Autoclave treatment of the classical scrapie agent US No. 13-7 and experimental inoculation to susceptible VRQ/ARQ sheep via the oral route results in decreased transmission efficiency

Scrapie, a prion disease of sheep, is highly resistant to conventional deactivation. Numerous methods to deactivate scrapie have been tested in laboratory animal models, and adequate autoclave treatment can reduce or remove the infectivity of some classical scrapie strains depending on the heating parameters used. In this study, we autoclaved brain homogenate from a sheep with US scrapie strain 13–7 for 30 minutes at 121°C. Genetically susceptible VRQ/ARQ sheep were orally inoculated with 3 grams of the autoclaved brain homogenate. For comparison, a second group of sheep was inoculated with a non-autoclaved brain homogenate. Rectal biopsies were used to assess antemortem scrapie disease progression throughout the study. Five out of ten (5/10) sheep that received autoclaved inoculum ultimately developed scrapie after an experimental endpoint of 72 months. These sheep had a mean incubation period of 26.99 months. Two out of five (2/5) positive sheep had detectable PrP^Sc in antemortem rectal biopsies, and two (2/5) other sheep had PrP^Sc in postmortem rectal tissue. A single sheep (1/5) was positive for scrapie in the CNS, small intestine, and retropharyngeal lymph node but had negative rectal tissue. All of the sheep (10/10) that received non-autoclaved inoculum developed scrapie with a mean incubation period of 20.2 months and had positive rectal biopsies at the earliest timepoint (14.7 months post-inoculation). These results demonstrate that sheep are orally susceptible to US derived classical scrapie strain 13–7 after autoclave treatment at 121°C for 30 minutes. Differences in incubation periods and time interval to first positive rectal biopsies indicate a partial reduction in infectivity titers for the autoclaved inoculum group.

Correlation between infectivity and disease associated prion protein in the nervous system and selected edible tissues of naturally affected scrapie sheep

The transmissible spongiform encephalopathies (TSEs) or prion diseases are a group of fatal neurodegenerative disorders characterised by the accumulation of a pathological form of a host protein known as prion protein (PrP). The validation of abnormal PrP detection techniques is fundamental to allow the use of high-throughput laboratory based tests, avoiding the limitations of bioassays. We used scrapie, a prototype TSE, to examine the relationship between infectivity and laboratory based diagnostic tools. The data may help to optimise strategies to prevent exposure of humans to small ruminant TSE material via the food chain. Abnormal PrP distribution/accumulation was assessed by immunohistochemistry (IHC), Western blot (WB) and ELISA in samples from four animals. In addition, infectivity was detected using a sensitive bank vole bioassay with selected samples from two of the four sheep and protein misfolding cyclic amplification using bank vole brain as substrate (vPMCA) was also carried out in selected samples from one animal. Lymph nodes, oculomotor muscles, sciatic nerve and kidney were positive by IHC, WB and ELISA, although at levels 100-1000 fold lower than the brain, and contained detectable infectivity by bioassay. Tissues not infectious by bioassay were also negative by all laboratory tests including PMCA. Although discrepancies were observed in tissues with very low levels of abnormal PrP, there was an overall good correlation between IHC, WB, ELISA and bioassay results. Most importantly, there was a good correlation between the detection of abnormal PrP in tissues using laboratory tests and the levels of infectivity even when the titre was low. These findings provide useful information for risk modellers and represent a first step toward the validation of laboratory tests used to quantify prion infectivity, which would greatly aid TSE risk assessment policies.

