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Accumulation of Pathogenic Prion Protein (PrP^Sc) in Nervous and Lymphoid Tissues of Sheep with Subclinical Scrapie

Departments of Sheep and Goat Research, Sandnes, Norway (CE, MJU, MAT ¹) and Morphology, Genetics, and Aquatic Biology, Oslo, Norway (CE), The Norwegian School of Veterinary Science; and Department of Pathology, National Veterinary Institute, Oslo, Norway (SLB)

All sheep older than 1 year of age from a flock of the Rygja breed in which clinical scrapie was detected for the first time in two animals (4%) were examined for accumulation of pathogenic prion protein (PrP^Sc) by immunohistochemistry in the obex, the cerebellum, and the medial retrophayngeal lymph node. In addition, six lambs, 2–3 months old, all offspring of PrP^Sc-positive dams, were examined for PrP^Sc in the ileal Peyers' patch (IPP), the distal jejunal lymph node, the spleen, and the medial retropharyngeal lymph node (RPLN). In this flock, 35% (17/48) of the adult sheep showed accumulation of PrP^Sc, an eightfold increase compared with clinical disease. All positives carried susceptible PrP genotypes. Three sheep had deposits of PrP^Sc in the RPLN and not in the brain, suggesting that this organ, easily accessible at slaughter, is suitable for screening purposes. Two 7-year-old clinically healthy homozygous V₁₃₆Q₁₇₁ ewes showed sparse immunostaining in the central nervous system and may have been infected as adults. Further, two littermates, 86-days-old, showed PrP^Sc in the IPP. Interestingly, one of these lambs had the intermediate susceptible PrP genotype, VA₁₃₆QR₁₇₁. In addition to early immunolabeling in the dorsal motor nucleus of the vagal nerve, a few of the sheep had early involvement of the cerebellum. In fact, a 2-year-old sheep had sparse deposits of PrP^Sc in the cerebellum only. Because experimental bovine spongiform encephalopathy (BSE) in sheep seems to behave in a similar manner as natural scrapie, these results, particularly regarding spread of infectivity, may have implications for the handling of BSE should it be diagnosed in sheep.

Key words: Brain; immunohistochemistry; lymphoid tissue; PrP^Sc; sheep; subclinical scrapie.

Request reprints from Dr. M. J. Ulvund, Department of Sheep and Goat Research, The Norwegian School of Veterinary Science, Kyrkjevegen 332/334, N-4325 Sandnes (Norway). E-mail: martha.ulvund{at}veths.no.

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