Association between incubation time and genotype in sheep experimentally inoculated with scrapie-positive brain homogenate

Combinatorial treatment of brain samples from sheep with scrapie using sodium percarbonate, sodium dodecyl sulfate, and proteinase K increases survival time in inoculated susceptible sheep

The agent of scrapie is resistant to most chemical and physical methods of inactivation. Prions bind to soils, metals, and various materials and persist in the environment confounding the control of prion diseases. Most methods of prion inactivation require severe conditions such as prolong exposure to sodium hypochlorite or autoclaving, which may not be suitable for field conditions. We evaluated the efficacy of a combinatorial approach to inactivation of US scrapie strain x124 under the mild conditions of treating scrapie-affected brain homogenate with sodium percarbonate (SPC), sodium dodecyl sulfate (SDS), or in combination followed by proteinase K (PK) digestion at room temperature. Western blot analysis of treated brain homogenate demonstrates partial reduction in PrP^Sc immunoreactivity. Genetically susceptible VRQ/ARQ Suffolk sheep were oronasally inoculated with 1 g of SPC (n = 1), SDS (n = 2), SDS + PK (n = 2), and SPC + SDS + PK (n = 4) treated brain homogenate. Sheep were assessed daily for clinical signs, euthanized at the development of clinical disease, and tissues were assessed for accumulation of PrP^Sc. Scrapie status in all sheep was determined by western blot, enzyme immunoassay, and immunohistochemistry. Mean incubation periods (IPs) for SPC (11.9 months, 0% survival), SDS (12.6 months, 0% survival), SDS + PK (14.0 months, 0% survival), and SPC + SDS + PK (12.5 months, 25% survival) were increased compared to positive control sheep (n = 2, 10.7 months, 0% survival) by 1.2, 1.9, 3.3, and 1.8 months, respectively. Treatment did not influence PrP^Sc accumulation and distribution at the clinical stage of disease. Differences in mean IPs and survival indicates partial but not complete reduction in scrapie infectivity.

Efficient transmission of classical scrapie agent x124 by intralingual route to genetically susceptible sheep with a low dose inoculum

Scrapie is a naturally occurring prion disease of sheep and goats that results in accumulation of the misfolded prion protein (PrP^Sc) and progressive neurodegeneration. After inoculation with classical scrapie isolate x124, susceptibility and incubation period are associated with valine at codon 136 (V136) of the prion protein: VRQ/VRQ had the shortest incubation periods, followed by VRQ/ARQ sheep, while ARQ/ARQ sheep only developed disease after inoculation via the intracerebral route. Intralingual inoculation of TSE agents effectively transmits disease similar to intracranial inoculation; therefore, it is possible that oral lesions may facilitate susceptibility to scrapie transmission. In this study, investigated the infectivity of decreasing doses of the x124 scrapie agent (100 mg, 50 mg, 20 mg, and 10 mg) on incubation time and attack rate after experimental intralingual inoculation into VRQ/ARQ sheep. The lowest inoculum dose tested in this study effectively transmitted the x124 scrapie agent in VRQ/ARQ sheep with a 100% attack rate and no significant difference in incubation times among sheep inoculated with varying doses. Moreover, immunohistochemistry and western blot analysis revealed similar biochemical and immunohistochemical features among the four cohorts of sheep irrespective of inoculum dose. This study provides a starting point for further investigation to determine the minimum infectious dose of x124 scrapie in sheep and its effect on attack rate and incubation time, central for assessing the potential risk of scrapie occurrence in sheep flock.

Source genotype influence on cross species transmission of transmissible spongiform encephalopathies evaluated by RT-QuIC

Scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep and goats. This fatal neurodegenerative disease is caused by misfolding of the cellular prion protein to pathogenic β-rich conformers (PrPSc) that accumulate in higher order structures of the brain and other tissues. This conversion has been used for in vitro assays including serial protein misfolding amplification and real-time quaking induced conversion (RT-QuIC). RT-QuIC can be used for the detection of prions and for strain discrimination in a variety of biological tissues from humans and animals. In this study, we evaluated how PrPSc isolated from sheep of different genotypes after inoculation with the scrapie agent influence the fibril formation in vitro using RT-QuIC. We found that reaction mixtures seeded with PrPSc from genotype VRQ/VRQ sheep brains have better conversion efficiency with 132M elk substrate compared to reactions seeded with PrPSc from the brains of sheep with the ARQ/ARQ genotype no matter which strain of scrapie was used to seed the reactions. We also inoculated transgenic mice expressing 132M elk PRNP (Tg12) with the scrapie agent from different genotypes of sheep to compare with our RT-QuIC results. The bioassays support the data showing a significantly shorter incubation period for inoculum from VRQ/VRQ sheep when compared to inoculum from ARQ/ARQ sheep. Thus, we conclude that the genotype of both source and recipient can strongly influence transmission.

Comparison of Two US Sheep Scrapie Isolates Supports Identification as Separate Strains

Scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep and goats. There are different strains of sheep scrapie that are associated with unique molecular, transmission, and phenotype characteristics. However, in the United States, very little is known about the potential presence of scrapie strains. Scrapie strain and PRNP genotype could both affect susceptibility, potential for transmission, incubation period (IP), and control measures required for eliminating scrapie from a flock. The investigators evaluated 2 US scrapie isolates, No. 13-7 and x124, after intranasal inoculation to compare clinical signs, IPs, spongiform lesions, and patterns of PrPSc deposition in sheep with scrapie-susceptible PRNP genotypes (QQ171). After inoculation with x124, susceptibility and IP were associated with valine at codon 136 (V136) of the prion protein: VV136 sheep had short IPs (6.9 months), those in AV136 sheep were 11.9 months, and AA136 sheep did not develop scrapie. All No. 13-7 inoculated sheep developed scrapie, with IPs of 20.1 months for AA136 sheep, 22.8 months for AV136 sheep, and 26.7 months for VV136 sheep. Patterns of immunoreactivity in the brain were influenced by inoculum isolate and host genotype. Differences in PrPSc profiles versus isolate were most striking when examining brains from sheep with the VV136 genotype. Inoculation into C57BL/6 mice resulted in markedly different attack rates (90.5% for x124 and 5.9% for No. 13-7). Taken together, these data demonstrate that No. 13-7 and x124 represent 2 distinct strains of scrapie with different IPs, genotype susceptibilities, and PrPSc deposition profiles.

Evaluation of two commercial, rapid, ELISA kits testing for scrapie in retro-pharyngeal lymph nodes in sheep

AIMS

To estimate the number of cases of scrapie that would occur in sheep of different prion protein (PrP) genotypes if scrapie was to become established in New Zealand, and to compare the performance of two commercially available, rapid ELISA kits using ovine retro-pharyngeal lymph nodes (RLN) from non-infected and infected sheep of different PrP genotypes.

METHODS

Using published data on the distribution of PrP genotypes within the New Zealand sheep flock and the prevalence of cases of scrapie in these genotypes in the United Kingdom, the annual expected number of cases of scrapie per genotype was estimated, should scrapie become established in New Zealand, assuming a total population of 28 million sheep. A non-infected panel of RLN was collected from 737 sheep from New Zealand that had been culled, found in extremis or died. Brain stem samples were also collected from 131 of these sheep. A second panel of infected samples comprised 218 and 117 RLN from confirmed scrapie cases that had originated in Europe and the United States of America, respectively. All samples were screened using two commercial, rapid, transmissible spongiform encephalopathy ELISA kits: Bio-Rad TeSeE ELISA (ELISA-BR), and IDEXX HerdChek BSE-Scrapie AG Test (ELISA-ID).

RESULTS

If scrapie became established in New Zealand, an estimated 596 cases would occur per year; of these 234 (39%) and 271 (46%) would be in sheep carrying ARQ/ARQ and ARQ/VRQ PrP genotypes, respectively. For the non-infected samples from New Zealand the diagnostic specificity of both ELISA kits was 100%. When considering all infected samples, the diagnostic sensitivity was 70.4 (95% CI=65.3-75.3)% for ELISA-BR and 91.6 (95% CI=88.2-94.4)% for ELISA-ID. For the ARQ/ARQ genotype (n=195), sensitivity was 66.2% for ELISA-BR and 90.8% for ELISA-ID, and for the ARQ/VRQ genotype (n=107), sensitivity was 81.3% for ELISA-BR and 98.1% for ELISA-ID.