Diphenylpyrazole-derived compounds increase survival time of mice after prion infection

Transmissible spongiform encephalopathies (TSEs) represent a group of fatal neurodegenerative disorders that can be transmitted by natural infection or inoculation. TSEs include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. The emergence of a variant form of CJD (vCJD), which has been associated with BSE, produced strong pressure to search for effective treatments with new drugs. Up to now, however, TSEs have proved incurable, although many efforts have been made both in vitro and in vivo to search for potent therapeutic and prophylactic compounds. For this purpose, we analyzed a compound library consisting of 10,000 compounds with a cell-based high-throughput screening assay dealing with scrapie-infected scrapie mouse brain and ScN(2)A cells and identified a new class of inhibitors consisting of 3,5-diphenylpyrazole (DPP) derivatives. The most effective DPP derivative showed half-maximal inhibition of PrP(Sc) formation at concentrations (IC(50)) of 0.6 and 1.2 μM, respectively. This compound was subsequently subjected to a number of animal experiments using scrapie-infected wild-type C57BL/6 and transgenic Tga20 mice. The DPP derivative induced a significant increase of incubation time both in therapeutic and prophylactic experiments. The onset of the prion disease was delayed by 37 days after intraperitoneal and 42 days after oral application, respectively. In summary, we demonstrate a high in vitro efficiency of DPP derivatives against prion infections that was substantiated in vivo for one of these compounds. These results indicate that the novel class of DPP compounds should comprise excellent candidates for future therapeutic studies.

Experimental transmission of scrapie agent to susceptible sheep by intralingual or intracerebral inoculation

Scrapie, a transmissible spongiform encephalopathy (TSE), is a naturally occurring fatal neurodegenerative disease of sheep and goats. This study documents survival periods, pathological findings, and the presence of abnormal prion protein (PrP(Sc)) in genetically susceptible sheep inoculated with scrapie agent. Suffolk lambs (AA/RR/QQ at codons 136, 154, and 171, respectively) aged 4 mo were injected by the intralingual (IL) or intracerebral (IC) route with an inoculum prepared from a pool of scrapie-affected US sheep brains. The animals were euthanized when advanced clinical signs of scrapie were observed. Spongiform lesions in the brain and PrPsc deposits in the central nervous system (CNS) and lymphoid tissues were detected by immunohistochemical and Western blot (WB) testing in all the sheep with clinical prion disease. The mean survival period was 18.3 mo for the sheep inoculated by the IL route and 17.6 mo for those inoculated by the IC route. Since the IC method is occasionally associated with anesthesia-induced complications, intracranial hematoma, and CNS infections, and the IL method is very efficient, it may be more humane to use the latter. However, before this method can be recommended for inoculation of TSE agents, research needs to show that other TSE agents can also transmit disease via the tongue.

Explaining the heterogeneous scrapie surveillance figures across Europe: a meta-regression approach

Background

Two annual surveys, the abattoir and the fallen stock, monitor the presence of scrapie across Europe. A simple comparison between the prevalence estimates in different countries reveals that, in 2003, the abattoir survey appears to detect more scrapie in some countries. This is contrary to evidence suggesting the greater ability of the fallen stock survey to detect the disease. We applied meta-analysis techniques to study this apparent heterogeneity in the behaviour of the surveys across Europe. Furthermore, we conducted a meta-regression analysis to assess the effect of country-specific characteristics on the variability. We have chosen the odds ratios between the two surveys to inform the underlying relationship between them and to allow comparisons between the countries under the meta-regression framework. Baseline risks, those of the slaughtered populations across Europe, and country-specific covariates, available from the European Commission Report, were inputted in the model to explain the heterogeneity.

Results

Our results show the presence of significant heterogeneity in the odds ratios between countries and no reduction in the variability after adjustment for the different risks in the baseline populations. Three countries contributed the most to the overall heterogeneity: Germany, Ireland and The Netherlands. The inclusion of country-specific covariates did not, in general, reduce the variability except for one variable: the proportion of the total adult sheep population sampled as fallen stock by each country. A large residual heterogeneity remained in the model indicating the presence of substantial effect variability between countries.

Conclusion

The meta-analysis approach was useful to assess the level of heterogeneity in the implementation of the surveys and to explore the reasons for the variation between countries.

