Objective

To compare the effect of decontamination of slurry and sewage at 37°C and 42°C on rates of isolation of M. paratuberculosis and culture contamination.

Design

Animal slurry and raw sewage were sampled over 6 months at approximately weekly intervals and decontaminated before radiometric culture (RMC) using the double incubation method. One set of triplicate 50 mL samples of slurry and sewage was decontaminated at 37°C and the other set was decontaminated at 42°C. Cultures showing positive growth were subcultured onto solid media to evaluate mycobactin dependency and tested for IS900 by the polymerase chain reaction (PCR).

Results

M. paratuberculosis or its DNA was detected in 7 of 45 (15.5%) cultures of slurry decontaminated at 37°C and in 14 of 39 (35.9%) cultures of slurry decontaminated at 42°C. The contamination rates in cultures of slurry processed at 37°C and 42°C were 88.2% and 69.2%, respectively. M. paratuberculosis DNA was detected in one of 45 (2.2%) cultures of sewage decontaminated at 42°C. No M. paratuberculosis or its DNA was detected in 45 cultures of sewage decontaminated at 37°C. The contamination rates in samples of sewage processed at 37°C and 42°C were 84.4% and 4.4%, respectively.

Conclusions

The method of double incubation at 42°C was more selective and sensitive than the standard procedure at 37°C. This warrants further studies to evaluate the usefulness of the former method for the decontamination of faeces, tissues and milk.