Scrapie-free Merino and Poll Dorset sheep from Australia and New Zealand have normal frequencies of scrapie-susceptible PrP genotypes

Classic and atypical scrapie - a genetic perspective

Scrapie was the first prion disease to be recognised and the study of this disease in sheep and goats has provided a wealth of information not only for scrapie but also for the other prion diseases. All prion diseases are under strong genetic control of the prion gene PRNP, independent of whether they are typical or atypical scrapie and which of the different prion strains is causing infection. Decades of studies using experimental disease challenges and field surveys have established disease association models, in which species-specific amino acid variations in the prion or PrP protein, encoded by the PRNP gene, can predict disease susceptibility or resistance. PRNP genetics represents an important and successful basis for implementing scrapie eradication strategies in sheep and goats. In general terms these studies have revealed that there appear to be many more amino acid changes in PrP leading to increased resistance than to higher susceptibility. Most changes are in the globular part of PrP protein and three regions appear to have major influence. This knowledge can be transferred into prion diseases of other species to facilitate genetic control strategies. However, an obstacle remains with the lack of fully understanding the underlying molecular mechanism, impeding our ability to deal with the difference in the genetic control between typical and atypical forms of scrapie or to predict association in newly infected species. This chapter will discuss the advances in both typical and atypical scrapie from a genetic perspective.

Quantification of historical livestock importation into New Zealand 1860-1979

AIMS

To quantify the numbers of live cattle, sheep and poultry imported into New Zealand and, where possible, their country of origin from 1860 to 1979.

METHODS

Information on the origin and number of live animal importations into New Zealand was collected for cattle, sheep and poultry for the period 1868-1979 from the annual reports compiled by the New Zealand Registrar General's Office, Government Statistician's Office, Census and Statistics Office, Census and Statistics Department, Customs Department and Department of Statistics. Census data from 1851 to 1871 were also used to estimate the livestock population during this period. The number of animals imported and the mean population for each species in a decade were determined, and the major countries of origin were identified.

RESULTS

A large number of cattle (53,384) and sheep (604,525) were imported in the 1860s, and then there was a marked reduction in importations. Live poultry were imported in relatively small numbers (20,701) from 1880 to 1939, then 1,564,330 live poultry were imported between 1960 and 1979. Australia was the predominant country of origin for sheep between 1868 and 1959 (51,347/60,918; 84.3%) and of cattle between 1868 and 1979 (10,080/15,157; 66.5%). Only 6,712 (11.0%) sheep and 3,909 (25.8%) cattle were imported from the United Kingdom over the same periods, and even fewer from other countries.

CONCLUSIONS

The collated data and historical reports show that from 1860 to 1979 Australia has been the main source of livestock introduced into New Zealand. The pattern of importation showed that large numbers of cattle and sheep were initially imported in the 1860s, probably in response to rapid agricultural expansion. Thereafter importations continued at much reduced numbers. In contrast, relatively small numbers of poultry were introduced until the 1960s when large numbers were imported as part of the development of a modern high-production industry. The overall pattern for both cattle and sheep was of a bottleneck event, as initially a relatively limited number of animals arrived from outside populations, followed by population expansion with ongoing but limited immigration (admixture). Investigation into the genetic population structure of New Zealand's cattle and sheep, as well as their host-associated microorganisms, could reflect the impact of these early historical events.

A Bayesian framework to assess the potential for controlling classical scrapie in sheep flocks using a live diagnostic test

Current strategies to control classical scrapie remove animals at risk of scrapie rather than those known to be infected with the scrapie agent. Advances in diagnostic tests, however, suggest that a more targeted approach involving the application of a rapid live test may be feasible in future. Here we consider the use of two diagnostic tests: recto-anal mucosa-associated lymphatic tissue (RAMALT) biopsies; and a blood-based assay. To assess their impact we developed a stochastic age- and prion protein (PrP) genotype-structured model for the dynamics of scrapie within a sheep flock. Parameters were estimated in a Bayesian framework to facilitate integration of a number of disparate datasets and to allow parameter uncertainty to be incorporated in model predictions. In small flocks a control strategy based on removal of clinical cases was sufficient to control disease and more stringent measures (including the use of a live diagnostic test) did not significantly reduce outbreak size or duration. In medium or large flocks strategies in which a large proportion of animals are tested with either live diagnostic test significantly reduced outbreak size, but not always duration, compared with removal of clinical cases. However, the current Compulsory Scrapie Flocks Scheme (CSFS) significantly reduced outbreak size and duration compared with both removal of clinical cases and all strategies using a live diagnostic test. Accordingly, under the assumptions made in the present study there is little benefit from implementing a control strategy which makes use of a live diagnostic test.

