A centuries-long epidemic of scrapie in British sheep?

Evaluation of Resistance and Susceptibility to Prion Diseases in Pelibuey Sheep from Mexico

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases with a chronic and fatal course, which are caused by a misfolded form of the cellular prion protein that is encoded by the host. The purpose of this work was to evaluate the resistance and genetic susceptibility to prion diseases in Pelibuey sheep from Mexico. The sequences of 99 Pelibuey sheep from the central and southwestern regions of Mexico were analyzed to determine the polymorphisms related to resistance and genetic susceptibility to scrapie, the Hardy-Weinberg equilibrium test and the D-Tajima test was used to identify the effect of evolutionary forces on the PRNP gene. Twelve non-synonymous polymorphisms Q101R, M112T, A116P, G127A, A136V, M137T, L141F, H143R, R154H, Q171R, Q171H, N176K were detected, in addition two synonymous substitutions 231R (agg/cgg) and 237L (ctc/ctg) were found. As a result of the sequence analysis, the ARR allele was not under the Hardy-Weinberg equilibrium, indicating that there is an evolutionary force at work, and the D-Tajima suggest the existence of purifying genetic selection. In conclusion, Pelibuey sheep exhibit genotypes for the PRNP gene that make them resistant to developing classic scrapie; at least 98% of the population is a carrier of an allele related to scrapie resistance; while for atypical scrapie there is a very high probability that an outbreak occurred in the herds, since the ARQ allele in combination with the L substitution at codon 141 confers susceptibility to carrier sheep.

Excessive replacement changes drive evolution of global sheep prion protein (PRNP) sequences

Sheep prion protein (PRNP) is the major host genetic factor responsible for susceptibility to scrapie. We aimed to understand the evolutionary history of sheep PRNP, and primarily focused on breeds from Turkey and Ethiopia, representing genome-wise ancient sheep populations. Population molecular genetic analyses are extended to European, South Asian, and East Asian populations, and for the first time to scrapie associated haplotypes. 1178 PRNP coding region nucleotide sequences were analyzed. High levels of nucleotide diversity driven by extensive low-frequency replacement changes are observed in all populations. Interspecific analyses were conducted using mouflon and domestic goat as outgroup species. Despite an abundance of silent and replacement changes, lack of silent or replacement fixations was observed. All scrapie-associated haplotype analyses from all populations also showed extensive low-frequency replacement changes. Neutrality tests did not indicate positive (directional), balancing or strong negative selection or population contraction for any of the haplotypes in any population. A simple negative selection history driven by prion disease susceptibility is not supported by the population and haplotype based analyses. Molecular function, biological process enrichment, and protein-protein interaction analyses suggested functioning of PRNP protein in multiple pathways, and possible other functional constraint selections. In conclusion, a complex selection history favoring excessive replacement changes together with weak purifying selection possibly driven by frequency-dependent selection is driving PRNP sequence evolution. Our results is not unique only to the Turkish and Ethiopian samples, but can be generalized to global sheep populations.

Routes of Transmission in the Food Chain

More than 250 different foodborne diseases have been described to date, annually affecting about one-third of the world's population. The incidence of foodborne diseases has been underreported and underestimated, and the asymptomatic presentation of some of the illnesses, worldwide heterogeneities in reporting, and the alternative transmission routes of certain pathogens are among the factors that contribute to this. Globalization, centralization of the food supply, transportation of food products progressively farther from their places of origin, and the multitude of steps where contamination may occur have made it increasingly challenging to investigate foodborne and waterborne outbreaks. Certain foodborne pathogens may be transmitted directly from animals to humans, while others are transmitted through vectors, such as insects, or through food handlers, contaminated food products or food-processing surfaces, or transfer from sponges, cloths, or utensils. Additionally, the airborne route may contribute to the transmission of certain foodborne pathogens. Complicating epidemiological investigations, multiple transmission routes have been described for some foodborne pathogens. Two types of transmission barriers, primary and secondary, have been described for foodborne pathogens, each of them providing opportunities for preventing and controlling outbreaks. Primary barriers, the most effective sites of prophylactic intervention, prevent pathogen entry into the environment, while secondary barriers prevent the multiplication and dissemination of pathogens that have already entered the environment. Understanding pathogen dynamics, monitoring transmission, and implementing preventive measures are complicated by the phenomenon of superspreading, which refers to the concept that, at the level of populations, a minority of hosts is responsible for the majority of transmission events.