Prevention of prion propagation by dehydrocholesterol reductase inhibitors in cultured cells and a therapeutic trial in mice

In prion diseases, the normal cellular form of prion protein (PrP(C)) is converted into the disease-associated isoforms (PrP(Sc)) which accumulate in the infected tissues. Although the precise mechanism of this conversion remains unsolved, drugs of various categories have been reported to reduce the accumulation of PrP(Sc) in prion-infected cultured cells. We here show that AY-9944 (a 7-dehydrocholesterol reductase inhibitor) and U18666A (a 24-dehydrocholesterol reductase inhibitor) prevent PrP(Sc) from accumulating in prion-infected mouse neuroblastoma cells (ScN2a), with an ED50 of about 0.5 microM and 10 nM, respectively. In order to evaluate the efficacy of these two inhibitors in vivo, C57BL/6J mice inoculated with mouse-adapted scrapie-prion received repetitive intraperitoneal injections of U18666A (10 mg/kg) or a mixture of U18666A (10 mg/kg) and AY-9944 (12 mg/kg). By contrast to the potent anti-prion effects observed in ScN2a cells, the in vivo trial was abortive with neither drug halting the progression of the disease.

Advanced survival models for risk-factor analysis in scrapie

Because of the confounding effects of long incubation duration and flock management, accurate epidemiological studies of scrapie outbreaks are difficult to carry out. In this study, 641 Manech red-faced sheep from six scrapie-affected field flocks in Pyrénées Atlantiques, France, were monitored for clinical scrapie over a 6–9 year period. Over this period, 170 scrapie clinical cases were recorded and half of the culled animals were submitted for post-mortem transmissible spongiform encephalopathy diagnosis to assess their infectious status. Collected data were analysed using a ‘mixture cure model’ approach, which allowed for the discriminating effect of PrP genotype and flock origin on incidence and incubation period. Simulations were performed to evaluate the applicability of such a statistical model to the collected data. As expected, ARR heterozygote sheep were less at risk of becoming infected than ARQ/ARQ individuals and had a greater age at clinical onset. Conversely, when compared with ARQ/ARQ, the VRQ haplotype was associated with an increased infection risk, but not a shorter incubation period. Considering the flock effect, we observed that a high incidence rate was not associated with shorter incubation periods and that the incubation period could be significantly different in flocks harbouring similar infection risks. These results strongly support the conclusion that other parameters, such as the nature of the agent or flock management, could interfere with epidemiological dynamics of the infection in scrapie-affected flocks.

Enhanced antiscrapie effect using combination drug treatment

Combination treatment with pentosan polysulfate and Fe(III)meso-tetra(4-sulfonatophenyl)porphine in mice beginning 14 or 28 days after scrapie inoculation significantly increased survival times. This increase may be synergistic, implying that the compounds act cooperatively in vivo. Combination therapy may therefore be more effective for treatment of transmissible spongiform encephalopathies and other protein-misfolding diseases.

Using genotype probabilities in survival analysis: a scrapie case

The objective was to evaluate the potential use of genotype probabilities to handle records of non-genotyped animals in the context of survival analysis. To do so, the risks associated with the PrP genotype and other transmission factors in relation to clinical scrapie were estimated. Data from 4049 Romanov sheep affected by natural scrapie were analyzed using survival analysis techniques. The original data set included 1310 animals with missing genotypes; five of those had uncensored records. Different missing genotype-information patterns were simulated for uncensored and censored records. Three strategies differing in the way genotype information was handled were tested. Firstly, records with unknown genotypes were discarded (P1); secondly, those records were grouped in an unknown class (P2). Finally the probabilities of genotypes were assigned (P3). Whatever the strategy, the ranking of relative risks for the most susceptible genotypes (VRQ-VRQ, ARQ-VRQ and ARQ-ARQ) was similar even when the non-genotyped animals were not a negligible part of uncensored records. However, P3 had a more efficient way of handling missing genotype information. As compared to P1, either P2 or P3 avoided discarding the records of non-genotyped animals; however, P3 eliminated the unknown class and the risk associated with this group. Genotype probabilities were shown to be a useful technique to handle records of individuals with unknown genotype.