Transmissible encephalopathy agents: virulence, geography and clockwork

Transmissible spongiform encephalopathies (TSEs) are caused by infectious agents with stable virulence characteristics that are not encoded by the host. Agent-specific features of virulence include variable disease latency and tissue pathology in a given host, as well as the ability to spread to many species. Such cross-species infections contradict predictions based on the prion hypothesis. Recent transmissions of several human agents to normal mice and to monotypic neural cells in culture, underscore the existence of unique agent clades that are prevalent in particular geographic regions. Examples include the epidemic UK bovine agent (BSE) and the New Guinea kuru agent. The virus-like biology of unique TSE agents, including epidemic spread, mutation, and superinfection, can be used to systematically define features of virulence that distinguish common endemic from newly emerging strains.

The role of mathematical modelling in understanding the epidemiology and control of sheep transmissible spongiform encephalopathies: a review

To deal with the incompleteness of observations and disentangle the complexities of transmission much use has been made of mathematical modelling when investigating the epidemiology of sheep transmissible spongiform encephalopathies (TSE) and, in particular, scrapie. Importantly, these modelling approaches allow the incidence of clinical disease to be related to the underlying prevalence of infection, thereby overcoming one of the major difficulties when studying these diseases. Models have been used to investigate the epidemiology of scrapie within individual flocks and at a regional level; to assess the efficacy of different control strategies, especially selective breeding programmes based on prion protein (PrP) genotype; to interpret the results of scrapie surveillance; and to inform the design of surveillance programmes. Furthermore, mathematical modelling has played an important role when assessing the risk to human health posed by the possible presence of bovine spongiform encephalopathy in sheep. Here, we review the various approaches that have been taken when developing and analysing mathematical models for the epidemiology and control of sheep TSE and assess their impact on our understanding of these diseases. We also identify areas that require further work, discuss future challenges and identify data gaps.

Agent-specific Shadoo responses in transmissible encephalopathies

Transmissible spongiform encephalopathies (TSE) are neurodegenerative diseases caused by an infectious agent with viral properties. Host prion protein (PrP), a marker of late stage TSE pathology, is linked to a similar protein called Shadoo (Sho). Sho is reduced in mice infected with the RML scrapie agent, but has not been investigated in other TSEs. Although PrP is required for infection by TSE agents, it is not known if Sho is similarly required. Presumably Sho protects cells from toxic effects of misfolded PrP. We compared Sho and PrP changes after infection by very distinct TSE agents including sporadic CJD, Asiatic CJD, New Guinea kuru, vCJD (the UK epidemic bovine agent) and 22L sheep scrapie, all passaged in standard mice. We found that Sho reductions were agent-specific. Variable Sho reductions in standard mice could be partly explained by agent-specific differences in regional neuropathology. However, Sho did not follow PrP misfolding in any quantitative or consistent way. Tga20 mice with high murine PrP levels revealed additional agent-specific differences. Sho was unaffected by Asiatic CJD yet was markedly reduced by the kuru agent in Tga20 mice; in standard mice both agents induced the same Sho reductions. Analyses of neural GT1 cells demonstrated that Sho was not essential for TSE infections. Furthermore, because all infected GT1 cells appeared as healthy as uninfected controls, Sho was not needed to protect infected cells from their "toxic" burden of abundant abnormal PrP and intracellular amyloid.

