Codon 141 polymorphisms of the ovine prion protein gene affect the phenotype of classical scrapie transmitted from goats to sheep.

BACKGROUND: A study to investigate transmission of classical scrapie via goat milk was carried out in sheep: firstly, lambs were challenged orally with goat scrapie brain homogenate to confirm transmission of scrapie from goats to sheep. In the second study phase, milk from scrapie-infected goats was fed to lambs. Lambs were selected according to their prion protein gene (PRNP) genotype, which was either VRQ/VRQ or ARQ/ARQ, with or without additional polymorphisms at codon 141 (FF141, LF141 or LL141) of the ovine PRNP. This report describes the clinical, pathological and molecular phenotype of goat scrapie in those sheep that progressed to clinical end-stage. RESULTS: Ten sheep (six VRQ/VRQ and four ARQ/ARQ, of which three FF141 and one LL141) challenged with one of two scrapie brain homogenates, and six pairs of sheep (ARQ, of which five LL141 and seven LF141) fed milk from six different goats, developed clinical disease, which was characterised by a pruritic (all VRQ/VRQ and LL141 sheep) or a non-pruritic form (all LF141 and FF141 sheep). Immunohistochemical (IHC) examination revealed that the pattern of intra- and extracellular accumulation of disease-associated prion protein in the brain was also dependent on PRNP polymorphisms at codon 141, which was similar in VRQ and LL141 sheep but different from LF141 and FF141 sheep. The influence of codon 141 was also seen in discriminatory Western blot (WB), with LF141 and FF141 sheep showing a bovine spongiform encephalopathy-like profile (diminished reactivity with P4 antibody) on brain tissue. However, discriminatory WB in lymphoid tissues, and IHC pattern and profile both in lymphoid and brain tissue was consistent with classical scrapie in all sheep. CONCLUSIONS: This study provided further evidence that the clinical presentation and the pathological and molecular phenotypes of scrapie in sheep are influenced by PRNP polymorphisms, particularly at codon 141. Differences in the truncation of disease-associated prion protein between LL141 sheep and those carrying the F141 allele may be responsible for these observations.

L-BSE experimentally transmitted to sheep presents as a unique disease phenotype.

Apart from prion protein genotype, the factors determining the host range and susceptiblity for specific transmissible spongiform encephalopathy agents remain unclear. It is known that bovine atypical L-BSE can transmit to a range of species including primates and humanised transgenic mice. It is important, therefore, that there is as broad an understanding as possible of how such isolates might present in food animal species and how robust they are on inter- and intra-species transmission to inform surveillance sytems and risk assessments. This paper demonstrates that L-BSE can be intracerebrally transmitted to sheep of several genotypes, with the exception of ARR/ARR animals. Positive animals mostly present with a cataplectic form of disease characterized by collapsing episodes and reduced muscle tone. PrP accumulation is confined to the nervous system, with the exception of one animal with lymphoreticular involvement. In Western blot there was maintenance of the low molecular mass and glycoform profile associated with L-BSE, irrespective of ovine host genotype, but there was a substantially higher N-terminal antibody signal relative to the core-specific antibody, which is similar to the ratio associated with classical scrapie. The disease phenotype was maintained on experimental subpassage, but with a shortened survival time indicative of an original species barrier and subsequent adaptation. Passive surveillance approaches would be unlikely to identify such cases as TSE suspects, but current statutory active screening methods would be capable of detecting such cases and classifying them as unusual and requiring further investigation if they were to occur in the field.

Abnormalities in Brainstem Auditory Evoked Potentials in Sheep with Transmissible Spongiform Encephalopathies and Lack of a Clear Pathological Relationship.

