Sensitivities of a bulk-tank milk ELISA and composite fecal qPCR to detect various seroprevalence levels of paratuberculosis in cattle herds in Normandy, France.

This study evaluated an ELISA on bulk tank milk (BTM) samples and a qPCR on a single composite fecal sample to detect paratuberculosis seropositive cattle dairy herds. Individual serum (n = 15 372), BTM and composite fecal samples were obtained from 192 herds. The within-herd apparent seroprevalence was categorized and compared with BTM ELISA and fecal qPCR results. The BTM ELISA had poor overall sensitivity (16%) to detect seropositive herds but higher sensitivity (53%) in the higher apparent seroprevalence group of > 9%. Using an optimized cut-off point (5.0% S/P), sensitivities overall and in the high apparent seroprevalence group were 53% and 88%, respectively. The BTM ELISA gave 5% positive results in seronegative herds and 25% using the optimized cut-off. Fecal qPCR had 72% sensitivity to detect seropositive herds and 88% in the higher apparent seroprevalence group, but gave 25% positive results in fully seronegative herds. The combination of BTM ELISA and composite fecal qPCR improved the sensitivity to detect seropositive herds.