Evidence for maternal transmission of scrapie in naturally affected flocks

It has been known for many years that the offspring of scrapie affected ewes are at increased risk of developing scrapie but whether this is simply the result of an increased genetic susceptibility or transmission of infection has always been unclear. To contribute to clarify this we analysed the data collected in a detailed study of scrapie occurrence in a number of naturally affected commercial sheep flocks in Great Britain (GB) to investigate the association between PrP genotype and parental scrapie status and the incidence of scrapie. Our analyses confirmed the strong association between PrP genotype and the incidence of scrapie found in previous studies and a low incidence of scrapie in animals carrying the ARR allele and a high risk in homozygous VRQ animals. However, we also demonstrate an increased incidence of scrapie in the offspring of scrapie affected ewes controlling for the confounding effect of PrP genotype, but no increased scrapie incidence in the offspring of scrapie affected sires. Our results suggest that some of the increased incidence of scrapie in the offspring of scrapie affected ewes is the result of transmission of infection from mother to offspring. Our results confirm that a breeding policy aimed at decreasing the genetic susceptibility of the population should decrease the incidence of scrapie and that removing the offspring of scrapie affected animals from affected flocks could contribute to the control of this disease.

The kuru infectious agent is a unique geographic isolate distinct from Creutzfeldt-Jakob disease and scrapie agents

Human sporadic Creutzfeldt-Jakob disease (sCJD), endemic sheep scrapie, and epidemic bovine spongiform encephalopathy (BSE) are caused by a related group of infectious agents. The new U.K. BSE agent spread to many species, including humans, and clarifying the origin, specificity, virulence, and diversity of these agents is critical, particularly because infected humans do not develop disease for many years. As with viruses, transmissible spongiform encephalopathy (TSE) agents can adapt to new species and become more virulent yet maintain fundamentally unique and stable identities. To make agent differences manifest, one must keep the host genotype constant. Many TSE agents have revealed their independent identities in normal mice. We transmitted primate kuru, a TSE once epidemic in New Guinea, to mice expressing normal and approximately 8-fold higher levels of murine prion protein (PrP). High levels of murine PrP did not prevent infection but instead shortened incubation time, as would be expected for a viral receptor. Sporadic CJD and BSE agents and representative scrapie agents were clearly different from kuru in incubation time, brain neuropathology, and lymphoreticular involvement. Many TSE agents can infect monotypic cultured GT1 cells, and unlike sporadic CJD isolates, kuru rapidly and stably infected these cells. The geographic independence of the kuru agent provides additional reasons to explore causal environmental pathogens in these infectious neurodegenerative diseases.

Epidemiological characteristics of classical scrapie outbreaks in 30 sheep flocks in the United Kingdom

Background

Most previous analyses of scrapie outbreaks have focused on flocks run by research institutes, which may not reflect the field situation. Within this study, we attempt to rectify this deficit by describing the epidemiological characteristics of 30 sheep flocks naturally-infected with classical scrapie, and by exploring possible underlying causes of variation in the characteristics between flocks, including flock-level prion protein (PrP) genotype profile. In total, the study involved PrP genotype data for nearly 8600 animals and over 400 scrapie cases.

Methodology/Principal Findings

We found that most scrapie cases were restricted to just two PrP genotypes (ARQ/VRQ and VRQ/VRQ), though two flocks had markedly different affected genotypes, despite having similar underlying genotype profiles to other flocks of the same breed; we identified differences amongst flocks in the age of cases of certain PrP genotypes; we found that the age-at-onset of clinical signs depended on peak incidence and flock type; we found evidence that purchasing infected animals is an important means of introducing scrapie to a flock; we found some evidence that flock-level PrP genotype profile and flock size account for variation in outbreak characteristics; identified seasonality in cases associated with lambing time in certain flocks; and we identified one case that was homozygous for phenylalanine at codon 141, a polymorphism associated with a very high risk of atypical scrapie, and 28 cases that were heterozygous at this codon.

Conclusions/Significance

This paper presents the largest study to date on commercially-run sheep flocks naturally-infected with classical scrapie, involving 30 study flocks, more than 400 scrapie cases and over 8500 PrP genotypes. We show that some of the observed variation in epidemiological characteristics between farms is related to differences in their PrP genotype profile; although much remains unexplained and may instead be attributed to the stochastic nature of scrapie dynamics.

