Histamine as an aid to biopsy of third eyelid lymphoid tissue in sheep.
An improved third eyelid sampling technique using histamine/proparacaine eye drops for the diagnosis of scrapie is presented. A field trial was conducted on 78 sheep in Alamo, New Mexico, using an eye drop formulation of 1% histamine and 0.5% proparacaine hydrochloride in an aqueous base (Verticare) before third eyelid biopsy. 67 (97.4%) sheep provided sufficient numbers of lymphoid follicles in their third eyelids. The sheep were checked on a regular basis for the next 12 months, and no adverse reactions from the biopsy were reported. Five other sheep flocks have been tested, totalling 131 sheep, 13 of which gave insufficient numbers of lymphoid follicles for scrapie diagnosis. The left and right eyelids of 5 sheep from a scrapie-positive research flock were treated with topical 0.5% sterile ophthalmic proparacaine hydrochloride and histamine/proparacaine eye drops, respectively, prior to biopsy. The lymphoid tissue of the right third eyelid increased in apparent size relative to the left eyelid after application of histamine. Four of the 5 sheep tested positive for PrPSc in the lymphoid follicles of both third eyelids by immunohistochemistry. In conclusion, histamine-containing eye drops improve the biopsy collection of third eyelid tissue, with increased numbers of lymphoid follicles, and do not interfere with the detection of PrPSc. The use of histamine-containing eye drops would greatly increase the accuracy of preclinical scrapie diagnosis using third eyelid tissue.