine (HH 154 ). All cases except two were homozygous glutamine/glutamine at codon 171 (QQ 171 ), the two exceptions being heterozygous glutamine/arginine (QR 171 ). More than 80 % of all scrapie cases in Norway have occurred in a Cheviot-related crossbred type of sheep called Rygja. This type of sheep, which is largely restricted to the south-western coast, carries the V 136 allele at a higher frequency than do other breeds of Norwegian sheep. Polymorphisms at codons 138 and 151 are also described.

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