Successful Application of the Gamma-Interferon Assay in a Bovine Tuberculosis Eradication Program: The French Bullfighting Herd Experience

Abstract

In the French Camargue region, where bovine tuberculosis had been enzootic for several years in bullfighting cattle herds, the gamma-interferon (IFN) assay was used since 2003 in parallel with the intradermal test in order to increase overall disease detection sensitivity in infected herds. This study presents the results of a field-evaluation of the assay during a 10-year period (2004-2014) of disease control and surveillance program and explores the particular pattern of IFN assay results in bullfight herds in comparison to cattle from other regions of France. The low sensitivity [59.2% (50.6; 67.3)] of IFN assay using the tuberculin stimulation could be related to the poor gamma-IFN production from bullfight cattle blood cells which is significantly lower than in animals of conventional breeds. The characteristics of the assay were progressively adapted to the epidemiological situation and the desired strategic applications. Data analysis with a receiver operating characteristic curve based on a simple S/P value algorithm allowed for the determination of a new cutoff adapted for a global screening, giving a high specificity of 99.9% results and a high accuracy of the assay. Having regularly risen to above 5% since 2005, with a peak around 10% in 2010, the annual incidence dropped to under 1% in 2014. The positive predictive value relative to the bacteriological confirmation evolved during the years, from 33% in 2009 to 12% during the last screening period, a normal trend in a context of decreasing prevalence. The estimated rate of false-positive reactions during screening campaigns was 0.67%, confirming the high specificity of the test, measured in bTB negative herds, in this epidemiological context. The proportion of false-positive reactions decreased with the age and was higher in males than in females. Although these results indicate that the IFN assay is accurate in the field, it also emphasizes great differences between interferon quantities produced by bullfight cattle blood samples compared to those of classical bovine breeds, which underlines the necessity to adapt the algorithms and combinations of the assay according to local epidemiological contexts.

Keywords: bovine tuberculosis; eradication; field performances; gamma-interferon assay; screening; strategic use.

Figure 1

Distribution of optical density (OD) values for PPDB antigen in two infected populations: bullfighting (N = 142) and conventional cattle (N = 44). The upper lines in the boxes represent the 75th percentile (P75), the middle line represents the median (P50), and the lower line in the box represents the 25th percentile (P25). The ends of the whiskers represent minimum and maximum OD values.

Figure 2

Receiver operating characteristic (ROC) curve established by using the S/P quantitative value.

Figure 3

Evolution of the apparent prevalence and incidence rates during the two global screening periods. During these periods, half of the farms was screened with IFN test (cattle over 24 months) each year, while the other half was screened with the skin test (cattle over 12 months), except during the 2011/2012 period when all farms were screened only with the skin test.