Nora Hunter

Transmission of BSE by blood transfusion in sheep

We have shown that it is possible to transmit bovine spongiform encephalopathy (BSE) to a sheep by transfusion with whole blood taken from another sheep during the symptom-free phase of an experimental BSE infection. BSE and variant Creutzfeldt-Jakob disease (vCJD) In human beings are caused by the same infectious agent, and the sheep-BSE experimental model has a similar pathogenesis to that of human vCJD. Although UK blood transfusions are leucodepleted--a possible protective measure against any risk from blood transmission--this report suggests that blood donated by symptom-free vCJD-infected human beings may represent a risk of spread of vCJD infection among the human population of the UK.

Transmission of prion diseases by blood transfusion

Attempts to detect infectivity in the blood of humans and animals affected with transmissible spongiform encephalopathies (TSEs or prion diseases) have often been inconclusive because of the limitations of cross-species bioassays and the small volumes of blood that can be injected by the intracerebral route. A model has been developed for the experimental study of TSE transmission by blood transfusion using sheep experimentally infected with bovine spongiform encephalopathy (BSE) or natural scrapie as donors and susceptible scrapie-free sheep as recipients. Donors and recipients of the same species greatly increase the sensitivity of the bioassay and in sheep large volumes of blood can be injected by the intravenous (i.v.) route. Transmission of BSE to a single animal using this approach was reported recently. This study confirms this result with a second transmission of BSE and four new cases of transmission of natural scrapie. Positive transmissions occurred with blood taken at pre-clinical and clinical stages of infection. Initial studies indicate that following such infection by the i.v. route, deposition of the abnormal prion protein isoform, PrP(Sc), in peripheral tissues may be much more limited than is seen following oral infection. These results confirm the risks of TSE infection via blood products and suggest that the measures taken to restrict the use of blood in the UK have been fully justified.

PrP genotype and agent effects in scrapie: change in allelic interaction with different isolates of agent in sheep, a natural host of scrapie

Man and sheep are the two species in which spongiform encephalopathies occur naturally, and in which there are recognized genetic components that predispose an individual person or sheep to clinical disease. In both species mutations/polymorphisms in the PrP gene have been linked to the incidence of natural disease, but only in sheep is it possible to investigate by deliberate exposure to infection whether these polymorphisms are directly correlated with survival time. Cheviot sheep of different PrP genotypes were challenged with one of two isolates of scrapie or an isolate of bovine spongiform encephalopathy and the survival time and incidence of disease were monitored. Genotype analysis showed that dimorphisms in codons 136 and 171 of the ovine PrP gene correlated with control of disease incidence and modulation of incubation time. Crucially, the functional effects of these domains of PrP were shown to alternate depending on the isolate of infecting agent.

Natural scrapie in a closed flock of Cheviot sheep occurs only in specific PrP genotypes

SummaryNatural scrapie in a closed flock of South Country Cheviot sheep has resulted in 45 deaths between 1986 and 1995. Of these cases, 35 sheep have been analysed for disease-linked PrP gene polymorphisms and all encode valine at codon 136 on at least one allele with 77% homozygous (VV136) and 23% valine/alanine heterozygotes (VA136). Mean survival time was 907 and 1482 days for VV136 and VA136 scrapie affected animals respectively. VV136 animals were all at great risk of disease if allowed to live long enough. However scrapie occurred only in a specific subgroup of VA136 sheep, survival advantage depending on VA136 animals being heterozygous for other polymorphisms at codons 154 or 171. The flock history has been recorded in great detail since its foundation in 1960 however there was no strong evidence for simple maternal or paternal transmission of disease other than inheritance of PrP genotype.

Different forms of the bovine PrP gene have five or six copies of a short, G-C-rich element within the protein-coding exon.