CONCLUSIONS

In this study, the ELISA-ID kit demonstrated a higher diagnostic sensitivity for detecting scrapie in samples of RLN from sheep carrying scrapie-susceptible PrP genotypes than the ELISA-BR kit at comparable diagnostic specificity.

CLINICAL RELEVANCE

The diagnostic performance of the ELISA-ID kit using ovine RLN merits the consideration of including this assay in the national scrapie surveillance programme in New Zealand.

Evidence in sheep for pre-natal transmission of scrapie to lambs from infected mothers

Natural scrapie transmission from infected ewes to their lambs is thought to occur by the oral route around the time of birth. However the hypothesis that scrapie transmission can also occur before birth (in utero) is not currently favoured by most researchers. As scrapie is an opportunistic infection with multiple infection routes likely to be functional in sheep, definitive evidence for or against transmission from ewe to her developing fetus has been difficult to achieve. In addition the very early literature on maternal transmission of scrapie in sheep was compromised by lack of knowledge of the role of the PRNP (prion protein) gene in control of susceptibility to scrapie. In this study we experimentally infected pregnant ewes of known PRNP genotype with a distinctive scrapie strain (SSBP/1) and looked for evidence of transmission of SSBP/1 to the offspring. The sheep were from the NPU Cheviot flock, which has endemic natural scrapie from which SSBP/1 can be differentiated on the basis of histology, genetics of disease incidence and strain typing bioassay in mice. We used embryo transfer techniques to allow sheep fetuses of scrapie-susceptible PRNP genotypes to develop in a range of scrapie-resistant and susceptible recipient mothers and challenged the recipients with SSBP/1. Scrapie clinical disease, caused by both natural scrapie and SSBP/1, occurred in the progeny but evidence (including mouse strain typing) of SSBP/1 infection was found only in lambs born to fully susceptible recipient mothers. Progeny were not protected from transmission of natural scrapie or SSBP/1 by washing of embryos to International Embryo Transfer Society standards or by caesarean derivation and complete separation from their birth mothers. Our results strongly suggest that pre-natal (in utero) transmission of scrapie may have occurred in these sheep.

Relationships between PrPSc stability and incubation time for United States scrapie isolates in a natural host system

Transmissible spongiform encephalopathies (TSEs), including scrapie in sheep (Ovis aries), are fatal neurodegenerative diseases caused by the misfolding of the cellular prion protein (PrP^C) into a â-rich conformer (PrP^Sc) that accumulates into higher-order structures in the brain and other tissues. Distinct strains of TSEs exist, characterized by different pathologic profiles upon passage into rodents and representing distinct conformations of PrP^Sc. One biochemical method of distinguishing strains is the stability of PrP^Sc as determined by unfolding in guanidine hydrochloride (GdnHCl), which is tightly and positively correlated with the incubation time of disease upon passage into mice. Here, we utilize a rapid, protease-free version of the stability assay to characterize naturally occurring scrapie samples, including a fast-acting scrapie inoculum for which incubation time is highly dependent on the amino acid at codon 136 of the prion protein. We utilize the stability methodology to identify the presence of two distinct isolates in the inoculum, and compare isolate properties to those of a host-stabilized reference scrapie isolate (NADC 13-7) in order to assess the stability/incubation time correlation in a natural host system. We demonstrate the utility of the stability methodology in characterizing TSE isolates throughout serial passage in livestock, which is applicable to a range of natural host systems, including strains of bovine spongiform encephalopathy and chronic wasting disease.