Scrapie is transmissible spongiform encephalopathy (TSE), which causes neurological signs in sheep, but confirmatory diagnosis is usually made postmortem on examination of the brain for TSE-associated markers like vacuolar changes and disease-associated prion protein (PrP(Sc)). The objective of this study was to evaluate whether testing of brainstem auditory evoked potentials (BAEPs) at two different sound levels could aid in the clinical diagnosis of TSEs in sheep naturally or experimentally infected with different TSE strains [classical and atypical scrapie and bovine spongiform encephalopathy (BSE)] and whether any BAEP abnormalities were associated with TSE-associated markers in the auditory pathways. BAEPs were recorded from 141 clinically healthy sheep of different breeds and ages that tested negative for TSEs on postmortem tests to establish a reference range and to allow comparison with 30 sheep clinically affected or exposed to classical scrapie (CS) without disease confirmation (test group 1) and 182 clinically affected sheep with disease confirmation (test group 2). Abnormal BAEPs were found in 7 sheep (23%) of group 1 and 42 sheep (23%) of group 2. The proportion of sheep with abnormalities did not appear to be influenced by TSE strain or PrP(Sc) gene polymorphisms. When the magnitude of TSE-associated markers in the auditory pathways was compared between a subset of 12 sheep with and 12 sheep without BAEP abnormalities in group 2, no significant differences in the total PrP(Sc) or vacuolation scores in the auditory pathways could be found. However, the data suggested that there was a difference in the PrP(Sc) scores depending on the TSE strain because PrP(Sc) scores were significantly higher in sheep with BAEP abnormalities infected with classical and L-type BSE, but not with CS. The results indicated that BAEPs may be abnormal in sheep infected with TSEs but the test is not specific for TSEs and that neither vacuolation nor PrP(Sc) accumulation appears to be responsible for the clinical abnormalities.

Further characterisation of transmissible spongiform encephalopathy phenotypes after inoculation of cattle with two temporally separated sources of sheep scrapie from Great Britain.

BACKGROUND: The infectious agent responsible for the bovine spongiform encephalopathy (BSE) epidemic in Great Britain is a transmissible spongiform encephalopathy (TSE) strain with uniform properties but the origin of this strain remains unknown. Based on the hypothesis that classical BSE may have been caused by a TSE strain present in sheep, cattle were inoculated intracerebrally with two different pools of brains from scrapie-affected sheep sourced prior to and during the BSE epidemic to investigate resulting disease phenotypes and characterise their causal agents by transmission to rodents. RESULTS: As reported in 2006, intracerebral inoculation of cattle with pre-1975 and post-1990 scrapie brain pools produced two distinct disease phenotypes, which were unlike classical BSE. Subsequent to that report none of the remaining cattle, culled at 10 years post inoculation, developed a TSE. Retrospective Western immunoblot examination of the brains from TSE cases inoculated with the pre-1975 scrapie pool revealed a molecular profile similar to L-type BSE. The inoculation of transgenic mice expressing the bovine, ovine, porcine, murine or human prion protein gene and bank voles with brains from scrapie-affected cattle did not detect classical or atypical BSE strains but identified two previously characterised scrapie strains of sheep. CONCLUSIONS: Characterisation of the causal agents of disease resulting from exposure of cattle to naturally occurring scrapie agents sourced in Great Britain did not reveal evidence of classical or atypical BSE, but did identify two distinct previously recognised strains of scrapie. Although scrapie was still recognizable upon cattle passage there were irreconcilable discrepancies between the results of biological strain typing approaches and molecular profiling methods, suggesting that the latter may not be appropriate for the identification and differentiation of atypical, particularly L-type, BSE agents from cattle experimentally infected with a potential mixture of classical scrapie strains from sheep sources.

Phenotype shift from atypical scrapie to CH1641 following experimental transmission in sheep.

The interactions of host and infecting strain in ovine transmissible spongiform encephalopathies are known to be complex, and have a profound effect on the resulting phenotype of disease. In contrast to classical scrapie, the pathology in naturally-occurring cases of atypical scrapie appears more consistent, regardless of genotype, and is preserved on transmission within sheep homologous for the prion protein (PRNP) gene. However, the stability of transmissible spongiform encephalopathy phenotypes on passage across and within species is not absolute, and there are reports in the literature where experimental transmissions of particular isolates have resulted in a phenotype consistent with a different strain. In this study, intracerebral inoculation of atypical scrapie between two genotypes both associated with susceptibility to atypical forms of disease resulted in one sheep displaying an altered phenotype with clinical, pathological, biochemical and murine bioassay characteristics all consistent with the classical scrapie strain CH1641, and distinct from the atypical scrapie donor, while the second sheep did not succumb to challenge. One of two sheep orally challenged with the same inoculum developed atypical scrapie indistinguishable from the donor. This study adds to the range of transmissible spongiform encephalopathy phenotype changes that have been reported following various different experimental donor-recipient combinations. While these circumstances may not arise through natural exposure to disease in the field, there is the potential for iatrogenic exposure should current disease surveillance and feed controls be relaxed. Future sheep to sheep transmission of atypical scrapie might lead to instances of disease with an alternative phenotype and onward transmission potential which may have adverse implications for both public health and animal disease control policies.