Prevalence of sheep infected with classical scrapie in Great Britain: integrating multiple sources of surveillance data for 2002

Estimates for the prevalence of sheep infected with classical scrapie are essential for assessing the efficacy of control strategies that have been implemented in Great Britain (GB). Here a back-calculation approach was used to estimate the prevalence in the GB national flock by integrating data on reported cases and the results of abattoir and fallen stock surveys for 2002. Prevalence estimates ranged from 0.33 to 2.06%, depending on the estimates used for the frequencies of prion protein (PrP) genotypes in the national flock and the stage of incubation at which the diagnostic tests used are able to detect infected animals. The risk of infection was found to be higher than that of clinical disease, especially in those PrP genotypes that have a later age at onset of clinical disease. Moreover, results suggest that a high proportion (more than 55%) of infected animals surviving to disease onset die on farm before clinical signs become apparent, which helps account for the high observed prevalence in the fallen stock compared with the abattoir survey. The analyses indicated that attention needs to be given to identifying the stage of incubation at which diagnostic tests are able to detect infected animals and obtaining better demographic data for the GB national flock.

Distribution of prion protein genotypes in breeds of sheep in New Zealand

AIM

To use an established high through-put genotyping procedure to gain an estimate of the frequency of alleles of the prion protein (PrP) gene in some common sheep breeds in New Zealand.

METHODS

Using a genotyping procedure based on matrix-assisted laser desorption ionisation-time of flight (MALDI-TOF), DNA samples from 3,024 sheep from New Zealand, including breeds such as Romney, Texel, Coopworth, Merino and mixed breed, were isolated, genotyped and the results analysed.

RESULTS

The 15 scrapie genotypes commonly reported, and derived from the five commonly reported allelic variants (ARR, ARQ, AHQ, ARH and VRQ), were all observed in the samples analysed. The estimates were indicative of the frequencies in the population of alleles present in breeds of sheep in New Zealand. There was a significant difference between the frequencies of alleles between breeds, but the ARQ, followed by the ARR allele, were, except in Carwell sheep, the most common alleles present.

CONCLUSION

This study gave an indication of the percentages of PrP gene alleles in sheep in New Zealand, including data previously unreported from breeds in this country. It is of interest because of the relatively large size of the sheep population in New Zealand compared with many countries, and it provides some useful information on the genetic susceptibility or resistance of the sheep population in New Zealand to scrapie. The frequencies of the alleles can be different for an individual breed compared between countries.

A 25 nm virion is the likely cause of transmissible spongiform encephalopathies

The transmissible spongiform encephalopathies (TSEs) such as endemic sheep scrapie, sporadic human Creutzfeldt-Jakob disease (CJD), and epidemic bovine spongiform encephalopathy (BSE) may all be caused by a unique class of "slow" viruses. This concept remains the most parsimonious explanation of the evidence to date, and correctly predicted the spread of the BSE agent to vastly divergent species. With the popularization of the prion (infectious protein) hypothesis, substantial data pointing to a TSE virus have been largely ignored. Yet no form of prion protein (PrP) fulfills Koch's postulates for infection. Pathologic PrP is not proportional to, or necessary for infection, and recombinant and "amplified" prions have failed to produce significant infectivity. Moreover, the "wealth of data" claimed to support the existence of infectious PrP are increasingly contradicted by experimental observations, and cumbersome speculative notions, such as spontaneous PrP mutations and invisible strain-specific forms of "infectious PrP" are proposed to explain the incompatible data. The ability of many "slow" viruses to survive harsh environmental conditions and enzymatic assaults, their stealth invasion through protective host-immune defenses, and their ability to hide in the host and persist for many years, all fit nicely with the characteristics of TSE agents. Highly infectious preparations with negligible PrP contain nucleic acids of 1-5 kb, even after exhaustive nuclease digestion. Sedimentation as well as electron microscopic data also reveal spherical infectious particles of 25-35 nm in diameter. This particle size can accommodate a viral genome of 1-4 kb, sufficient to encode a protective nucleocapsid and/or an enzyme required for its replication. Host PrP acts as a cellular facilitator for infectious particles, and ultimately accrues pathological amyloid features. A most significant advance has been the development of tissue culture models that support the replication of many different strains of agent and can produce high levels of infectivity. These models provide new ways to rapidly identify intrinsic viral and strain-specific molecules so important for diagnosis, prevention, and fundamental understanding.

