Prion diseases are progressive and fatal, neurodegenerative disorders described in humans and animals. According to the "protein-only" hypothesis, the normal host-encoded prion protein (PrP C) is converted into a pathological and infectious form (PrP Sc) in these diseases. Transgenic knockout models have shown that PrP C is a prerequisite for the development of prion disease. In Norwegian dairy goats, a mutation (Ter) in the prion protein gene (PRNP) effectively blocks PrP C synthesis. We inoculated 12 goats (4 PRNP +/+ , 4 PRNP +/Ter , and 4 PRNP Ter/Ter) intracerebrally with goat scrapie prions. The mean incubation time until clinical signs of prion disease was 601 days post-inoculation (dpi) in PRNP +/+ goats and 773 dpi in PRNP +/Ter goats. PrP Sc and vacuolation were similarly distributed in the central nervous system (CNS) of both groups and observed in all brain regions and segments of the spinal cord. Generally, accumulation of PrP Sc was limited in peripheral organs, but all PRNP +/+ goats and 1 of 4 PRNP +/Ter goats were positive in head lymph nodes. The four PRNP Ter/Ter goats remained healthy, without clinical signs of prion disease, and were euthanized 1260 dpi. As expected, no accumulation of PrP Sc was observed in the CNS or peripheral tissues of this group, as assessed by immunohistochemistry, enzyme immunoassay, and real-time quaking-induced conversion. Our study shows for the first time that animals devoid of PrP C due to a natural mutation do not propagate prions and are resistant to scrapie. Clinical onset of disease is delayed in heterozygous goats expressing about 50% of PrP C levels.

Abstract. Twenty-one orally inoculated and seven naturally infected sheep with scrapie were examined for PrP Sc in peripheral tissues and in the central nervous system (CNS), using immunohistochemistry. In the inoculated group, VRQ (valine at codon 136, arginine at codon 154 and glutamine at codon 171)/VRQ sheep generally had a greater accumulation of the pathologic form of prion protein (PrP Sc ) in peripheral tissues, as compared with VRQ/ARQ (alanine at codon 136, arginine at codon 154, and glutamine at codon 171) animals at corresponding time points after inoculation. PrP Sc was not detected in the ileal Peyer's patch, the spleen, the superficial cervical lymph node, and peripheral nervous tissues of several inoculated VRQ/ARQ animals. All inoculated VRQ/VRQ sheep, but only one of eight inoculated VRQ/ARQ animals, were PrP Sc -positive in the CNS. Thus, the propagation of PrP Sc seemed slower and more limited in VRQ/ARQ animals. Tissue and cellular localization of PrP Sc suggested that PrP Sc was disseminated through three different routes. PrP Sc -positive cells in lymph node sinuses and in lymphatics indicated spreading by lymph. The sequential appearance of PrP Sc in the peripheral nervous system and the CNS, with satellite cells as early targets, suggested the periaxonal transportation of PrP Sc through supportive cells. Focal areas of vascular amyloid-like PrP Sc in the brain of five sheep, suggested the hematogenous dissemination of PrP Sc . There was a poor correlation between the amount of PrP Sc in the CNS and clinical signs. One subclinically affected sheep showed widespread PrP Sc accumulation in the CNS, whereas three sheep had early clinical signs without detectable PrP Sc in the CNS. A VV 136 (homozygous for valine at codon 136) sheep inoculated with ARQ/ARR (alanine at codon 136, arginine at codon 154, and arginine at codon 171) tissue succumbed to disease, demonstrating successful heterologous transmission. Less susceptible sheep receiving VRQ/VRQ or ARQ/ARR material were PrP Sc -negative by immunohistochemistry, enzyme-linked immunosorbent assay, and western blot.

Abstract. 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 V 136 Q 171 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, VA 136 QR 171 . 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-yearold 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.

Cataracts in farmed Atlantic salmon have been known for many years, but the aetiology and importance of the disease have not been clarified. A cross-sectional field study of 51 cages of Atlantic salmon at 49 randomly selected sea sites was performed during the summer of 1998. The target population was spring and autumn entry groups of the 1997 generation salmon. Approximately 15 fish from each cage, altogether 777 fish, were autopsied by the same person. Each eye of the fish was scored for cataracts on a scale from 0 to 4 using an otoscope lamp with magnification. The weight and length of each fish were measured. The prevalence of cataracts was 83% and 79% in spring entry groups and autumn entry groups, respectively. The overall mean cataract index (mean score of both eyes) was 1.23, being significantly higher in the spring entry groups (1.36) than the autumn entry groups (0.85). The final results in the spring entry groups showed that the fish groups with higher weight at sea transfer also had a higher cataract index at inspection. The risk of development of cataracts varied significantly among the offspring from the 5 strains represented in the study. Fish from sites located in 2 counties in the southern part of Norway had a significantly higher cataract index than fish farmed in the northernmost county in the study. For the autumn entry groups none of the explanatory variables was significant. In the spring entry groups a significant negative relationship was observed between the cataract score and the weight of the fish at the time of inspection (Pearson's r = -0.17), while the corresponding correlation for the autumn released groups was r = -0.10. Among the spring entry groups the average weight of the fish with the highest cataract score was estimated to about a third of the weight of the fish with no visible cataracts. KEY WORDS: Cataracts · Epidemiology · Atlantic salmon · Risk factors Resale or republication not permitted without written consent of the publisherDis Aquat Org 45: [229][230][231][232][233][234][235][236] 2001 (1) zinc deficiency (Ketola 1979, Barash et al. 1982, Richardson et al. 1985, (2) riboflavin deficiency (Poston et al. 1977, Hughes et al. 1981, Barash et al. 1982, (3) tryptophan deficiency (Poston & Rumsey 1983), (4) thiamine deficiency (Hughes 1985) and (5) methionine deficiency (Poston et al. 1977, Barash et al. 1982, Walton et al. 1982, Cowey et al. 1992. Other factors associated with cataract formation are rapid fluctuations in water temperature (Loewenstein & Bettelheim 1979, Bruno & Raynard 1994, Bjerkås & Bjørnestad 1999, rapid growth (Kincaid & Elrod 1991, fluctuations in the water salinity (Hargis 1991), triploid genetic constitution (Wall & Richards 1992), genetic identity (strain) (Kincaid & Elrod 1991), UV radiation (Cullen et al. 1993), cholinesterase inhibitors (Fraser et al. 1989) and electrolytic imbalance (Iwata et al. 1987).Cataracts in seawater farmed salmon have been noticed for many years, but the incidence apparently increased in the 1990s. On th...

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