Association between PrP genotypes and performance traits in a Welsh Mountain flock

The UK national scrapie plan (NSP) for sheep is based on selection for the resistant ARR/ARR genotype and elimination of susceptible types of the ovine prion protein (PrP) gene. The aim of this study was to estimate the possible association of the PrP genotype and performance traits by using data from the CAMDA Welsh Mountain flock. Four alleles (ARH, ARQ, ARR and VRQ) and 10 genotypes covering all five NSP risk groups were present in the CAMDA flock. Overall, the most common allele was ARR (35.2%), and VRQ was the least common (5.4%). The commonest genotypes were ARR/ARQ (23.7%) and ARR/AHQ (23.1%). The most resistant genotype, ARR/ARR, and the most susceptible genotype, VRQ/VRQ, were found in 10.2% and 0.3%, respectively, of the population tested. The associations of PrP genotypes with weight and ultrasonically scanned traits were investigated in three analyses, the first using genotypes, the second using risk categories and the third using number of alleles. These associations were evaluated by univariate analysis of each trait using an animal model with maternal effects where appropriate, and PrP was included as a fixed effect. Selection for scrapie resistance will not adversely affect progress in the traits considered and is consistent with improvements in muscle depth.

Generalized cerebral atrophy seen on MRI in a naturally exposed animal model for Creutzfeldt-Jakob disease

BACKGROUND

Magnetic resonance imaging has been used in the diagnosis of human prion diseases such as sCJD and vCJD, but patients are scanned only when clinical signs appear, often at the late stage of disease. This study attempts to answer the questions "Could MRI detect prion diseases before clinical symptoms appear?, and if so, with what confidence?"

METHODS

Scrapie, the prion disease of sheep, was chosen for the study because sheep can fit into a human sized MRI scanner (and there were no large animal MRI scanners at the time of this study), and because the USDA had, at the time of the study, a sizeable sample of scrapie exposed sheep, which we were able to use for this purpose. 111 genetically susceptible sheep that were naturally exposed to scrapie were used in this study.

RESULTS

Our MRI findings revealed no clear, consistent hyperintense or hypointense signal changes in the brain on either clinically affected or asymptomatic positive animals on any sequence. However, in all 37 PrPSc positive sheep (28 asymptomatic and 9 symptomatic), there was a greater ventricle to cerebrum area ratio on MRI compared to 74 PrPSc negative sheep from the scrapie exposed flock and 6 control sheep from certified scrapie free flocks as defined by immunohistochemistry (IHC).

CONCLUSIONS

Our findings indicate that MRI imaging can detect diffuse cerebral atrophy in asymptomatic and symptomatic sheep infected with scrapie. Nine of these 37 positive sheep, including 2 one-year old animals, were PrPSc positive only in lymph tissues but PrPSc negative in the brain. This suggests either 1) that the cerebral atrophy/neuronal loss is not directly related to the accumulation of PrPSc within the brain or 2) that the amount of PrPSc in the brain is below the detectable limits of the utilized immunohistochemistry assay. The significance of these findings remains to be confirmed in human subjects with CJD.

Quantitative estimation of genetic risk for atypical scrapie in French sheep and potential consequences of the current breeding programme for resistance to scrapie on the risk of atypical scrapie

BACKGROUND

Since 2002, active surveillance programmes have detected numerous atypical scrapie (AS) and classical scrapie cases (CS) in French sheep with almost all the PrP genotypes. The aim of this study was 1) to quantify the genetic risk of AS in French sheep and to compare it with the risk of CS, 2) to quantify the risk of AS associated with the increase of the ARR allele frequency as a result of the current genetic breeding programme against CS.

METHODS

We obtained genotypes at codons 136, 141, 154 and 171 of the PRNP gene for representative samples of 248 AS and 245 CS cases. We used a random sample of 3,317 scrapie negative animals genotyped at codons 136, 154 and 171 and we made inferences on the position 141 by multiple imputations, using external data. To estimate the risk associated with PrP genotypes, we fitted multivariate logistic regression models and we estimated the prevalence of AS for the different genotypes. Then, we used the risk of AS estimated for the ALRR-ALRR genotype to analyse the risk of detecting an AS case in a flock homogenous for this genotype.