An overview of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders of humans and animals associated with an accumulation of abnormal isoforms of prion protein (PrP) in nerve cells. The pathogenesis of TSEs involves conformational conversions of normal cellular PrP (PrPc) to abnormal isoforms of PrP (PrPSc). While the protein-only hypothesis has been widely accepted as a causal mechanism of prion diseases, evidence from more recent research suggests a possible involvement of other cellular component(s) or as yet undefined infectious agent(s) in PrP pathogenesis. Although the underlying mechanisms of PrP strain variation and the determinants of interspecies transmissibility have not been fully elucidated, biochemical and molecular findings indicate that bovine spongiform encephalopathy in cattle and new-variant Creutzfeldt–Jakob disease in humans are caused by indistinguishable etiological agent(s). Cumulative evidence suggests that there may be risks of humans acquiring TSEs via a variety of exposures to infected material. The development of highly precise ligands is warranted to detect and differentiate strains, allelic variants and infectious isoforms of these PrPs. This article describes the general features of TSEs and PrP, the current understanding of their pathogenesis, recent advances in prion disease diagnostics, and PrP inactivation.

Breeding programmes for TSE resistance in British sheep. II. Assessing the impact on the prevalence and incidence of scrapie

By establishing a breeding programme for transmissible spongiform encephalopathie (TSE) resistance, there are plans to eradicate sheep TSEs from member states of the European Union (EU). In this paper, we used a simple age- and genotype-structured model to assess the impact of four breeding strategies on the prevalence and incidence of scrapie in the British sheep flock. The strategies ranged from the minimum EU requirements to compulsory implementation of the current National Scrapie Plan for Great Britain (NSP). All four strategies were predicted to reduce the prevalence and incidence of disease, though there was likely to be a delay of several years between the implementation of a breeding programme and the reduction in incidence. There were differences in the efficacy of the strategies, with the most stringent resulting in the greatest reduction in prevalence and incidence. However, the magnitude of the differences was not great, largely because all four strategies eliminated the VRQ allele, which is associated with a markedly higher risk of disease than any of the other alleles. Sensitivity analyses indicated that the model results were robust to selection bias when estimating the risk of infection; and that the efficacy of a breeding programme was unlikely to be compromised, unless the risk of infection is substantially underestimated by data on clinical disease.

Prion protein gene polymorphisms in healthy and scrapie-affected Spanish sheep

The Rasa Aragonesa sheep is the second most important Spanish breed after the Merino breed. Reported here is the prion protein (PrP) haplotype frequency distribution for scrapie-related codons (136, 154 and 171) and a sequencing study of the complete PrP gene open reading frame for this breed and six other closely related breeds. The study includes four scrapie-affected sheep flocks belonging to Rasa Aragonesa and Rasa Navarra breeds. Thirty-eight scrapie-affected sheep, 502 healthy sheep from scrapie-affected flocks and 905 sheep from a breed survey were genotyped. The most frequent PrP haplotype in both scrapie and healthy flocks was ARQ, which was found at significantly higher frequency in scrapie-affected sheep. The susceptibility-associated VRQ haplotype was found at low frequencies in six out of eight breeds, but was not present in the 38 scrapie-affected sheep. The resistance-associated ARR haplotype was found in all breeds except one (Ojinegra) at frequencies >or=14 %. Fourteen amino acid polymorphisms were detected in these Spanish sheep, including the known amino acid substitutions at codons 112, 136, 141, 143, 154, 171 and 176, and new polymorphisms at codons 101 (Q-->R), 151 (R-->G), 151 (R-->H), 172 (Y-->D) and 175 (Q-->E). Most of the novel polymorphic codons show frequencies lower than 5 %.

Frequencies of prion protein (PrP) genotypes and distribution of ages in 15 scrapieaffected flocks in Great Britain

The frequencies of prion protein (PrP) genotypes were investigated in 15 scrapie-affected flocks in Great Britain. The flocks were heterogeneous in the frequencies of different genotypes and alleles, and in their age distributions. The median flock frequency of animals with VRQ-containing genotypes was 21 per cent (range 2 to 82 per cent, mean 25 per cent). The VRQ-containing and other non-ARR genotypes made up 11 to 82 per cent of a flock (median 46 per cent, mean 48 per cent). In comparison with data from the general population the scrapie-affected population had a lower frequency of the ARR/ARR genotype, and so of the ARR allele, and had a higher frequency of VRQ/non-ARR heterozygote genotypes, and thus of the VRQ allele.

