Between exposure to Mycobacterium avium subsp paratuberculosis (M. ptb) and expression of clinical disease, there is a long subclinical phase in which shedding of the organism in faeces occurs intermittently. Current diagnostic tests are unable to provide sensitive and specific diagnosis during the early stages of the disease.

Detection of M. ptb DNA in blood, and culture of M.ptb from milk, liver, mammary tissue, spleen, foetal tissues, reproductive tissue, and extra intestinal lymph nodes, indicate that at some stage in the disease, bacteraemia occurs. Demonstration of the organism in blood from infected sheep and cattle by PCR has sparked recent interest in developing a diagnostic test based on culture or molecular detection of the organism from blood.

Various methods of processing blood prior to culturing in Bactec 12B culture media were compared using whole blood spiked with a known quantity of a sheep strain of M.ptb. The culture results and factors influencing the ease of processing were used to choose a method to be applied to samples from naturally and experimentally infected sheep. The chosen method utilises the intracellular location of the organism, concentrating the M.ptb by collecting the white blood cells. This allows efficient removal of the red blood cells which can be inhibitory to the growth of M.ptb in liquid media.

Preliminary findings indicate that M.ptb can be isolated from blood of a low proportion of animals following exposure, and before development of clinical signs. Results will be presented from two trials with 152 sheep.