RESULTS

Genotypes most at risk for AS were those including an AFRQ or ALHQ allele while genotypes including a VLRQ allele were less commonly associated with AS. Compared to ALRQ-ALRQ, the ALRR-ALRR genotype was significantly at risk for AS and was very significantly protective for CS. The prevalence of AS among ALRR-ALRR animals was 0.6 per thousand and was not different from the prevalence in the general population.

CONCLUSION

In conclusion, further selection of ALRR-ALRR animals will not result in an overall increase of AS prevalence in the French sheep population although this genotype is clearly susceptible to AS. However the probability of detecting AS cases in flocks participating in genetic breeding programme against CS should be considered.

Surveillance and simulation of bovine spongiform encephalopathy and scrapie in small ruminants in Switzerland

BACKGROUND

After bovine spongiform encephalopathy (BSE) emerged in European cattle livestock in 1986 a fundamental question was whether the agent established also in the small ruminants' population. In Switzerland transmissible spongiform encephalopathies (TSEs) in small ruminants have been monitored since 1990. While in the most recent TSE cases a BSE infection could be excluded, for historical cases techniques to discriminate scrapie from BSE had not been available at the time of diagnosis and thus their status remained unclear. We herein applied state-of-the-art techniques to retrospectively classify these animals and to re-analyze the affected flocks for secondary cases. These results were the basis for models, simulating the course of TSEs over a period of 70 years. The aim was to come to a statistically based overall assessment of the TSE situation in the domestic small ruminant population in Switzerland.

RESULTS

In sum 16 TSE cases were identified in small ruminants in Switzerland since 1981, of which eight were atypical and six were classical scrapie. In two animals retrospective analysis did not allow any further classification due to the lack of appropriate tissue samples. We found no evidence for an infection with the BSE agent in the cases under investigation. In none of the affected flocks, secondary cases were identified. A Bayesian prevalence calculation resulted in most likely estimates of one case of BSE, five cases of classical scrapie and 21 cases of atypical scrapie per 100'000 small ruminants. According to our models none of the TSEs is considered to cause a broader epidemic in Switzerland. In a closed population, they are rather expected to fade out in the next decades or, in case of a sporadic origin, may remain at a very low level.

CONCLUSIONS

In summary, these data indicate that despite a significant epidemic of BSE in cattle, there is no evidence that BSE established in the small ruminant population in Switzerland. Classical and atypical scrapie both occur at a very low level and are not expected to escalate into an epidemic. In this situation the extent of TSE surveillance in small ruminants requires reevaluation based on cost-benefit analysis.

Implications of conflicting associations of the prion protein (PrP) gene with scrapie susceptibility and fitness on the persistence of scrapie

BACKGROUND

Existing mathematical models for scrapie dynamics in sheep populations assume that the PrP gene is only associated with scrapie susceptibility and with no other fitness related traits. This assumption contrasts recent findings of PrP gene associations with post-natal lamb survival in scrapie free Scottish Blackface populations. Lambs with scrapie resistant genotypes were found to have significantly lower survival rates than those with susceptible genotypes. The present study aimed to investigate how these conflicting PrP gene associations may affect the dynamic patterns of PrP haplotype frequencies and disease prevalence.

METHODOLOGY/PRINCIPAL FINDINGS

A deterministic mathematical model was developed to explore how the associations between PrP genotype and both scrapie susceptibility and postnatal lamb mortality affect the prevalence of scrapie and the associated change in PrP gene frequencies in a closed flock of sheep. The model incorporates empirical evidence on epidemiological and biological characteristics of scrapie and on mortality rates induced by causes other than scrapie. The model results indicate that unfavorable associations of the scrapie resistant PrP haplotypes with post-natal lamb mortality, if sufficiently strong, can increase scrapie prevalence during an epidemic, and result in scrapie persisting in the population. The range of model parameters, for which such effects were observed, is realistic but relatively narrow.

CONCLUSIONS/SIGNIFICANCE

The results of the present model suggest that for most parameter combinations an unfavourable association between PrP genotype and post-natal lamb mortality does not greatly alter the dynamics of scrapie and, hence, would not have an adverse impact on a breeding programme. There were, however, a range of scenarios, narrow, but realistic, in which such an unfavourable association resulted in an increased prevalence and in the persistence of infection. Consequently, associations between PrP genotypes and fitness traits should be taken into account when designing future models and breeding programmes.

PrP genotype: a flock-level risk factor for scrapie?