Prevalence of scrapie infection in Great Britain: interpreting the results of the 1997-1998 abattoir survey

An accurate estimate of the prevalence of scrapie infection in the Great Britain (GB) sheep flock is essential when assessing any potential risk to human health through exposure to sheep transmissible spongiform encephalopathies (TSEs). One method for assessing the prevalence is to sample sheep intended for human consumption using a diagnostic test capable of detecting infected animals prior to the onset of clinical signs. An abattoir survey conducted in Great Britain in 1997-1998 tested brain samples from 2809 apparently healthy sheep of which none was found to be positive for scrapie by histopathology or immunohistochemistry (IHC) although 10 were positive for scrapie-associated fibrils (SAF). Subsequently, the tonsils from a subset of the animals sampled were examined using IHC, one of which tested positive. To interpret these results we use a likelihood-based approach, which accounts for the variation in the prevalence of infection with age and test sensitivity and specificity with stage of infection. Combining the results for all of the diagnostic tests yields an estimate of the prevalence of scrapie infection in the GB sheep flock of 0.22% (95% confidence interval: 0.01-0.97%). Moreover, our analysis suggests that all of the diagnostic tests used are very specific (greater than 99%). Indeed, only SAF detection yields a specificity estimate of less than 100%, which helps to account for the high number of samples found to be positive for SAF.

The PrP genotype of sheep of the improved Valachian breed

In a worldwide majority of sheep breeds an excessive susceptibility to scrapie associated with the PrP gene alleles coding for valine (V; at the 136 codon) and glutamine (Q; at the 171 codon) (e.g., VRQ/VRQ, VRQ/ARQ, or ARQ/ARQ) was demonstrated. Particularly the PrPVRQ allele is closely associated with the high-risk development of the disease; the PrPARQ allele can also fulfill this function but under certain limited conditions. Polymorphism in the PrP gene sequences (conclusively related to the increased susceptibility of sheep to scrapie) of improved Valachian sheep from two Slovak regions, Orava and Spis, was determined. Examination of 735 sheep showed that ARR/ARQ was the most frequent genotype (45.2%). High-risk genotypes were determined in 32.4% of sheep (ARQ/ARQ 19.3, ARR/VRQ 9.0, ARR/VRQ 3.5, VRQ/VRQ 0.3, ARR/VRR 0.3). Low-risk genotypes were found in 67.7% of sheep (ARR/ARQ 45.2, ARR/ARR 10.9, ARR/AHQ 5.7, ARQ/ARQ 4.9, AHQ/AHQ 0.7, ARR/AHR 0.3). Despite the geographically distant flocks of improved Valachian sheep investigated no difference in the occurrence of individual PrP genotypes was observed.

Scrapie and experimental BSE in sheep

Scrapie is a natural disease of sheep, but it can also be successfully transmitted between sheep by experimental inoculation. Although BSE is primarily a disease of cattle, it has also infected humans (causing vCJD) and, in addition, can be transmitted orally to sheep bringing concerns that BSE might naturally have infected the UK sheep population. Because of this, scrapie and BSE are being compared and studied in detail in sheep. PrP genotype controls sheep susceptibility and resistance to scrapie and to BSE, and deposition of the disease-associated PrP(Sc), used as a marker of infection, has the potential to act as a means of identifying BSE-infected animals and describing different pathogenesis mechanisms. Sheep orally dosed with BSE show signs of infection in their blood and this model is of major importance in the study of the safety of blood products for use with human beings.

TSE agent strains and PrP: reconciling structure and function

The molecular structures of the infectious agents that cause transmissible spongiform encephalopathy (TSE) diseases are still not known despite the current wide acceptance of the prion hypothesis as the basis for their resolution. Here, data supporting and challenging the prion hypothesis in relation to both the biochemical and biological properties of TSE infectious agents are discussed. The need for the independent transmission of TSE agent-specific genetic information is described and the requirements for the molecule to carry this information are proposed. Such a molecule is likely to be a small nucleic acid encoding information to determine the diversity of the pathogenesis of TSE agents.

