Scrapie is a TSE of sheep and goats that is characterized by slowly progressive neurologic dysfunction and loss of body condition. Following the recognition of the link between bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease in humans,1–3 public sensitivity to TSEs has resulted in regulations and trade restrictions that have negatively impacted the sheep and goat industries. The NSEP was established to eradicate scrapie from sheep and goat populations in the United States.4
Scrapie surveillance activities in the United States include regulatory scrapie surveillance of sheep and goats at slaughter, a national scrapie flock certification program,5 voluntary testing of lymphoid tissue from third eyelids of susceptible animals, and postmortem testing of sheep and goats that have clinical signs of scrapie or neurologic signs with a negative test result for rabies. When an animal with a positive test result for scrapie is identified, field investigations are undertaken by the NSEP to identify epidemiologically related flocks. The flock in which an infected animal gave birth or was born is restricted until actions are completed to eliminate scrapie from that flock. In other epidemiologically associated flocks, the NSEP conducts testing to determine the scrapie status of animals and, if infection is detected, to direct interventions aimed at eliminating scrapie from infected flocks and identifying any additional potentially exposed flocks. Testing efforts are directed primarily toward high-risk subpopulations believed to be at risk of infection because of expression of clinical signs consistent with scrapie or a combination of genetic susceptibility and exposure to the disease.
Testing of high-risk sheep typically involves postmortem testing of specimens of obex, tonsil, and retropharyngeal lymph node by means of IHC to identify the accumulation of PrPSc. This diagnostic approach is capable of identifying infected sheep prior to the onset of clinical signs6 and is considered to have near perfect diagnostic sensitivity when applied to sheep with clinical disease.7 The NSEP has also used IHC testing for PrPSc in biopsy specimens of lymphoid tissue from third eyelids to detect subclinically infected sheep without indemnification. However, the quantity of lymphoid follicles in thirdeyelid specimens is limited, and a large proportion of specimens contain insufficient lymphoid tissue for evaluation (ie, < 6 follicles).8 Furthermore, IHC testing of third-eyelid biopsy specimens for PrPSc is fairly insensitive (72.5%), compared with testing of postmortem specimens of obex.9 A more reliable test for live sheep is needed.
Sheep experimentally or naturally infected with scrapie agent accumulate PrPSc within RAMALT at a point during the infectious process in which the agent accumulates in other lymphoid tissues.10, 11 In contrast to the third eyelid, rectal mucosa usually contains plentiful lymphoid tissue and is conveniently accessible for biopsy in live sheep. The purposes of the study reported here were to determine whether performing biopsy of RAMALT in sheep on farm settings is a suitable strategy for scrapie testing and to estimate the sensitivity of an IHC test for PrPSc in biopsy specimens of RAMALT relative to that for the same test applied to specimens of obex, tonsil, and retropharyngeal lymph node. A secondary objective was to compare the performance of the IHC test for PrPSc in biopsy specimens of RAMALT versus third-eyelid tissue.
National Scrapie Eradication Program
Prion protein gene
Scrapie-associated, protease-resistant prion protein
Rectoanal mucosa–associated lymphoid tissue
Transmissible spongiform encephalopathy
Target Retrieval Solution, Dako Corp, Carpinteria, Calif.
Decloaking Chamber, BioCare Medical, Walnut Creek, Calif.
BioCare Medical, Walnut Creek, Calif.
NexES, Ventana Medical Systems, Tucson, Ariz.
Anti-Prion 99, Ventana Medical Systems, Tucson, Ariz.
ultraView Universal Alkaline Phosphatase Red Detection Kit, Ventana Medical Systems, Tucson, Ariz.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
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USDA, APHIS. Voluntary scrapie flock certification program standards. APHIS 91-55-091. Washington, DC: USDA, 2007. Available at: www.aphis.usda.gov/animal_health/animal_diseases/scrapie/downloads/sfcp.pdf. Sep 12, 2008.
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