Comparative performance of three TSE rapid tests for surveillance in healthy sheep affected by scrapie.

Rapid tests specific for sheep and goats became part of European Union-wide active scrapie surveillance in 2006. Performance of three approved TSE rapid tests for the detection of sheep infected with scrapie in field cases in the pre-clinical stage of the disease was compared. The medulla oblongata of 969 asymptomatic sheep of various genotype and breed aged over 18 months from 23 Italian flocks affected with scrapie, were tested by the Bio-Rad TeSeE Sheep/Goat (A), the IDEXX HerdChek BSE-Scrapie Antigen Test Kit, EIA (B) and the Prionics(®)-Check Western Small Ruminant (C) rapid tests. Of 136 positive samples of classical scrapie, as confirmed by Western blot assay, 132 were positive with test A (Se 97.06%, CI 95% 92.64-99.19); 135 with test B (Se 99.26%, 95% CI 95.97-99.98) and 128 with test C (Se 94.12%, 95% CI 88.74-97.43). Tests A and B showed the best performance on analytical sensitivity. All three systems demonstrated good reproducibility: being the intrarater and interrater kappa coefficients always over 0.83. The one available atypical scrapie sample was positive with tests A and B, negative with test C. Considering the discrepant results in the detection of low PrP(sc) concentrations and of the atypical case, differences can be expected in the efficacy of an active surveillance system, depending on the test adopted.