Previous epidemiological studies of risk factors for classical scrapie at flock level have identified a variety of management and purchase related variables, along with increased flock size and, in some cases, breed effects. Although known as a risk factor at the individual animal level, PrP genotype frequencies at flock level have not yet been studied. In an unmatched case-control study, three measures of flock-level prion protein (PrP) frequency estimates were investigated with respect to the scrapie status of the flock in 293 British sheep flocks (195 control flocks and 98 case flocks). Flocks with positive frequencies (more than 0 per cent) of two genotypes (VRQ/VRQ and AHQ/VRQ), large frequencies (more than 10 per cent) of the ARR/VRQ genotype, and large frequencies (more than 5.2 per cent) of the VRQ allele were at increased odds of being affected with clinical classical scrapie. When adjusted for flock size, breed and sampling strategy the genotype and allele effects remained, except that for flocks with positive frequencies of VRQ/VRQ. The known effect of increased risk with increased flock size was confirmed. A measure of the flock PrP genotype frequency profile should thus be included in studies of risk factors for scrapie. It could also be integrated into risk-based surveillance strategies for identification of "at-risk-of scrapie" flocks.

Associations between lamb survival and prion protein genotype: analysis of data for ten sheep breeds in Great Britain

Background

Selective breeding programmes, based on prion protein (PrP) genotype, have been introduced throughout the European Union to reduce the risk of sheep transmissible spongiform encephalopathies (TSEs). These programmes could have negative consequences on other important traits, such as fitness and production traits, if the PrP gene has pleiotropic effects or is in linkage disequilibrium with genes affecting these traits. This paper presents the results of an investigation into associations between lamb survival and PrP genotype in ten mainstream sheep breeds in Great Britain (GB). In addition, the reasons for lamb deaths were examined in order to identify any associations between these and PrP genotype.

Results

Survival times from birth to weaning were analysed for over 38000 lambs (2427 dead and 36096 live lambs) from 128 flocks using Cox proportional hazard models for each breed, including additive animal genetic effects. No significant associations between PrP genotype and lamb survival were identified, except in the Charollais breed for which there was a higher risk of mortality in lambs of the ARR/VRQ genotype compared with those of the ARR/ARR genotype. Significant effects of birth weight, litter size, sex, age of dam and year of birth on survival were also identified. For all breeds the reasons for death changed significantly with age; however, no significant associations between reason for death and PrP genotype were found for any of the breeds.

Conclusion

This study found no evidence to suggest that a selective breeding programme based on PrP genotype will have a detrimental effect on lamb survival. The only significant effect of PrP genotype identified was likely to be of little consequence because an increased risk of mortality was associated with a genotype that is selected against in current breeding strategies.

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.

Lambs with scrapie susceptible genotypes have higher postnatal survival

Background

Prion protein (PrP) alleles associated with scrapie susceptibility persist in many sheep populations even with high frequencies despite centuries of selection against them. This suggests that scrapie susceptibility alleles have a pleiotropic effect or are associated with fitness or other traits that have been subject to selection.

Methodology/Principal Findings

We genotyped all lambs in two scrapie-free Scottish Blackface sheep flocks for polymorphisms at codons 136, 154 and 171 of the PrP gene. We tested potential associations of the PrP genotype with lamb viability at birth and postnatal survival using a complementary log-log link function and a Weibull proportional hazard model, respectively. Here we show there is an association between PrP genotype, as defined by polymorphisms at codons 154 ad 171, and postnatal lamb survival in the absence of scrapie. Sheep carrying the wild-type ARQ allele have higher postnatal survival rates than sheep carrying the more scrapie-resistant alleles (ARR or AHQ).

Conclusion

The PrP genotypes associated with higher susceptibility to scrapie are associated with improved postnatal survival in the absence of the disease. This association helps to explain the existence, and in many instances the high frequency, of the ARQ allele in sheep populations.