The pathological and molecular but not clinical phenotypes are maintained after second passage of experimental atypical bovine spongiform encephalopathy in cattle.

BACKGROUND: Atypical bovine spongiform encephalopathies (BSEs), classified as H-type and L-type BSE based on the Western immunoblot profiles, are naturally occurring diseases in cattle, which are phenotypically different to classical BSE. Transmission studies in cattle using the intracerebral route resulted in disease where the phenotypes were maintained irrespective of BSE type but clinically affected cattle with a shorter survival time displayed a nervous form whereas cattle with a longer survival time displayed a dull form. A second transmission study is reported here where four cattle were intracerebrally inoculated with brain tissue from experimentally infected cattle presenting with either the nervous or dull form of H- or L-type BSE to determine whether the phenotype is maintained. RESULTS: The four inoculated cattle were culled at 16.5-19.5 months post inoculation after presenting with difficulty getting up, a positive scratch response (all) and dullness (three cattle), which was not observed in two non-inoculated control cattle, each housed with either group of inoculated cattle. Only the inoculated cattle had detectable prion protein in the brain based on immunohistochemical examination, and the Western immunoblot profile was consistent with the H-type or L-type BSE of the respective donor cattle. CONCLUSIONS: Second passage of H-type and L-type BSE in cattle produced a TSE where the majority of cattle displayed the dull form regardless of clinical disease form of the donor cattle. The pathological and molecular phenotypes of H- and L-type BSE were maintained.

Experimental H-type and L-type bovine spongiform encephalopathy in cattle: observation of two clinical syndromes and diagnostic challenges.

BACKGROUND: The majority of atypical bovine spongiform encephalopathy (BSE) cases so far identified worldwide have been detected by active surveillance. Consequently the volume and quality of material available for detailed characterisation is very limiting. Here we report on a small transmission study of both atypical forms, H- and L-type BSE, in cattle to provide tissue for test evaluation and research, and to generate clinical, molecular and pathological data in a standardised way to enable more robust comparison of the two variants with particular reference to those aspects most relevant to case ascertainment and confirmatory diagnosis within existing regulated surveillance programmes. RESULTS: Two groups of four cattle, intracerebrally inoculated with L-type or H-type BSE, all presented with a nervous disease form with some similarities to classical BSE, which progressed to a more dull form in one animal from each group. Difficulty rising was a consistent feature of both disease forms and not seen in two BSE-free, non-inoculated cattle that served as controls. The pathology and molecular characteristics were distinct from classical BSE, and broadly consistent with published data, but with some variation in the pathological characteristics. Both atypical BSE types were readily detectable as BSE by current confirmatory methods using the medulla brain region at the obex, but making a clear diagnostic distinction between the forms was not consistently straightforward in this brain region. Cerebellum proved a more reliable sample for discrimination when using immunohistochemistry. CONCLUSIONS: The prominent feature of difficulty rising in atypical BSE cases may explain the detection of naturally occurring cases in emergency slaughter cattle and fallen stock. Current confirmatory diagnostic methods are effective for the detection of such atypical cases, but consistently and correctly identifying the variant forms may require modifications to the sampling regimes and methods that are currently in use.

Studies of the transmissibility of the agent of bovine spongiform encephalopathy to the domestic chicken.

BACKGROUND: Transmission of the prion disease bovine spongiform encephalopathy (BSE) occurred accidentally to cattle and several other mammalian species via feed supplemented with meat and bone meal contaminated with infected bovine tissue. Prior to United Kingdom controls in 1996 on the feeding of mammalian meat and bone meal to farmed animals, the domestic chicken was potentially exposed to feed contaminated with the causal agent of BSE. Although confirmed prion diseases are unrecorded in avian species a study was undertaken to transmit BSE to the domestic chicken by parenteral and oral inoculations. Transmissibility was assessed by clinical monitoring, histopathological examinations, detection of a putative disease form of an avian prion protein (PrP) in recipient tissues and by mouse bioassay of tissues. Occurrence of a progressive neurological syndrome in the primary transmission study was investigated by sub-passage experiments. RESULTS: No clinical, pathological or bioassay evidence of transmission of BSE to the chicken was obtained in the primary or sub-passage experiments. Survival data showed no significant differences between control and treatment groups. Neurological signs observed, not previously described in the domestic chicken, were not associated with significant pathology. The diagnostic techniques applied failed to detect a disease associated form of PrP. CONCLUSION: Important from a risk assessment perspective, the present study has established that the domestic chicken does not develop a prion disease after large parenteral exposures to the BSE agent or after oral exposures equivalent to previous exposures via commercial diets. Future investigations into the potential susceptibility of avian species to mammalian prion diseases require species-specific immunochemical techniques and more refined experimental models.

