Prevalence of Bovine Tuberculosis in Egyptian Cattle and the Standardization of the Interferon-gamma Assay as an Ancillary Test

Bovine tuberculosis (bTB) caused primarily by Mycobacterium bovis continues to cause significant losses in the cattle industry and is a major public health problem. Despite its worldwide application, the IFN-gamma assay has not been applied in Egypt. The aim of this study was to determine the appropriate cut-off value of IFN-gamma assay to complement the skin test screening in Egypt. The relative sensitivity (Ser) of PPD and antigen cocktail-based IFN-gamma assays (IFN-gamma-BA and IFN-gamma-EC) was analysed retrospectively, relative to bTB confirmatory tests (culture and PCR), using single cervical tuberculin (SCT) test reactors during 2011-2013. The absolute specificity (Sp) was studied using blood samples collected from cattle from one bTBfree herd. Analysis of the bTB database-generated sheets indicates the infection rate had decreased from 2009 to 2012 and then increased in 2013. The disease is concentrated in the Egyptian Nile Delta and Valley relative to elsewhere in the country. The cut-offs for IFN-gamma-EC assay could be optimized to provide higher sensitivity, comparable to cut-offs for IFN-gamma-BA assay. Data analysis suggests (PPDbOD > 0.1, PPDbOD - NILOD > 0.05 and PPDb(OD) > PPDa(OD)) and (ECOD - NILOD >= 0.1) cut-off strategies to get optimal IFN-gamma-BA and IFN-gamma-EC assays results respectively. To our knowledge, this is the first report describing the prevalence of bTB in cattle in Egypt and pointing out the appropriate cut-off criteria to optimize IFN-gamma assay as a routine ancillary test for diagnosis of bTB in Egypt.