Association analyses between the prion protein locus and reproductive and lamb weight traits in Ripollesa sheep1

The objective of this study was to analyze the association between the haplotypes of the prion protein (PrP) locus and several reproductive and lamb weight traits in Ripollesa sheep. Prion protein genotypes were available for a total of 310 sheep (7 rams, 114 ewes, and 189 lambs), all of them belonging to the purebred Ripollesa flock of the Universitat Autònoma of Barcelona, for which all sheep had a known pedigree. In addition, the genotype of 24 historical descendants of the previously genotyped adult individuals was reconstructed, provided that both parents were homozygous for PrP haplotypes. Only 3 haplotypes (ARR, ARQ, and ARH) were observed in the PrP locus of the sheep sampled. Reproductive traits included conception rate and litter size, whereas birth BW and 90-d BW were the lamb weight traits studied. The additive effect of PrP haplotypes was analyzed through Bayesian animal threshold and linear models, for reproduction and weight traits, respectively. Ewe reproductive data belonged to 89 ewes that gave 492 conception rate records and 440 litter size records. Analyses of BW at birth and at 90 d of age were made on 323 and 164 lamb records, respectively. No associations between PrP haplotypes and conception rate and BW traits were observed. For litter size, the effect of the ARH haplotype was greater than that of the ARQ haplotype. Differences between ARH and ARR haplotypes also suggested an advantage for the ARH. As a whole, our results indicated that the selection favorable to increase litter size in Ripollesa ewes may also increase the ARH haplotype frequency, which contradicts the recommendations of the current European Union legislation aiming to increase the genetic resistance to scrapie. As a consequence, scrapie genotyping needs to be included as a new selection criterion in the breed.

Molecular evolution of the sheep prion protein gene

Transmissible spongiform encephalopathies (TSEs) are infectious, fatal neurodegenerative diseases characterized by aggregates of modified forms of the prion protein (PrP) in the central nervous system. Well known examples include variant Creutzfeldt-Jakob Disease (vCJD) in humans, BSE in cattle, chronic wasting disease in deer and scrapie in sheep and goats. In humans, sheep and deer, disease susceptibility is determined by host genotype at the prion protein gene (PRNP). Here I examine the molecular evolution of PRNP in ruminants and show that variation in sheep appears to have been maintained by balancing selection, a profoundly different process from that seen in other ruminants. Scrapie eradication programs such as those recently implemented in the UK, USA and elsewhere are based on the assumption that PRNP is under positive selection in response to scrapie. If, as these data suggest, that assumption is wrong, eradication programs will disrupt this balancing selection, and may have a negative impact on the fitness or scrapie resistance of national flocks.

A modelling framework to describe the spread of scrapie between sheep flocks in Great Britain

My aim was to develop a stochastic, spatial model describing the spread of scrapie between sheep flocks in Great Britain; I wanted a model, which could subsequently be used to assess the efficacy of different control strategies. The structure of the model reflects the demography of the British sheep flock, including a description of the contact structure between flocks. The dynamics of scrapie were incorporated through two probabilities associated with each flock: of acquiring infection and of experiencing a within-flock outbreak following exposure. The acquisition of infection depends on whether or not a flock buys-in sheep and, if it does, whether or not it trades with an affected flock. Once a flock is exposed, the probability of a within-flock outbreak occurring and its duration depend on the basic reproductive number, the prion-protein (PrP) genotype profile and the flock size. The model was validated using regional data from two postal surveys conducted in 1998 and 2002, which demonstrated that the model captures the spatial dynamics of scrapie (at least at a regional level). Moreover, the predicted distribution for the duration of a within-flock outbreak reflects the duration of outbreaks reported in the literature. Using the model to predict long-term trends in the proportion of affected flocks suggested that, even without control measures beyond the removal of animals with clinical signs, scrapie ultimately will disappear from the national flock, though it is likely to be decades before the disease is eliminated. However, there were scenarios consistent with the available data which suggested that scrapie could remain endemic within the British sheep flock. Consequently, it is essential to take this uncertainty in the long-term dynamics of scrapie into account when considering the efficacy of control strategies. Although control strategies were not explicitly examined, the model suggests two aspects important for control: larger flocks remain affected for longer and provide infection for other, smaller flocks and animal movements must be traceable.

