Objectives: To evaluate the usefulness of the radiometric culture of faecal slurry for rapid and inexpensive screening of dairy herds for paratuberculosis.

Experimental design: Samples of faecal slurry were collected from yards immediately after milking from 70 herds in which paratuberculosis had previously been diagnosed. Results of the last blood ELISA test that was performed as part of the Victorian Bovine Johne's Disease Test and Control Program were also obtained from each herd.

Faecal material was pushed together across the full length and breadth of the yard and mixed using a shovel or a shed scraper and triplicate samples were collected from each farm. After initial processing, two sub-samples were derived from each replicate. Subsequently, 6 sub-samples from each herd were submitted for culture and decontaminated using the double incubation method. Three of the 6 sub-samples were decontaminated at 37ºC and the other 3 were decontaminated at 42ºC. After decontamination, each sub-sample was inoculated into a BACTEC 12B bottle supplemented with egg yolk, mycobactin J and PANTA, and incubated for at least 12 weeks at 37ºC. Cultures showing growth were subcultured to demonstrate mycobactin dependency and tested by the polymerase chain reaction for the presence of IS900.

Results: M. paratuberculosis, or its DNA, was identified in faecal slurry from 50 of the 70 (71.4%) herds with paratuberculosis. The radiometric culture of slurry detected 53% of herds with low (0 to 1.5%) seroprevalence, 75% of herds with moderate (1.6 to 3%) seroprevalence and 100% of herds with high (>3%) seroprevalence. The growth of M. paratuberculosis in cultures of slurry from herds with high seroprevalence was detected significantly earlier than that observed in cultures from herds with low and moderate seroprevalences. These results show that this test is a good indicator of the prevalence of infection.

Of the 210 paired sub-samples of slurry that were decontaminated at 37ºC and 42ºC, 175 produced concordant results. Among the 35 pairs of sub-samples with discordant results, M. paratuberculosis was detected in 23 (65.7%) sub-samples decontaminated at 42ºC and in 12 (34.3%) sub-samples decontaminated at 37ºC. This indicates that the decontamination at 42ºC offers a sensitivity advantage.

From these preliminary findings using replicate samples it has been estimated that about 50% of infected herds would have been identified with the culture of a single sample.

Conclusions: The radiometric culture of farm slurry has the potential to be a useful and inexpensive tool to screen dairy herds for paratuberculosis.