Experimental oral transmission of atypical scrapie to sheep.

To investigate the possibility of oral transmission of atypical scrapie in sheep and determine the distribution of infectivity in the animals' peripheral tissues, we challenged neonatal lambs orally with atypical scrapie; they were then killed at 12 or 24 months. Screening test results were negative for disease-specific prion protein in all but 2 recipients; they had positive results for examination of brain, but negative for peripheral tissues. Infectivity of brain, distal ileum, and spleen from all animals was assessed in mouse bioassays; positive results were obtained from tissues that had negative results on screening. These findings demonstrate that atypical scrapie can be transmitted orally and indicate that it has the potential for natural transmission and iatrogenic spread through animal feed. Detection of infectivity in tissues negative by current surveillance methods indicates that diagnostic sensitivity is suboptimal for atypical scrapie, and potentially infectious material may be able to pass into the human food chain.

Detection and localisation of PrP(Sc) in the liver of sheep infected with scrapie and bovine spongiform encephalopathy.

Prions are largely contained within the nervous and lymphoid tissue of transmissible spongiform encephalopathy (TSE) infected animals. However, following advances in diagnostic sensitivity, PrP(Sc), a marker for prion disease, can now be located in a wide range of viscera and body fluids including muscle, saliva, blood, urine and milk, raising concerns that exposure to these materials could contribute to the spread of disease in humans and animals. Previously we demonstrated low levels of infectivity in the liver of sheep experimentally challenged with bovine spongiform encephalopathy. In this study we show that PrP(Sc) accumulated in the liver of 89% of sheep naturally infected with scrapie and 100% of sheep challenged with BSE, at both clinical and preclinical stages of the disease. PrP(Sc) was demonstrated in the absence of obvious inflammatory foci and was restricted to isolated resident cells, most likely Kupffer cells.

Atypical scrapie/Nor98 in a sheep from New Zealand.

In a consignment of sheep brains from New Zealand, to be used in Europe as negative control material in scrapie rapid screening test evaluations, brain samples from 1 sheep (no. 1512) gave the following initially confusing results in various screening tests: the brainstem repeatedly produced negative results in 2 very similar screening kits (enzyme-linked immunosorbent assay [ELISA]-1, ELISA-2), a macerate made from brainstem and cerebellum returned a clearly positive result in ELISA-2, and the macerate and a brainstem sample gave negative results in a third screening test (ELISA-3). In subsequent testing, cerebellum tissue alone tested strongly positive in ELISA-1 and produced a banding pattern very similar to atypical scrapie/Nor98 in a confirmatory Western blot (WB). The macerate showed weak staining in the confirmatory WB but presented a staining pattern identical to atypical scrapie/Nor98 in the scrapie-associated fibril WB. The latter test confirmed conclusively the first case of atypical scrapie/Nor98 in a sheep from New Zealand. Other parts of the brain either tested negative or very weak positive in ELISA-2 and in WBs, or tested with negative results by histopathology and immunohistochemistry. It appears that sheep no. 1512 is a case of atypical scrapie/Nor98 in which the abnormal prion protein was detected mainly in the cerebellum. This case emphasizes the need to retain brainstem, and cerebral and cerebellar tissues, as frozen and fixed materials, for conclusive confirmatory testing. Furthermore, consideration should be given to which screening method to use.

The natural atypical scrapie phenotype is preserved on experimental transmission and sub-passage in PRNP homologous sheep.