New Zealand sheep with scrapie-susceptible PrP genotypes succumb to experimental challenge with a sheep-passaged scrapie isolate (SSBP/1)

Scrapie does not occur in New Zealand (NZ), although PrP gene alleles associated with susceptibility to the disease are found at relatively high frequencies in NZ sheep. The hypothesis that scrapie is a genetic disease of sheep is thus unlikely to be true. To confirm that NZ sheep are actually susceptible to scrapie infection, NZ sheep of various PrP genotypes were challenged by subcutaneous inoculation with a sheep-passaged scrapie isolate (SSBP/1). Showing similar PrP genetics to that seen in UK sheep, all NZ sheep carrying the VRQ PrP allele developed clinical signs typical of scrapie, with characteristic neurodegenerative changes and PrP(Sc) evident on histopathological examination of their brains and lymphoid tissues. The incubation periods recorded in NZ sheep were generally shorter than those found in UK sheep. The results confirm that New Zealand sheep are as susceptible as their UK counterparts to experimental scrapie infection by subcutaneous inoculation.

Influence of the prion protein gene, Prnp, on scrapie susceptibility in sheep

Natural scrapie in sheep occurs through a complex interplay between host genetic elements and various strains of the infectious scrapie agent. Scrapie-related polymorphisms in the coding region of the prion protein (PrP) gene, Prnp, have been studied in a number of breeds. The disease-promoting V136 allele, and the susceptibility-reducing R171 allele, have proved to be most important. However, variation in the coding region of Prnp cannot alone explain the diverse patterns of scrapie susceptibility in various breeds. For instance, in many breeds plagued with scrapie, the V136 allele appears to be a rarity. The R171 allele greatly reduces scrapie susceptibility This lays the molecular foundation for marker-assisted breeding for reduced scrapie susceptibility now underway in many countries. Although potentially important, and still under investigation, variable expression level and pattern of the ovine Prnp appears to be of little importance for the occurrence of natural scrapie. Studies of scrapie in mice also indicate that genetic elements other than Prnp may have a strong influence on scrapie incubation time, and hence susceptibility. Narrowing down the search to focus on these elements and identification of candidate genes are important tasks for future research in sheep scrapie.

A stochastic model to estimate the prevalence of scrapie in Great Britain using the results of an abattoir-based survey

In 1997/1998, an abattoir survey was conducted to determine the likely exposure of the human population to transmissible spongiform encephalopathy (TSE) infection in sheep submitted for slaughter in Great Britain. The survey examined brain material from 2809 sheep processed through British abattoirs. Sampling was targeted by age: 45% of animals tested were > or =15 months old. All samples of adequate quality (98%) were tested for signs of scrapie infection using histopathology and scrapie-associated fibril (SAF) detection and 500 were tested using immunohistochemistry (IHC). No conclusive positive animals were found using either histology or IHC. Ten animals were positive by SAF. Standard statistical analyses suggest (with 95% confidence) that the prevalence of detectable (by histopathology) infection in the slaughter population was < or =0.11%. However, the incubation period of scrapie is long (usually around 2-3 years) and none of the tests used in the survey is capable of detecting scrapie infection in the early stages of infection. We present an age-structured stochastic model incorporating parameters for the incubation period of scrapie, prevalence of infection by age and test sensitivity. Using the model, we demonstrate that the negative results obtained for all samples using IHC and histopathology are consistent with a true prevalence of infection in the slaughter population of up to 11%. This suggests that up to 300 of the animals tested might have been infected but the infection was not sufficiently advanced in these animals to be detectable by IHC or histopathology. The survey was designed to detect a prevalence of 1% with a precision of +/-0.5% and a confidence level of 95% in each age group assuming that diagnostic tests were 100% specific and sensitive from a known stage in the incubation period. The results of the model demonstrate that to estimate a true prevalence of scrapie infection of 1% with an accuracy of +/-0.5% would have required a far larger sample size. An accurate estimate of the required sample size is complicated by uncertainty about test sensitivity and the underlying infection dynamics of scrapie. A pre-requisite for any future abattoir survey is validation of the diagnostic tests used in relation to both stage of incubation and genotype. Sampling in the <15-month age group was of no value in this survey because the diagnostic tests used were thought to be ineffective in most of the animals in this age group.