High incidence of scrapie induced by repeated injections of subinfectious prion doses

To clarify the mechanisms leading to the development of Creutzfeldt-Jakob disease in some recipients of pituitary-derived human growth hormone (hGH), we investigated the effects of repeated injections of low prion doses in mice. The injections were performed, as in hGH-treated children, by a peripheral route at short intervals and for an extended period. Twelve groups of 24 mice were intraperitoneally inoculated one, two, or five times per week for 200 days with 2 x 10(-5) to 2 x 10(-8) dilutions of brain homogenate containing the mouse-adapted C506M3 scrapie strain. Sixteen control mice were injected once a week for 200 days with a 2 x 10(-4) dilution of normal brain homogenate. Of mice injected in a single challenge with a scrapie inoculum of a 2 x 10(-4), 2 x 10(-5), or 2 x 10(-6) dilution, 2/10, 1/10, and 0/10 animals developed scrapie, respectively. Control mice remained healthy. One hundred thirty-five of 135 mice injected with repeated prion doses of a 2 x 10(-5) or 2 x 10(-6) dilution succumbed to scrapie. Of mice injected with repeated scrapie doses of a 2 x 10(-7) or 2 x 10(-8) dilution, 52/59 and 38/67 animals died of scrapie, respectively. A high incidence of scrapie was observed in mice receiving repeated doses at low infectivity, whereas there was no disease in mice that were injected once with the same doses. Repeated injections of low prion doses thus constitute a risk for development of prion disease even if the same total dose inoculated in a single challenge does not induce the disease.

Genotype-level variation in lifetime breeding success, litter size and survival of sheep in scrapie-affected flocks

Five different sheep flocks with natural outbreaks of scrapie were examined to determine associations between individual performance (lifetime breeding success, litter size and survival) and scrapie infection or PrP genotype. Despite different breed composition and forces of infection, consistent patterns were found among the flocks. Regardless of the flock, scrapie-infected sheep produced on average 34 % fewer offspring than non-scrapie-infected sheep. The effect of scrapie on lifetime breeding success appears to be a function of lifespan as opposed to fecundity. Analysis of litter size revealed no overall or genotype differences among the five sheep flocks. Survival, however, depends on the individual's scrapie status (infected or not) and its PrP genotype. Susceptible genotypes appear to perform less well in lifetime breeding success and life expectancy even if they are never affected with clinical scrapie. One possible explanation for these results is the effect of pre-clinical scrapie. Additional evidence supporting this hypothesis is discussed.

Unchanged survival rates of 14-3-3gamma knockout mice after inoculation with pathological prion protein

The diagnosis of sporadic Creutzfeldt-Jakob disease (CJD) is based on typical clinical findings and is supported by a positive 14-3-3 Western blot of cerebrospinal fluid. However, it is not clear whether 14-3-3 indicates general neuronal damage or is of pathophysiological relevance in CJD. The fact that the 14-3-3 isoform spectrum in cerebrospinal fluid does not correspond to that found in the brain points to a regulated process. To investigate a possible role of 14-3-3 proteins in transmissible spongiform diseases, we generated a 14-3-3gamma-deficient mutant mouse line by using a classical knockout strategy. The anatomy and cage behavior of the mutant mice were normal. Western blot analyses of brain homogenates revealed no changes in the protein expression of other 14-3-3 isoforms (epsilon, beta, zeta, and eta). Proteomic analyses of mouse brains by two-dimensional differential gel electrophoresis showed that several proteins, including growth hormone, 1-Cys peroxiredoxin, CCT-zeta, glucose-6-phosphate isomerase, GRP170 precursor, and alpha-SNAP, were differentially expressed. Mutant and wild-type mice were inoculated either intracerebrally or intraperitoneally with the Rocky Mountain Laboratory strain of scrapie, but no differences were detected in the postinoculation survival rates. These results indicate that 14-3-3gamma is unlikely to play a causal role in CJD and related diseases.