Simulation of the options for a national control programme to eradicate scrapie from Great Britain

Because of the risk to public health posed by the potential presence of bovine spongiform encephalopathy (BSE) in sheep, there are plans to eradicate transmissible spongiform encephalopathies (TSEs) from the British sheep population. We used a mathematical model for the spread of scrapie between sheep flocks to assess the efficacy of five control strategies at eradicating the infection from the national flock. These range from ram-genotyping schemes through whole-flock genotyping with selective culling to whole-flock slaughter. The impact of control was considered under three scenarios for the long-term dynamics of scrapie in GB: two in which scrapie is ultimately eliminated (with different median extinction times) and one in which scrapie remains endemic. Results suggested that it is feasible to eradicate scrapie from the British sheep flock, but that any national control programme will take decades to eliminate the disease and be costly. The most-effective strategy, measured in terms of the probability of eradication and time taken for eradication, was predicted to be whole-flock culling, which was effective under all three scenarios for the long-term dynamics of scrapie. Strategies involving whole-flock genotyping with selective culling were also effective, though they were predicted to take longer to eradicate scrapie than whole-flock culling. Ram-genotyping schemes were effective in some instances, but not for the scenario where scrapie remained endemic in the national flock. At low levels of reporting of clinical disease (< 20%) the probability of eradication within 100 years was predicted to be < 100% and, consequently, low levels of reporting could compromise the effectiveness of a control programme. Moreover, the predicted time taken to eradicate scrapie would increase markedly if the reporting compliance decreased.

BSE - a wolf in sheep's clothing?

The entire sheep flock in the UK has been threatened with slaughter if BSE is found in farmed sheep, largely on the grounds that an epidemic of BSE in sheep could be harder to contain than was the case for cattle, and that lamb could present a greater risk to consumers than beef. However, identifying BSE in a sheep is not straightforward, because of its similarities to the related disease, scrapie. Here, we review the likelihood that any UK sheep have BSE, how they might have got it, how a case could be identified and what the Government is doing in terms of surveillance and possible control methods.

Biological and biomedical implications of the co-evolution of pathogens and their hosts

Co-evolution between host and pathogen is, in principle, a powerful determinant of the biology and genetics of infection and disease. Yet co-evolution has proven difficult to demonstrate rigorously in practice, and co-evolutionary thinking is only just beginning to inform medical or veterinary research in any meaningful way, even though it can have a major influence on how genetic variation in biomedically important traits is interpreted. Improving our understanding of the biomedical significance of co-evolution will require changing the way in which we look for it, complementing the phenomenological approach traditionally favored by evolutionary biologists with the exploitation of the extensive data becoming available on the molecular biology and molecular genetics of host-pathogen interactions.

Human prion diseases

Transmissible spongiform encephalopathy (TSE) is a group of rare, sub-acute, fatal neurodegenerative diseases in humans and animals. TSE includes Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, fatal familial insomnia, and Kuru (a disease confined to the Fore linguistic group, a tribe in Papua-New Guinea). From the onset, it was recognized that some of these spongiform diseases occur in clusters, in an inherited, familial manner. This article describes these diseases, current treatment modalities, and suggests directions for future research.

The potential size and duration of an epidemic of bovine spongiform encephalopathy in British sheep

Because there is a theoretical possibility that the British national sheep flock is infected with bovine spongiform encephalopathy (BSE), we examined the extent of a putative epidemic. An age cohort analysis based on numbers of infected cattle, dose responses of cattle and sheep to BSE, levels of exposure to infected feed, and number of BSE-susceptible sheep in the United Kingdom showed that at the putative epidemic peak in 1990, the number of cases of BSE-infected sheep would have ranged from fewer than 10 to about 1500. The model predicts that fewer than 20 clinical cases of BSE in sheep would be expected in 2001 if maternal transmission occurred at a rate of 10%. Although there are large uncertainties in the parameter estimates, all indications are that current prevalence is low; however, a simple model of flock-to-flock BSE transmission shows that horizontal transmission, if it has occurred, could eventually cause a large epidemic.

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.

Modelling the national scrapie eradication programme in the UK

In accordance with a policy to eliminate all transmissible spongiform encephalopathies from the food chain, a national untargeted ram breeding programme to eliminate scrapie in the UK is in the final stages of planning. Here we formulate a model of flock-to-flock scrapie transmission, in order to consider the effect of a targeted breeding programme which is in the early stages of consideration. We estimate the size of the susceptible flock population, and discuss implications for potential control programmes. Targeting all rams and ewes in highly susceptible flocks rather than rams in all flocks will eradicate scrapie more quickly, and so is likely to be beneficial as long as suitable penalties or incentives are available to facilitate their identification. A more restricted programme aimed only at highly affected flocks would be much easier to implement and crucially will eradicate scrapie just as quickly. This will leave behind a residue population of susceptible sheep, which could then be gradually removed by a more general breeding programme.