Salivary prions in sheep and deer

Scrapie of sheep and chronic wasting disease (CWD) of cervids are transmissible prion diseases. Milk and placenta have been identified as sources of scrapie prions but do not explain horizontal transmission. In contrast, CWD prions have been reported in saliva, urine and feces, which are thought to be responsible for horizontal transmission. While the titers of CWD prions have been measured in feces, levels in saliva or urine are unknown. Because sheep produce ~17 L/day of saliva, and scrapie prions are present in tongue and salivary glands of infected sheep, we asked if scrapie prions are shed in saliva. We inoculated transgenic (Tg) mice expressing ovine prion protein, Tg(OvPrP) mice, with saliva from seven Cheviot sheep with scrapie. Six of seven samples transmitted prions to Tg(OvPrP) mice with titers of -0.5 to 1.7 log ID₅₀ U/ml. Similarly, inoculation of saliva samples from two mule deer with CWD transmitted prions to Tg(ElkPrP) mice with titers of -1.1 to -0.4 log ID₅₀ U/ml. Assuming similar shedding kinetics for salivary prions as those for fecal prions of deer, we estimated the secreted salivary prion dose over a 10-mo period to be as high as 8.4 log ID₅₀ units for sheep and 7.0 log ID₅₀ units for deer. These estimates are similar to 7.9 log ID₅₀ units of fecal CWD prions for deer. Because saliva is mostly swallowed, salivary prions may reinfect tissues of the gastrointestinal tract and contribute to fecal prion shedding. Salivary prions shed into the environment provide an additional mechanism for horizontal prion transmission.

PrP(Sc) detection and infectivity in semen from scrapie-infected sheep

A scrapie-positive ewe was found in a flock that had been scrapie-free for 13 years, but housed adjacent to scrapie-positive animals, separated by a wire fence. Live animal testing of the entire flock of 24 animals revealed seven more subclinical scrapie-positive ewes. We hypothesized that they may have contracted the disease from scrapie-positive rams used for breeding 4 months prior, possibly through the semen. The genotypes of the ewe flock were highly scrapie-susceptible and the rams were infected with the 'Caine' scrapie strain having a short incubation time of 4.3-14.6 months in sheep with 136/171 VQ/VQ and AQ/VQ genotypes. PrP(Sc) accumulates in a variety of tissues in addition to the central nervous system. Although transmission of prion diseases, or transmissible spongiform encephalopathies, has been achieved via peripheral organ or tissue homogenates as well as by blood transfusion, neither infectivity nor PrP(Sc) have been found in semen from scrapie-infected animals. Using serial protein misfolding cyclic amplification followed by a surround optical fibre immunoassay, we demonstrate that semen from rams infected with a short-incubation-time scrapie strain contains prion disease-associated-seeding activity that generated PrP(Sc) in sPMCA (serial protein misfolding cyclic amplification). Injection of the ovinized transgenic mouse line TgSShpPrP with semen from scrapie-infected sheep resulted in PrP(Sc)-seeding activity in clinical and, probably as a result of the low titre, non-clinical mouse brain. These results suggest that the transmissible agent, or at least the seeding activity, for sheep scrapie is present in semen. This may be a strain-specific phenomenon.

Prion disease detection, PMCA kinetics, and IgG in urine from sheep naturally/experimentally infected with scrapie and deer with preclinical/clinical chronic wasting disease

Prion diseases, also known as transmissible spongiform encephalopathies, are fatal neurodegenerative disorders. Low levels of infectious agent and limited, infrequent success of disease transmissibility and PrP(Sc) detection have been reported with urine from experimentally infected clinical cervids and rodents. We report the detection of prion disease-associated seeding activity (PASA) in urine from naturally and orally infected sheep with clinical scrapie agent and orally infected preclinical and infected white-tailed deer with clinical chronic wasting disease (CWD). This is the first report on PASA detection of PrP(Sc) from the urine of naturally or preclinical prion-diseased ovine or cervids. Detection was achieved by using the surround optical fiber immunoassay (SOFIA) to measure the products of limited serial protein misfolding cyclic amplification (sPMCA). Conversion of PrP(C) to PrP(Sc) was not influenced by the presence of poly(A) during sPMCA or by the homogeneity of the PrP genotypes between the PrP(C) source and urine donor animals. Analysis of the sPMCA-SOFIA data resembled a linear, rather than an exponential, course. Compared to uninfected animals, there was a 2- to 4-log increase of proteinase K-sensitive, light chain immunoglobulin G (IgG) fragments in scrapie-infected sheep but not in infected CWD-infected deer. The higher-than-normal range of IgG levels found in the naturally and experimentally infected clinical scrapie-infected sheep were independent of their genotypes. Although analysis of urine samples throughout the course of infection would be necessary to determine the usefulness of altered IgG levels as a disease biomarker, detection of PrP(Sc) from PASA in urine points to its potential value for antemortem diagnosis of prion diseases.