Polygenic variation and transmission factors involved in the resistance/susceptibility to scrapie in a Romanov flock

Data from 4049 Romanov sheep belonging to a flock affected by natural scrapie were analysed by using survival-analysis techniques. Failure time was defined as the period of time between first exposure to infection and the date that animals left the flock with scrapie signs. Four hundred and forty-seven sheep were identified as ‘scrapie animals’. Several models, including level of exposure as a time-dependent effect, PrP genotype, sex, age at first exposure, litter size and factors related to vertical transmission, were tested. The best model was extended to a sire–dam frailty model, in order to estimate the polygenic variation in addition to that in the Prnp gene. A combined effect of rearing type and the dam's disease status was detected. Thus, only sheep with a low degree of exposure to infection as lambs (lambs reared artificially and born out of a healthy dam) showed less risk than others. Animals first exposed to infection at older ages seemed to be less susceptible to scrapie. In this Romanov population, new genotypes (AHQ/AHQ, AHQ/VRQ, ARR/VRQ and ARR/ARQ) were associated with risk, suggesting the effect of genotypes on the incubation period of animals. Polygenic variance was responsible for 21 % of the total genetic variability that was related to susceptibility to scrapie. Therefore, the genetic susceptibility to scrapie may be explained by the joint effect of point mutations at the Prnp major gene and a number of genes that modulate its effect.

Reduced proteinase K resistance and infectivity of prions after pressure treatment at 60 °C

High hydrostatic pressure is a mild technology compared with high temperatures and is commonly used for food pasteurization. Crude brain homogenates of terminally diseased hamsters infected with scrapie 263K strain were heated at 60 degrees C and/or pressurized up to 1000 MPa for 2 h. Prion proteins were analysed for their proteinase K sensitivity using a Western blot technique. PrP(Sc) pressurized with 500 MPa or above proved to be proteinase K sensitive. To test the remaining infectivity of the pressurized material, hamsters were infected intracerebrally. Results showed a greatly delayed onset of disease (from 80 up to 153 days) when samples had been pressurized at 500 MPa and above. An increase in the survival rate was also observed: 47 % survival over 180 days was seen following infection with homogenates pressurized at 700-1000 MPa.

Prophylactic and therapeutic effects of phthalocyanine tetrasulfonate in scrapie-infected mice

The transmissible spongiform encephalopathy (TSE) diseases are rare, neurodegenerative diseases that include scrapie in sheep, bovine spongiform encephalopathy, and Creutzfeldt-Jakob disease in humans. There are no effective treatments available for clinical use in humans. We now demonstrate that, in 2 different rodent models of scrapie, multiple pretreatments with the cyclic tetrapyrrole phthalocyanine tetrasulfonate (PcTS) were as effective at delaying disease as multiple treatments starting at the time of infection. At low doses of scrapie infectivity, PcTS also protected some mice from peripheral scrapie infection, even if treatment was initiated several weeks after infection. Furthermore, PcTS completely inactivated low levels of scrapie infectivity when incubated with the infectious inoculum. Thus, PcTS has a broad range of antiscrapie activities. These findings suggest that cyclic tetrapyrroles may be useful both prophylactically and therapeutically against TSE diseases in vivo, as well as for inactivation of TSE infectivity suspended in solution.

