The weak response detected to PPDb by non-infected cattle in both ex vivo and long-term cultures was likely due to prior exposure to non-tuberculous mycobacteria (NTM, a common occurrence in US dairy cattle) as pre-infection whole blood (18 h stimulation) IFN-γ responses to M. avium PPD (PPDa) exceeded (P < 0. The gene discussed is IFNG; the disease is infection.