A novel method for preclinical detection of PrPSc in blood

In this study, we demonstrate that a moderate amount of protein misfolding cyclic amplification (PMCA) coupled to a novel surround optical fibre immunoassay (SOFIA) detection scheme can be used to detect the disease-associated form of the prion protein (PrP(Sc)) in protease-untreated plasma from preclinical and clinical scrapie sheep, and white-tailed deer with chronic wasting disease, following natural and experimental infection. PrP(Sc), resulting from a conformational change of the normal (cellular) form of prion protein (PrP(C)), is considered central to neuropathogenesis and serves as the only reliable molecular marker for prion disease diagnosis. While the highest levels of PrP(Sc) are present in the central nervous system, the development of a reasonable diagnostic assay requires the use of body fluids that characteristically contain exceedingly low levels of PrP(Sc). PrP(Sc) has been detected in the blood of sick animals by means of PMCA technology. However, repeated cycling over several days, which is necessary for PMCA of blood material, has been reported to result in decreased specificity (false positives). To generate an assay for PrP(Sc) in blood that is both highly sensitive and specific, we have utilized limited serial PMCA (sPMCA) with SOFIA. We did not find any enhancement of sPMCA with the addition of polyadenylic acid nor was it necessary to match the genotypes of the PrP(C) and PrP(Sc) sources for efficient amplification.

Characterization of a US Sheep Scrapie Isolate with Short Incubation Time

Scrapie is a naturally occurring fatal neurodegenerative disease of sheep and goats. Susceptibility to the disease is partly dependent upon the genetic makeup of the host. In a previous study it was shown that sheep intracerebrally inoculated with US scrapie inoculum (No. 13–7) developed terminal disease within an average of 19 months. We have since produced an inoculum, No. x124 from pooled brains of US-origin sheep scrapie, that results in incubations nearly threefold shorter. The present study documents clinicopathologic findings and the distribution of abnormal prion proteins (PrPSc) by immunohistochemical (IHC) and Western blot (WB) techniques, in tissues of sheep inoculated with No. x124. All inoculated sheep developed clinical disease and were euthanatized within an average of 7.7 months postinoculation (MPI). Sheep that had valine/valine or alamine/valine at codon 136 of prion protein ( PRNP) gene developed the disease faster and were euthanatized at an average of 4.3 and 5.6 MPI, respectively. Also, the inoculum was able to induce disease in a short time (7 MPI) in a sheep that was relatively resistant (QR at codon 171) to scrapie. This indicates that inoculum No. x124 appears to induce scrapie in shorter time than inoculum No. 13–7, especially in sheep homozygous or heterozygous for valine at codon 136.

Serial passage of sheep scrapie inoculum in Suffolk sheep

Scrapie is a naturally occurring fatal neurodegenerative disease of sheep and goats. Susceptibility to the disease is partly dependent upon the genetic makeup of the host. In a recent study, it was shown that sheep intracerebrally inoculated with a US scrapie agent (No. 13-7) developed scrapie and survived for an average of 19 months post inoculation. In the present study, when this scrapie inoculum was further passaged for 3 successive generations, the survival time was reduced by approximately 8 months in scrapie-susceptible (QQ on prion protein gene [PRNP] at codon 171) Suffolk sheep. It is concluded that inoculum No. 13-7 appears to have been stabilized in susceptible (171 QQ) Suffolk sheep and may be considered a specific isolate of sheep scrapie agent in the USA and therefore that it can be used to evaluate other isolates of sheep scrapie in this country.

What veterinary practitioners should know about scrapie

Scrapie is the longest known and most widely spread of the transmissible spongiform encephalopathies and remains the model for much of the research regarding these diseases. Because scrapie is a reportable disease and the subject of an active eradication program in the United States, veterinary practitioners should have a basic understanding of the disease.