Scrapie epidemic in a fully PrP-genotyped sheep flock

In scrapie-affected sheep flocks, host PrP genotype plays a vital role in determining which sheep will succumb to scrapie and the incubation period. Consequently, within-flock scrapie dynamics is best understood within the context of the genotype profile of the flock. Here we describe a 17 month epidemic of scrapie in a commercially farmed flock of 230 genotyped Texel sheep. At the start of the study, 70% of the sheep were of three genotypes only: ARR/ARQ, ARH/ARQ and ARQ/ARQ. Only 15% of sheep encoded the disease-associated VRQ allele and only a single sheep (0.4%) was of the most susceptible VRQ/VRQ genotype. For susceptible genotypes there was a marked deficit (P<0.025) of older animals (> or =3 years), implying that some cases of scrapie had occurred previously. In the ensuing 17 months, 18 sheep of known genotype were confirmed positive for the disease: seven VRQ/ARQ, six VRQ/ARH, two VRQ/ARR, three ARQ/ARQ. Median ages at death were 2.7, 2.8, 4.2 and 3.8 years respectively. Mortality rates were 55, 86, 13 and 3% respectively. Survival analysis revealed a highly significant effect of genotype on survivorship, but no difference between VRQ/ARQ and VRQ/ARH, or between VRQ/ARR and ARQ/ARQ. There was no difference in the survivorship of middle- and older-age cohorts of susceptible sheep. Scrapie risk group (as defined by PrP genotype) was not associated with submission as a scrapie suspect but later found to be negative, or with dying of unknown causes on the farm.

Sensitivity of the Western blot detection of prion protein PrPres in natural sheep scrapie

The sensitivity of Western blot detection of PrPres using two different extraction procedures on brain material of 30 scrapie-affected sheep was compared. Whereas PrPres could be detected in all sheep after extraction with the first method, 30% did not give any signal after extraction with the second method. However, the second method, when positive, permitted the detection of PrPres from smaller amounts of infected brain tissue. When used with the ruminant specific monoclonal antibody p4, the second method gave positive signals corresponding to less than 12.5 microg of scrapie-infected brain, that, up to now, is the highest sensitivity described for PrPres detection from naturally infected ruminant brains. The overall results showed highly variable levels of PrPres between sheep and are presented in relation to breed, survival time and animal genotype data. Further progress can thus be expected for PrPres detection in prion diseases, if more efficient extraction procedures and more sensitive immunological reagents are used. Such technical improvements could contribute to more accurate diagnosis in animals affected naturally.

Effect of repeated oral infection of hamsters with scrapie

The development of a transmissible spongiform encephalopathy upon uptake of the infectious agent in feed was studied in the model system scrapie in hamsters. Compared to single dosing, repeated dosing caused disease at a considerable higher incidence. The risk of infection was higher when the time interval between repetitive dosing was short. There was a statistically significant trend of clearance of infectivity with time.

Astrocyte-specific expression of hamster prion protein (PrP) renders PrP knockout mice susceptible to hamster scrapie

Transmissible spongiform encephalopathies are characterized by spongiosis, astrocytosis and accumulation of PrPSc, an isoform of the normal host protein PrPC. The exact cell types responsible for agent propagation and pathogenesis are still uncertain. To determine the possible role of astrocytes, we generated mice devoid of murine PrP but expressing hamster PrP transgenes driven by the astrocyte-specific GFAP promoter. After inoculation with hamster scrapie, these mice accumulated infectivity and PrPSc to high levels, developed severe disease after 227 +/- 5 days and died 7 +/- 4 days later. Therefore, astrocytes could play an important role in scrapie pathogenesis, possibly by an indirect toxic effect on neurons. Interestingly, mice expressing the same transgenes but also endogenous murine PrP genes propagated infectivity without developing disease.

PrP genotype contributes to determining survival times of sheep with natural scrapie

Several allelic variants of the sheep PrP gene are associated with scrapie susceptibility. However, it is not known whether, and to what extent, the PrP genotype contributes to determining survival times of scrapie sheep. We therefore determined the PrP genotype and life spans of over 50 Flemish and Swifter sheep within a single scrapie-affected flock. Eighty-three per cent of the scrapie sheep were homozygous for the PrP(VQ) allele (polymorphic amino acids at codons 136 and 171 are indicated) and these sheep died from scrapie at a mean age of 25 months. In sheep heterozygous for PrP(VQ), development of scrapie was delayed or did not occur. Sheep with at least one PrP(AR) allele, including PrP(VQ)/PrP(AR) sheep, did not develop scrapie. No scrapie sheep were found without a PrP(VQ) allele. We conclude that the PrP genotype contributes to determining survival times of sheep with natural scrapie. Additionally, we describe two novel sheep PrP allelic variants.

