Objectives: To evaluate the usefulness of the polymerase chain reaction (IS900 PCR) and radiometric culture of bulk milk for screening of dairy herds for paratuberculosis.

Experimental design: Samples of bulk milk from 70 herds with paratuberculosis and 15 herds considered as free of infection were tested for Mycobacterium paratuberculosis using the IS900 PCR and radiometric culture.

Results: From each herd triplicate samples of milk were tested by the PCR. Of the 210 samples from the 70 herds with paratuberculosis, one gave a positive PCR reaction. The survey of milking practices revealed that a filter of a milking system on the farm that had the positive PCR result was always dirty. The PCR gave negative results in all 45 samples from the 15 herds considered as free of infection. No M. paratuberculosis was detected in any cultures of bulk milk from the 70 herds with paratuberculosis or the 15 herds considered as free of the disease.

The analytical sensitivities of the PCR and culture are 1 organism/mL of milk and 10 organisms/mL of milk, respectively.

The positive PCR result was obtained from only one of three samples of the bulk milk so the estimated concentration of M. paratuberculosis in bulk milk from the herd tested positive by the PCR was less than one organism/mL.

Conclusions: M. paratuberculosis could not be readily detected in the bulk milk from herds with infected cattle by either PCR or culture. This indicates that the hygiene measures in milk collection are effective and that in Australian conditions testing of bulk milk may not be a useful method to identify herds with paratuberculosis.