Current models of the virus-like agents of scrapie and bovine spongiform encephalopathy (BSE) have to take into account that structural changes in a host-encoded protein (PrP protein) exhibit an effect on the time course of these diseases and the survival time of any man or animal exposed to these pathogens. We report here the sequence of different forms of the bovine PrP gene which contain either five or six copies of a short, G-C-rich element which encodes the octapeptide Pro-His-Gly-Gly-Gly-Trp-Gly-Gln or its longer variants Pro-Gln/His-Gly-Gly-Gly-Gly-Trp-Gly-Gln. Out of 12 cattle, we found eight animals homozygous for genes with six copies of the Gly-rich peptide (6:6), while four were heterozygous (6:5). Two confirmed cases of BSE occurred in (6:6) homozygous animals.

PrP genetics in sheep and the implications for scrapie and BSE

The strong links between PrP genotype and the occurrence of scrapie in sheep strengthen evidence supporting the central importance of the PrP protein in the development of transmissible spongiform encephalopathies, despite the fact that the cattle PrP gene has, so far, failed to show any association between PrP alleles and susceptibility to BSE.

Natural scrapie and PrP genotype: case-control studies in British sheep.

Natural scrapie in sheep is associated with polymorphisms of the PrP gene, particularly at amino acid codons 136, 154 and 171. This paper reports the results of nine scrapie case-control studies in Bleu du Maine, Herdwick, Merino x Shetland, Poll Dorset, Scottish Halfbred, Shetland, Soay, Suffolk and Swaledale sheep from British flocks affected by scrapie. In some outbreaks, scrapie was found to occur only in animals with at least one PrP allele encoding valine at codon 136 (V136), usually a relatively rare allele in healthy controls In other outbreaks, the v136, PrP aUlele was either not found or was not an absolute prerequisite for scrapie to develop. Although scrapie had a strong tendency to affect sheep with PrP genotypes homozygous for glutamine at codon 171 (QQ136), these genotypes (QQ136 but varying at other codon positions) were relatively common in healthy controls. The reliable prediction of scrapie susceptibility in previously uninvestigated sheep flocks will therefore require information at least about PrP genotypes at codons 136 and 171.

Novel polymorphisms in the caprine PrP gene: a codon 142 mutation associated with scrapie incubation period.

Age at disease onset and rate of progression of transmissible spongiform encephalopathies in man, sheep and mice are modulated by the host genome, in particular by the PrP gene and its allelic forms. Analysis of the caprine PrP gene revealed several different alleles. Four PrP protein variants were found, three of which were goat specific with single amino acid changes at codons 142, 143 and 240. The fourth was identical to the most common sheep PrP protein variant (Ala136-Arg154-Gln171). The dimorphism at codon 142 (Ile --> Met) appeared to be associated with differing disease incubation periods in goats experimentally infected with isolates of bovine spongiform encephalopathy, sheep scrapie CH1641 or sheep-passaged ME7 scrapie.

Molecular analysis of ovine prion protein identifies similarities between BSE and an experimental isolate of natural scrapie, CH1641

New variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE) are caused by the same strain of pathogen and, as sheep can develop experimental BSE, this has raised concern that humans may be at risk from eating mutton if BSE has naturally transmitted to sheep. Biochemical typing of abnormal prion proteins (PrPsc) has been suggested to detect BSE in sheep. Although this approach is ingenuous, we can now report biochemical evidence of strain variation in contemporary and archival brain tissue from cases of experimental BSE or experimental and natural scrapie in sheep. Interestingly, we found at least one isolate of natural scrapie (CH 1641) with a very similar, but not identical, PrPsc profile to BSE but which differs from BSE in its transmission characteristics to mice.

Association between natural scrapie and PrP genotype in a flock of Suffolk sheep in Scotland

The incidence of natural scrapie in sheep is associated with polymorphisms of the PrP gene, particularly those at codons 136, 154 and 171. In many breeds, the PrP allele encoding valine at codon 136 confers an extremely high risk of scrapie, but in Suffolk sheep this allele is vanishingly rare. In this study of a single closed flock of Suffolk sheep in Scotland, scrapie occurred primarily in animals which were homozygous for glutamine at codon 171, a genotype which was significantly less frequent in healthy flockmates. However, the apparent linkage between glutamine at codon 171 and scrapie was not completely recessive because two of 64 scrapie cases were heterozygous glutamine/arginine. These results suggest that breeding for increased resistance to scrapie in Suffolks by the selection of animals according to their PrP genotype is a feasible option.