High prion and PrPSc levels but delayed onset of disease in scrapie-inoculated mice heterozygous for a disrupted PrP gene

BACKGROUND

It has been proposed that the prion, the infectious agent of transmissible spongiform encephalopathies, is PrPSc, a post-translationally modified form of the normal host protein PrPC. We showed previously that mice devoid of PrPC (Prn-p0/0) are completely resistant to scrapie. We now report on the unexpected response of heterozygous (Prn-p0/+) mice to scrapie infection.

MATERIALS AND METHODS

Prn-p0/+, Prn-p0/0 and Prn-p+/+ mice were obtained from crosses of Prn-p0/+ mice. Mice were inoculated intracerebrally with mouse-adapted scrapie agent and the clinical progression of the disease recorded. Mice were sacrificed at intervals, PrPSc was determined as protease-resistant PrP and the prion titer by the incubation time assay.

RESULTS

Prn-p0/+ mice, which have about half the normal level of PrPC in their brains, show enhanced resistance to scrapie, as manifested by a significant delay in onset and progression of clinical disease. However, while in wild type animals an increase in prion titer and PrPSc levels is followed within weeks by scrapie symptoms and death, heterozygous Prn-p0/+ mice remain free of symptoms for many months despite similar levels of scrapie infectivity and PrPSc.

CONCLUSIONS

Our findings extend previous reports showing an inverse relationship between PrP expression level and incubation time for scrapie. However, contrary to expectation, overall accumulation of PrPSc and prions to a high level do not necessarily lead to clinical disease. These findings raise the question whether high titers of prion infectivity could also persist for long periods under natural circumstances in the absence of clinical symptoms.

Polymorphisms of a scrapie-associated fibril protein (PrP) gene and their association with susceptibility to experimentally induced scrapie in Cheviot sheep in the United States

The duration of the incubation period for scrapie, a fatal transmissible neurodegenerative disorder of sheep and goats, is mainly determined by the Sip gene, which has 2 alleles (sA--susceptible and pA--resistant). A diagnostic test is not available to detect scrapie in live animals. We analyzed genomic DNA extracted from frozen sheep brains collected from Cheviot sheep of the United States that had been inoculated with the SSBP/1 scrapie inoculum. Digestion of the DNA with EcoRI or HindIII followed by the addition of a scrapie-associated fibril protein (PrP)-specific marker probe, yielded fragments of 6.8 (e1) and 4.0 (e3) kb, or 5.0 (h1) and 3.4 (h2) kb, respectively. Fragments e1 and h2 were associated with the histopathologic diagnosis of scrapie, and fragments e3 and h1 were associated with survival. A valine/alanine polymorphism within the PrP coding region that resulted in a BspHI site was further used to determine the genotype of these Cheviot sheep. Digestion of polymerase chain reaction fragments with BspHI resulted in an undigested fragment b- (0.840 kb), digested fragments b+ (0.460 and 0.380 kb), or both types of fragments. Survival time of b+/b+ homozygous sheep was significantly (P < 0.01) shorter (218 +/- 26.0 days) than survival time for b-/b- sheep (> 700 days after inoculation). Results indicated that b+ and b- are markers for the Sip sA and pA alleles, respectively. The intermediate duration of the incubation period for heterozygous sheep (b+/b-; 342.9 +/- 25.3 days) indicated that the Sip sA allele is expressed codominantly to the Sip pA allele.