Sheep and goats: natural and experimental TSEs and factors influencing incidence of disease

The major factor influencing incidence of disease following challenge with transmissible spongiform encephalopathies (TSEs) in sheep is the allotype at amino acid numbers 136, 154 and 171 of the PrP protein. There are at least two groups of TSEs, one which targets the amino acid encoded at position 136 and the other which is more influenced by the amino acid at codon 171. Within these groups of TSE types, there may additionally be sub-types, as resistance to some, but not all, "136-type" TSEs can also be affected by the amino acid at codon 154. In goats, there are also PrP polymorphisms which apparently influence incubation period of TSE disease, however, this has not found to be true for cattle and BSE incidence. Sheep PrP amino acid codons 136, 154 and 171 do not explain everything about, for example, natural scrapie occurrence in sheep flocks, and attention is now turning to the flanking regions of the PrP gene looking for sequence differences in gene expression control motifs which may also have an influence on disease development. The sheep PrP gene produces two mRNAs in peripheral tissues, the result of alternative polyadenylation in the 3' untranslated region of the gene. Results from transfection assays of murine neuroblastoma cells with constructs expressing different regions of ovine PrP mRNA have revealed the presence of sequences in the 3' untranslated region that modulate protein synthesis and have therefore the potential to affect disease progression.

An approach to the control of disease transmission in pig-to-human xenotransplantation

Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved.

Susceptibility of sheep for scrapie as assessed by in vitro conversion of nine naturally occurring variants of PrP

Polymorphisms in the prion protein (PrP) gene are associated with phenotypic expression differences of transmissible spongiform encephalopathies in animals and humans. In sheep, at least 10 different mutually exclusive polymorphisms are present in PrP. In this study, we determined the efficiency of the in vitro formation of protease-resistant PrP of nine sheep PrP allelic variants in order to gauge the relative susceptibility of sheep for scrapie. No detectable spontaneous protease-resistant PrP formation occurred under the cell-free conditions used. All nine host-encoded cellular PrP (PrP(C)) variants had distinct conversion efficiencies induced by PrP(Sc) isolated from sheep with three different homozygous PrP genotypes. In general, PrP allelic variants with polymorphisms at either codon 136 (Ala to Val) or codon 141 (Leu to Phe) and phylogenetic wild-type sheep PrP(C) converted with highest efficiency to protease-resistant forms, which indicates a linkage with a high susceptibility of sheep for scrapie. PrP(C) variants with polymorphisms at codons 171 (Gln to Arg), 154 (Arg to His), and to a minor extent 112 (Met to Thr) converted with low efficiency to protease-resistant isoforms. This finding indicates a linkage of these alleles with a reduced susceptibility or resistance for scrapie. In addition, PrP(Sc) with the codon 171 (Gln-to-His) polymorphism is the first variant reported to induce higher conversion efficiencies with heterologous rather than homologous PrP variants. The results of this study strengthen our views on polymorphism barriers and have further implications for scrapie control programs by breeding strategies.

PrP gene polymorphism and natural scrapie in Icelandic sheep

The association between scrapie and polymorphism of the prion protein (PrP) gene was studied in the Icelandic sheep breed. Polymorphism of the three codons, 136, 154 and 171, that are important for scrapie susceptibility was determined. A BspHI restriction analysis was used to study the alleles of codons 136 and 154, while density gradient gel electrophoresis (DGGE) was used to analyse codon 171 and detect new polymorphisms. The PrP allelic variant, VRQ (amino acids at codons 136, 154 and 171), was found to be highly statistically associated with scrapie, whereas the allelic variant, AHQ, was never found in scrapie-affected animals, a finding that is statistically significant. Iceland has a few scrapie-free regions, which are a part of a quarantine network. Homozygotes for the VRQ variant were found there at a low frequency, indicating that genetic susceptibility is not enough for scrapie to develop and further evidence for the infectious nature of the disease. A comparison of PrP genotypes between sheep outside and within the scrapie-free zones revealed an increase in the AHQ allelic variant in the latter. No polymorphism was found at codon 171 in a total of 932 sheep studied, all individuals having the glutamine allele. Two novel, rare PrP alleles were found using DGGE at codons 138 and 151, i.e. S138N and R151C. Their relevance to scrapie is still unclear, but the former was found in scrapie-affected sheep as well as healthy sheep, whereas the latter was only found in healthy sheep.