BACKGROUND: Atypical scrapie was first identified in Norwegian sheep in 1998 and has subsequently been identified in many countries. Retrospective studies have identified cases predating the initial identification of this form of scrapie, and epidemiological studies have indicated that it does not conform to the behaviour of an infectious disease, giving rise to the hypothesis that it represents spontaneous disease.However, atypical scrapie isolates have been shown to be infectious experimentally, through intracerebral inoculation in transgenic mice and sheep. The first successful challenge of a sheep with 'field' atypical scrapie from an homologous donor sheep was reported in 2007. RESULTS: This study demonstrates that atypical scrapie has distinct clinical, pathological and biochemical characteristics which are maintained on transmission and sub-passage, and which are distinct from other strains of transmissible spongiform encephalopathies in the same host genotype. CONCLUSIONS: Atypical scrapie is consistently transmissible within AHQ homozygous sheep, and the disease phenotype is preserved on sub-passage.

Transmissibility studies of vacuolar changes in the rostral colliculus of pigs.

BACKGROUND: Histopathological examinations of brains from healthy pigs have revealed localised vacuolar changes, predominantly in the rostral colliculus, that are similar to the neuropil vacuolation featured in the transmissible spongiform encephalopathies and have been described in pigs challenged parenterally with the agent causing bovine spongiform encephalopathy (BSE). Feedstuff containing BSE-contaminated meat and bone meal (MBM) may have been fed to pigs prior to the ban of mammalian MBM in feed of farmed livestock in the United Kingdom in 1996, but there is no evidence of the natural occurrence of a transmissible spongiform encephalopathy (TSE) in the domestic pig. Furthermore, experimental transmission of BSE to pigs by the oral route has been unsuccessful. A study was conducted to investigate whether the localised vacuolar changes in the porcine brain were associated with a transmissible aetiology and therefore biologically significant. Two groups of ten pigs were inoculated parenterally with vacuolated rostral colliculus from healthy pigs either born before 1996 or born after 1996. Controls included ten pigs similarly inoculated with rostral colliculus from New Zealand-derived pigs and nine pigs inoculated with a bovine BSE brain homogenate. RESULTS: None of the pigs inoculated with rostral colliculus developed a TSE-like neurological disease up to five years post inoculation when the study was terminated, and disease-associated prion protein, PrPd, was not detected in the brains of these pigs. By contrast, eight of nine BSE-inoculated pigs developed neurological signs, two of which had detectable PrPd by postmortem tests. No significant histopathological changes were detected to account for the clinical signs in the PrPd-negative, BSE-inoculated pigs. CONCLUSION: The findings in this study suggest that vacuolation in the porcine rostral colliculus is not caused by a transmissible agent and is probably a clinically insignificant change. The presence of neurological signs in pigs inoculated with BSE without detectable PrPd raises the possibility that the BSE agent may produce a prion disease in pigs that remains undetected by the current postmortem tests.

Immunohistochemical and biochemical characteristics of BSE and CWD in experimentally infected European red deer (Cervus elaphus elaphus).

BACKGROUND: The cause of the bovine spongiform encephalopathy (BSE) epidemic in the United Kingdom (UK) was the inclusion of contaminated meat and bone meal in the protein rations fed to cattle. Those rations were not restricted to cattle but were also fed to other livestock including farmed and free living deer. Although there are no reported cases to date of natural BSE in European deer, BSE has been shown to be naturally or experimentally transmissible to a wide range of different ungulate species. Moreover, several species of North America's cervids are highly susceptible to chronic wasting disease (CWD), a transmissible spongiform encephalopathy (TSE) that has become endemic. Should BSE infection have been introduced into the UK deer population, the CWD precedent could suggest that there is a danger for spread and maintenance of the disease in both free living and captive UK deer populations. This study compares the immunohistochemical and biochemical characteristics of BSE and CWD in experimentally-infected European red deer (Cervus elpahus elaphus). RESULTS: After intracerebral or alimentary challenge, BSE in red deer more closely resembled natural infection in cattle rather than experimental BSE in small ruminants, due to the lack of accumulation of abnormal PrP in lymphoid tissues. In this respect it was different from CWD, and although the neuropathological features of both diseases were similar, BSE could be clearly differentiated from CWD by immunohistochemical and Western blotting methods currently in routine use. CONCLUSION: Red deer are susceptible to both BSE and CWD infection, but the resulting disease phenotypes are distinct and clearly distinguishable.

Atypical scrapie in sheep from a UK research flock which is free from classical scrapie.