Pathogenesis and pathology of scrapie after stereotactic injection of strain 22L in intact and bisected cerebella

The mechanisms involved in the spread of scrapie within the brain remain unclear. To examine this issue the 22L scrapie strain was injected in one side of the cerebellum of mice in which the cerebellum had been bisected prior to injection. Another group of animals received the same injection into intact cerebella, i.e. without prior bisection. We found that bisection of the cerebella delayed the spread of scrapie agent from the injection site to the contralateral side of the cerebellum and that the occurrence of vacuolization was not as extensive and was markedly delayed in the uninjected side compared to its occurrence after injection in the intact cerebellum. Replication of agent in an area preceded the development of vacuolization in that area by several weeks. There was marked loss of Purkinje cells on the injected side of bisected cerebella, with no loss seen on the uninjected side. The incubation period of scrapie disease in mice injected after cerebellar bisection was significantly longer than after the injection of intact cerebella. The results in this study suggest that the scrapie agent spreads along intact nerve cell tracts, probably by axonal transport.

Vacuolization, incubation period and survival time analyses in three mouse genotypes injected stereotactically in three brain regions with the 22L scrapie strain

In previous studies we showed that C57BL mice injected stereotactically in the cerebellum with the 22L scrapie strain had a significantly shorter incubation period than those injected with the same agent in other brain regions. In mice injected in the cerebellum, vacuolization was limited to the cerebellum, medulla and mesencephalon, whereas injection into forebrain regions resulted in vacuolization in all brain regions. The studies suggested that the cerebellum had a selective vulnerability for 22L. In this study we examined the interaction between host genotype and selective vulnerability of specific brain regions. The mouse gene that has the most profound effect on pathogenesis, particularly incubation period, is termed Sinc (scrapie incubation). Groups of mice with three genotypes of Sinc (s7s7, p7p7 and their F1 cross, s7p7) were injected with 22L into the cerebral cortex, thalamus or cerebellum. Analysis of incubation periods showed that, regardless of the host genotype, the cerebellum injection group had a significantly shorter incubation period than groups injected in other regions. After cerebellum injection vacuolization was limited to the cerebellum, medulla and mesencephalon in all three host genotypes. The location of vacuoles within the cerebellum differed depending upon the host strain. Vacuolization developed almost exclusively in grey matter in s7s7 mice, mainly in white matter in p7p7 mice, and in both grey and white matter in F1 mice. These results demonstrate that the selective vulnerability of the cerebellum to induction of clinical disease by 22L does not depend on host genotype, but host genotype does affect lesion distribution within the cerebellum.

Age at death from natural scrapie in a flock of Suffolk sheep

This study deals with natural scrapie in a flock of Suffolk sheep being bred to maximise the incidence of the disease and shows that with succeeding generations the incubation period of the disease became progressively shorter until the flock died out. The most likely explanation for this phenomenon is considered to be an increase in the load of infection.

Determination of scrapie agent titer from incubation period measurements in hamsters

Progress in studies on the scrapie agent has been hampered by the slow and tedious endpoint titration assays in rodents. A new assay based on incubation period measurements has been developed. The incubation period is defined as the time interval from inoculation to the onset of clinically detectable neurological illness or as the interval from inoculation to death. Both of these intervals or incubation periods are inversely proportional to the size of the dose injected intracerebrally into random bred weanling syrian hamsters. The incubation period was found to be a linear function of the logarithm of the inoculum size over a wide range of dosages (10(2) - 10(8) ID50 units). From these studies an equation relating the titer of the inoculum to the dilution of the sample and the length of the incubation period has been developed facilitating the use of a computerized record system. Validation of the assay was provided by comparing samples where the agent was measured both by endpoint titration and incubation period methods. Agreement between the 2 methods was generally within +/- 0.5 log10 ID50 units. In addition, no differences between the molecular properties of the agents from hamster and murine sources have been detected using primarily the incubation period method with the former and endpoint titration with the latter. The advantages of the incubation period assay are considerable with respect to time and economy.

Interferon system in mice infected with the scrapie agent

Therapy with polyinosinic-polycytidylic acid did not protect scrapie-infected mice; scrapie infection did not affect the interferon response to polyinosinic . polycytidyl acid or Newcastle disease virus.