Histamine as an aid to biopsy of third eyelid lymphoid tissue in sheep

TRANSMISSIBLE spongiform encephalopathies, of which ovine scrapie is a prototype, are neurodegenerative disorders characterised by the accumulation of an abnormal isoform (PrP-scrapie or PrPSc) of a normal cellular protein (PrPC) (Prusiner 1982). Characteristics of the disease include long incubation periods, and a progressive and chronic clinical course resulting in death (Fraser 1976). PrPsc is detectable during the preclinical stages of scrapie in peripheral lymph nodes (Ikegami and others 1991, Muramatsu and others 1993), in lymphoid follicles of the tonsils (Schreuder and others 1996) and in lymphoid follicles of the nictitating membrane or third eyelid (O\u27Rourke and others 1998b). Diagnosis of scrapie in sheep by third eyelid lymphoid tissue biopsy and PrPsc assay, as described by O\u27Rourke and others (1998b, 2000), is a practical test for live sheep and is useful for the preclinical diagnosis of scrapie in surveillance programmes (O\u27Rourke and others 2002). Although the lymphoid tissue of the third eyelid is accessible and can be anaesthetised locally, one obstacle to using this diagnostic test is obtaining third eyelid samples with sufficient numbers of lymphoid follicles to enable accurate scrapie diagnosis. This is complicated by the irregular distribution of the lymphoid tissue within the third eyelid and the sparse amount of lymphoid tissues in older sheep. A previous study reported lymphoid follicle mean (se) numbers of 0•7 (0•16), 0•78 (0•27) and 5•32 (0•7) in three areas of the third eyelids of sheep (Thuring and others 2000). Suitable samples must contain at least six lymphoid follicles per section for the diagnosis of scrapie (O\u27Rourke and others 2002). This short communication reports an improved third eyelid sampling technique using histamine/proparacaine eye drops