BACKGROUND: In the wake of the epidemic of bovine spongiform encephalopathy the British government established a flock of sheep from which scrapie-free animals are supplied to laboratories for research. Three breeds of sheep carrying a variety of different genotypes associated with scrapie susceptibility/resistance were imported in 1998 and 2001 from New Zealand, a country regarded as free from scrapie. They are kept in a purpose-built Sheep Unit under strict disease security and are monitored clinically and post mortem for evidence of scrapie. It is emphasised that atypical scrapie, as distinct from classical scrapie, has been recognised only relatively recently and differs from classical scrapie in its clinical, neuropathological and biochemical features. Most cases are detected in apparently healthy sheep by post mortem examination. RESULTS: The occurrence of atypical scrapie in three sheep in (or derived from) the Sheep Unit is reported. Significant features of the affected sheep included their relatively high ages (6 y 1 mo, 7 y 9 mo, 9 y 7 mo respectively), their breed (all Cheviots) and their similar PRNP genotypes (AFRQ/AFRQ, AFRQ/ALRQ, and AFRQ/AFRQ, respectively). Two of the three sheep showed no clinical signs prior to death but all were confirmed as having atypical scrapie by immunohistochemistry and Western immunoblotting. Results of epidemiological investigations are presented and possible aetiologies of the cases are discussed. CONCLUSION: By process of exclusion, a likely explanation for the three cases of atypical scrapie is that they arose spontaneously and were not infected from an exterior source. If correct, this raises challenging issues for countries which are currently regarded as free from scrapie. It would mean that atypical scrapie is liable to occur in flocks worldwide, especially in older sheep of susceptible genotypes. To state confidently that both the classical and atypical forms of scrapie are absent from a population it is necessary for active surveillance to have taken place.

Pruritus is a common feature in sheep infected with the BSE agent.

BACKGROUND: The variability in the clinical or pathological presentation of transmissible spongiform encephalopathies (TSEs) in sheep, such as scrapie and bovine spongiform encephalopathy (BSE), has been attributed to prion protein genotype, strain, breed, clinical duration, dose, route and type of inoculum and the age at infection. The study aimed to describe the clinical signs in sheep infected with the BSE agent throughout its clinical course to determine whether the clinical signs were as variable as described for classical scrapie in sheep. The clinical signs were compared to BSE-negative sheep to assess if disease-specific clinical markers exist. RESULTS: Forty-seven (34%) of 139 sheep, which comprised 123 challenged sheep and 16 undosed controls, were positive for BSE. Affected sheep belonged to five different breeds and three different genotypes (ARQ/ARQ, VRQ/VRQ and AHQ/AHQ). None of the controls or BSE exposed sheep with ARR alleles were positive. Pruritus was present in 41 (87%) BSE positive sheep; the remaining six were judged to be pre-clinically infected. Testing of the response to scratching along the dorsum of a sheep proved to be a good indicator of clinical disease with a test sensitivity of 85% and specificity of 98% and usually coincided with weight loss. Clinical signs that were displayed significantly earlier in BSE positive cases compared to negative cases were behavioural changes, pruritic behaviour, a positive scratch test, alopecia, skin lesions, teeth grinding, tremor, ataxia, loss of weight and loss of body condition. The frequency and severity of each specific clinical sign usually increased with the progression of disease over a period of 16-20 weeks. CONCLUSION: Our results suggest that BSE in sheep presents with relatively uniform clinical signs, with pruritus of increased severity and abnormalities in behaviour or movement as the disease progressed. Based on the studied sheep, these clinical features appear to be independent of breed, affected genotype, dose, route of inoculation and whether BSE was passed into sheep from cattle or from other sheep, suggesting that the clinical phenotype of BSE is influenced by the TSE strain more than by other factors. The clinical phenotype of BSE in the genotypes and breed studied was indistinguishable from that described for classical scrapie cases.

Experimental transmission of chronic wasting disease agent from mule deer to cattle by the intracerebral route.

