Specificity of the comparative skin test for bovine tuberculosis in Great Britain

Abstract

A novel method for estimating specificity of the Single Intradermal Comparative Cervical Tuberculin (SICCT) test for bovine tuberculosis (bTB) using surveillance tests results is reported. The specificity of the SICCT test at three cut-offs was estimated from the dates, locations and skinfold measurements of all routine tests carried out in Officially TB Free (OTF) cattle herds in Great Britain (GB) between 2002 and 2008, according to their separation (by distance and time) from known infected (OTF-withdrawn) herds. The proportion of animals that tested positive was constant (P>0.20) when the distance between tested herds and nearest infected herd exceeded 8 km. For standard cut-off, calculated specificity was 99.98 per cent (95 per cent confidence interval ±0.004 per cent), equating to one false positive result per 5000 uninfected animals tested. For severe cut-off it was 99.91 per cent (±0.013 per cent) and for ultrasevere cut-off (selecting all reactors and inconclusive reactors) it was 99.87 per cent (±0.017 per cent). The estimated positive predictive value of the test averaged 91 per cent and varied by regional prevalence. This study provides further evidence of the high specificity of the SICCT test under GB conditions, suggests that over 90 per cent of cattle currently culled using this test in GB were infected, and endorses slaughter of at least these cattle for bTB control.

Keywords: Bovine tuberculosis; Cattle; Comparative Tuberculin Skin Test; Diagnostics; Great Britain; Specificity.

FIG 1:

Skin test results: the cut-offs used for combinations of 72-hour increases in Single Intradermal Comparative Cervical Tuberculin (SICCT) test reactions to bovine and avian tuberculin (mm), where oedema was not present at the injection site. Based on the TB64 diagram for England and Scotland issued by the predecessor of APHA in January 2010. The arrows show the direction in which the diagram would be expanded for measurements greater than 7 mm. APHA, Animal and Plant Health Agency

FIG 2:

Estimates of specificity for standard, severe and ultrasevere interpretations, when the calculation is performed at various distances from the nearest herd with Officially TB Free (OTF) status withdrawn (currently, or in the previous or subsequent 60 months). Error bars represent 95% confidence intervals. Only the lower bar is shown for ultrasevere interpretation, to avoid clutter

FIG 3:

Estimates of apparent specificity for standard interpretation, when the calculation is performed at various distances from the nearest herd with Officially TB Free (OTF) status withdrawn (currently, or in the previous or subsequent 60 months). In comparison with Fig 2, the vertical axis has been expanded. Error bars represent 95% confidence intervals

FIG 4:

Regular 100 km² hexagonal cells in Great Britain classified according to the distance between herds Single Intradermal Comparative Cervical Tuberculin (SICCT)-tested for bovine tuberculosis (bTB) surveillance in 2002–2008 and the nearest herd that had its Officially TB Free (OTF) status withdrawn within a±60-month window. Cells where most of the herds were used for estimating specificity are marked with dots in shades of green, and other cells are marked in shades of red. In unmarked areas, no herds were surveillance-tested