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SHEEP scrapie is the prototype of a heterogeneous group of transmissible spongiform encephalopathies (TSES) which occur in humans, cattle, cats, mink and cervids. TSEs are characterised by the deposition of prion proteins (Prp-Scrapie or PrpSc) in the central nervous system of affected individuals (Prusiner 1982). PrPSc and infectivity have also been detected in the lymphoreticular system of sheep well before symptoms occur (Hadlow and others 1982, Race and others 1992). Detection of PrpSc in lymphoid tissue, such as tonsils or peripheral lymph nodes, has been proposed as a preclinical diagnostic test for scrapie (Ikegami and others 1991, Schreuder and others 1996). This short communication reports the use of a non-invasive diagnostic assay based on biopsy of lymphoid nodules in the nictitating membrane of sheep. The nictitating membrane or third eyelid (palpebra tertia) of ruminant animals consists of a cartilaginous sheet with superficial lymphoid follicles and a seromucinous secretory gland beneath the conjunctiva of the bulbar surface (Slatter and Mills 1981). The lymphoid nodules are easily biopsied under local anaesthesia. Sheep were pretreated by topical administration of proparacaine hydrochloride (Ophthetic; Allergan) and the nictitating membrane was everted with disposable 1 x 2-toothed, 5½-inch forceps (Sklar 78055-1). Typically two clusters of lymphoid tissue were visualised above the more pale glandular tissue. A biopsy of one cluster of lymphoid tissue was collected using disposable Metzenbaum scissors (Sklar 30455) and a second set of forceps. Biopsied tissue was fixed in formalin and paraffin embedded using conventional methods. Sections typically contained four to seven lymphoid follicles.