Submitted to: Clinical and Diagnostic Laboratory Immunology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2002
Publication Date: 9/1/2002
Citation: O'Rourke, K.I., Duncan, J.V., Logan, J.R., Anderson, A.K., Norden, D.K., Williams, E.S., Combs, B.A., Stobart, R.H., Moss, G.E., Sutton, D.L. 2002. Active surveillance for scrapie utilizing third eyelid biopsy and genetic susceptibility testing in flocks of sheep in Wyoming. Clinical and Diagnostic Laboratory Immunology. 9(5):966-971. Interpretive Summary: Scrapie is a fatal neurologic disease of sheep. The disease has a long incubation period and sheep may transmit disease before it is apparent that they are infected. A live animal test to identify infected sheep early in disease has been described. The test is based on examination of the lymphoid tissue of the third eyelid for PrP-Sc, the diagnostic marker for scrapie. In this study, the feasibility and efficacy of third eyelid testing for identification of infected flocks and individual infected animals was evaluated in a large sample of privately owned sheep. Sheep were also evaluated for potential risk of scrapie due to contact with an infected ewe and for relative genetic susceptibility to scrapie. The study demonstrated that the third eyelid sampling is feasible (80% of the sheep in the study had suitable tests). The study identified 13 infected animals of 600 sampled, well over the predicted prevalence of 0.07%. All positive animals were in the highly susceptible genetic group, although some of the sheep had no reported exposure to scrapie. The study demonstrated that targeted testing of genetically susceptible sheep is a useful addition to other monitoring and surveillance programs in a comprehensive eradication program.
Technical Abstract: 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. In 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 in 80% of the sheep tested, with a mean of 18.1 lymphoid follicles (germinal centers) per histologic section. 311 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. 10 PrPSc-positive sheep were identified through the passive surveillance program and an additional 3 PrP-Sc-positive sheep, including 2 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.