EU-approved rapid tests for bovine spongiform encephalopathy detect atypical forms: a study for their sensitivities

Abstract

Since 2004 it become clear that atypical bovine spongiform encephalopthies (BSEs) exist in cattle. Whenever their detection has relied on active surveillance plans implemented in Europe since 2001 by rapid tests, the overall and inter-laboratory performance of these diagnostic systems in the detection of the atypical strains has not been studied thoroughly to date. To fill this gap, the present study reports on the analytical sensitivity of the EU-approved rapid tests for atypical L- and H-type and classical BSE in parallel. Each test was challenged with two dilution series, one created from a positive pool of the three BSE forms according to the EURL standard method of homogenate preparation (50% w/v) and the other as per the test kit manufacturer's instructions. Multilevel logistic models and simple logistic models with the rapid test as the only covariate were fitted for each BSE form analyzed as directed by the test manufacturer's dilution protocol. The same schemes, but excluding the BSE type, were then applied to compare test performance under the manufacturer's versus the water protocol. The IDEXX HerdChek ® BSE-scrapie short protocol test showed the highest sensitivity for all BSE forms. The IDEXX® HerdChek BSE-scrapie ultra short protocol, the Prionics®--Check WESTERN and the AJ Roboscreen® BetaPrion tests showed similar sensitivities, followed by the Roche® PrionScreen, the Bio-Rad® TeSeE™ SAP and the Prionics®--Check PrioSTRIP in descending order of analytical sensitivity. Despite these differences, the limit of detection of all seven rapid tests against the different classes of material set within a 2 log(10) range of the best-performing test, thus meeting the European Food Safety Authority requirement for BSE surveillance purposes. These findings indicate that not many atypical cases would have been missed surveillance since 2001 which is important for further epidemiological interpretations of the sporadic character of atypical forms.

Figure 1. Test performance compared to the IDEXX® HerdCheck BSE-scrapie short protocol.

The vertical axis reports the different rapid tests challenged. The horizontal axis reflects the odds ratio magnitude using IDEXX® HerdCheck BSE-scrapie short protocol as reference test. Panels A, B, C, D (left column) report the results obtained under the w/v protocol; panels E, F, G, H (right column) display the results under the manufacturers' instructions. BSE forms studied: panels A, E: strong C-type; panels B, F: weak C-type; panels C, G: H-type; panels D, H: L-type. All the weak C type water dilutions series tested negative with Bio-Rad® TeSeE™ SAP (notably, the optical densities were for all the three replicates of the 1∶2 dilution just under the cut-off value), thereby, the odd ratio could not be calculated (Subfigure B).

Figure 2. Comparison of test performance under the manufacturer's dilution protocol versus the 50% w/v protocol.

The vertical axis reports the different rapid tests challenged. The horizontal axis reflects the odds ratio magnitude.