This communication reports final observations on experimental transmission of chronic wasting disease (CWD) from mule deer to cattle by the intracerebral route. Thirteen calves were inoculated intracerebrally with brain suspension from mule deer naturally affected with CWD. Three other calves were kept as uninoculated controls. The experiment was terminated 6 years after inoculation. During that time, abnormal prion protein (PrP(res)) was demonstrated in the central nervous system (CNS) of 5 cattle by both immunohistochemistry and Western blot. However, microscopic lesions suggestive of spongiform encephalopathy (SE) in the brains of these PrP(res)-positive animals were subtle in 3 cases and absent in 2 cases. Analysis of the gene encoding bovine PRNP revealed homozygosity for alleles encoding 6 octapeptide repeats, serine (S) at codon 46, and S at codon 146 in all samples. Findings of this study show that although PrP(res) amplification occurred after direct inoculation into the brain, none of the affected animals had classic histopathologic lesions of SE. Furthermore, only 38% of the inoculated cattle demonstrated amplification of PrP(res). Although intracerebral inoculation is an unnatural route of exposure, this experiment shows that CWD transmission in cattle could have long incubation periods (up to 5 years). This finding suggests that oral exposure of cattle to CWD agent, a more natural potential route of exposure, would require not only a much larger dose of inoculum but also may not result in amplification of PrP(res) within CNS tissues during the normal lifespan of cattle.

Transmission of sheep scrapie to elk (Cervus elaphus nelsoni) by intracerebral inoculation: final outcome of the experiment.

This is a final report of an experimental transmission of sheep scrapie agent by intracerebral inoculation to Rocky Mountain elk (Cervus elaphus nelsoni). It documents results obtained in experimental (n = 6) and control (n = 2) elk. During the first 2 years postinoculation (PI), 3 animals died or were euthanized because of infection or injuries other than spongiform encephalopathy (SE). In years 3 and 4 PI, 3 other inoculated elk died after brief terminal neurological episodes. Necropsy of these animals revealed moderate weight loss but no other gross lesions. Microscopically, characteristic lesions of SE were seen throughout the brain and spinal cord, and the tissue was positive for proteinase K-resistant prion protein (PrPres) by immunohistochemistry (IHC) and by Western blot. Scrapie-associated fibrils (SAF) were observed by negative-stain electron microscopy in the brain of elk with neurologic signs. PrPres and SAF were not detected in the 3 inoculated elk necropsied during the first 2 years or in the 2 control animals. Retrospective analysis of the gene-encoding cervid PrP revealed a polymorphism at codon 132. The elk with SE were either homozygous (MM) or heterozygous (LM). These findings confirm that intracerebral inoculation of sheep scrapie agent results in SE with accumulations of PrPres in the central nervous system of elk. Based on morphologic and IHC findings, the experimentally induced SE cannot be distinguished from chronic wasting disease of elk with currently available diagnostic techniques.

Transmission of transmissible mink encephalopathy to raccoons (Procyon lotor) by intracerebral inoculation.

To determine the transmissibility of transmissible mink encephalopathy (TME) agent to raccoons and to provide information about clinical course, lesions, and suitability of currently used diagnostic procedures for detection of transmissible spongiform encephalopathies (TSEs) in raccoons, 4 raccoon kits were inoculated intracerebrally with a brain suspension from mink experimentally infected with TME. One uninoculated raccoon kit served as a control. All 4 animals in the TME-inoculated group showed clinical signs of neurologic disorder and were euthanized between 21 and 23 weeks postinoculation (PI). Necropsy examinations revealed no gross lesions. Spongiform encephalopathy was observed by light microscopy, and the presence of protease-resistant prion protein (PrPres) was detected by immunohistochemistry and Western blot techniques. Scrapie-associated fibrils were observed by negative-stain electron microscopy in the brains of 3 of the 4 inoculated raccoons. These findings confirm that TME is experimentally transmissible to raccoons and that diagnostic techniques currently used for TSE in livestock detect prion protein in raccoon tissue. According to previously published data, the incubation period of sheep scrapie in raccoons is 2 years, whereas chronic wasting disease (CWD) had not shown transmission after 3 years of observation. Because incubation periods for the 3 US TSEs (scrapie, TME, and CWD) in raccoons appear to be markedly different, it may be possible to use raccoons for differentiating unknown TSE agents. Retrospective genotyping of raccoons using frozen spleens showed that the raccoon PrP gene is identical to the mink gene at codons 179 and 224. Further studies, such as the incubation periods of bovine spongiform encephalopathy and other isolates of scrapie, CWD, and TME in raccoons, are needed before the model can be further characterized for differentiation of TSE agents.