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

BACKGROUND: Magnetic resonance imaging has been used in the diagnosis of human prion diseases such as sCJD and vCJD, but patients are scanned only when clinical signs appear, often at the late stage of disease. This study attempts to answer the questions "Could MRI detect prion diseases before clinical symptoms appear?, and if so, with what confidence?" METHODS: Scrapie, the prion disease of sheep, was chosen for the study because sheep can fit into a human sized MRI scanner (and there were no large animal MRI scanners at the time of this study), and because the USDA had, at the time of the study, a sizeable sample of scrapie exposed sheep, which we were able to use for this purpose. 111 genetically susceptible sheep that were naturally exposed to scrapie were used in this study. RESULTS: Our MRI findings revealed no clear, consistent hyperintense or hypointense signal changes in the brain on either clinically affected or asymptomatic positive animals on any sequence. However, in all 37 PrPSc positive sheep (28 asymptomatic and 9 symptomatic), there was a greater ventricle to cerebrum area ratio on MRI compared to 74 PrPSc negative sheep from the scrapie exposed flock and 6 control sheep from certified scrapie free flocks as defined by immunohistochemistry (IHC). CONCLUSIONS: Our findings indicate that MRI imaging can detect diffuse cerebral atrophy in asymptomatic and symptomatic sheep infected with scrapie. Nine of these 37 positive sheep, including 2 one-year old animals, were PrPSc positive only in lymph tissues but PrPSc negative in the brain. This suggests either 1) that the cerebral atrophy/neuronal loss is not directly related to the accumulation of PrPSc within the brain or 2) that the amount of PrPSc in the brain is below the detectable limits of the utilized immunohistochemistry assay. The significance of these findings remains to be confirmed in human subjects with CJD.

Evaluation of immunohistochemical detection of prion protein in rectoanal mucosa-associated lymphoid tissue for diagnosis of scrapie in sheep.

OBJECTIVE: To determine the suitability and estimate the sensitivity of an immunohistochemical (IHC) test for disease-associated prion protein (PrP(Sc)) in biopsy specimens of rectoanal mucosa-associated lymphoid tissue (RAMALT) for diagnosis of scrapie in sheep. ANIMALS: 762 sheep at high risk for having scrapie and indemnified by the National Scrapie Eradication Program. PROCEDURES: The IHC test for PrP(Sc) was applied to 2 RAMALT and 2 third-eyelid biopsy specimens and a postmortem RAMALT specimen from each sheep. Results were compared with those of a reference test in which results for tissues from obex and retropharyngeal lymph nodes, tonsil, or both were considered in parallel. RESULTS: The reference test identified 139 sheep as having scrapie. Biopsy-related complications occurred in 3 sheep. Sensitivity of the IHC test in RAMALT ranged from 85.3% to 89.4%, depending on the anatomic location from which RAMALT was obtained. Results for the test applied to 1 RAMALT specimen were similar to results interpreted in parallel for 2 third-eyelid specimens (sensitivity, 87.0%). The proportion of inconclusive test results attributable to insufficient lymphoid follicles in biopsy specimens was lower when considering results for 2 RAMALT specimens in parallel (10.1%) than when considering results for 2 third-eyelid specimens in parallel (23.7%). Specimens of RAMALT that were inappropriately collected from an area caudal to the rectoanal interface yielded a high proportion of inconclusive results (33.3% to 50.0%). CONCLUSIONS AND CLINICAL RELEVANCE: The IHC test for PrP(Sc) in RAMALT was an effective means of detecting subclinical scrapie in live, high-risk sheep.

Resistance of domestic cats to a US sheep scrapie agent by intracerebral route.

Feline spongiform encephalopathy (FSE) is thought to have resulted from consumption of food contaminated with bovine spongiform encephalopathy and the latter is believed to result from the consumption of food contaminated with scrapie. However, no direct experimental documentation exists to indicate that the scrapie agent is capable of amplifying in cats, and, therefore, crossing the species barrier. During 1979, 6 cats ranging in age from 3.5 to 18 months were intracerebrally inoculated with sheep scrapie (inoculum G-639-PP) and were observed for an extended period. Inoculated cats did not develop neurologic disease, and microscopic lesions of spongiform encephalopathy were not evident. Immunohistochemistry and Western blot techniques failed to detect the abnormal form of prion protein (PrP(res)). These results indicate that the sheep scrapie agent (G-639-PP) used in this study was not capable of amplifying in cats and therefore was unable to cross the species barrier to produce FSE.

Polymorphic distribution of the ovine prion protein (PrP) gene in scrapie-infected sheep flocks in which embryo transfer was used to circumvent the transmissions of scrapie.

The genetic sequence of the ovine prion protein (PrP) gene between codons 102 and 175 with emphasis on ovine PrP gene codons 136 and 171 was determined, and the polymorphic distribution of the ovine PrP gene in the scrapie-exposed Suffolk embryo donors and offspring from these donors that were transferred to scrapie-free recipient ewes was investigated in this study. The most common genotype was AA(136)QQ(171) (70% and 63% in the donor and offspring flocks, respectively), which is considered a high risk genotype in US Suffolk sheep. Although embryos were collected from scrapie-positive donors and many embryos had the high risk genotype, no scrapie occurred in the resulting offspring. Based upon the results of this study, we conclude that vertical transmission of scrapie can be circumvented using embryo transfer procedures even when the offspring have the high risk genotype.

Active surveillance for scrapie by third eyelid biopsy and genetic susceptibility testing of flocks of sheep in Wyoming.

Control of scrapie, an ovine transmissible spongiform encephalopathy or prion disorder, has been hampered by the lack of conventional antemortem diagnostic tests. Currently, scrapie is diagnosed by postmortem examination of the brain and lymphoid tissues for PrP(Sc), the protein marker for this group of disorders. For live, asymptomatic sheep, diagnosis using tonsil or third-eyelid lymphoid tissue biopsy and PrP(Sc) assay has been described. To evaluate the feasibility and efficacy of third-eyelid testing for identification of infected flocks and individual infected sheep, 690 sheep from 22 flocks were sampled by third-eyelid lymphoid tissue biopsy and immunohistochemistry. Sheep were further evaluated for relative genetic susceptibility and potential contact exposure to scrapie. Third-eyelid testing yielded suitable samples for 80% of the sheep tested, with a mean of 18.1 lymphoid follicles (germinal centers) per histologic section. Three hundred eleven of the sheep were sampled through passive surveillance programs, in which only sheep with potential contact with an infected sheep at a lambing event were tested, regardless of their scrapie susceptibility genotype. In addition, 141 genetically susceptible sheep with no record of contact with an infected animal at a lambing event were sampled through a targeted active surveillance program. Ten PrP(Sc)-positive sheep were identified through the passive surveillance program, and an additional three PrP(Sc)-positive sheep, including two from flocks with no history of scrapie, were identified through the active surveillance program. All PrP(Sc)-positive sheep had the highly susceptible PrP genotype. Third-eyelid testing is a useful adjunct to flock monitoring programs, slaughter surveillance, and mandatory disease reporting in a comprehensive scrapie eradication and research program.