The results of using faecal culture as confirmation test of paratuberculosis-seropositive dairy cattle

A total of 15,822 cattle aged 3 years and older, belonging to 378 randomly selected herds, were tested for paratuberculosis using an absorbed enzyme-linked immunosorbent assay (ELISA); 3.3% tested positive. This percentage was lowest for the group of cattle aged 3-4 years (2.3%) and highest for cattle with the age of 5-6 years (4.5%). The mean Sample to Positive (S/P) ratio of seropositive cattle vaccinated against paratuberculosis was higher (0.75 +/- 0.33) than that of seropositive, non-vaccinated cattle (0.58 +/- 0.26). Faecal samples of 422 ELISA-positive cattle were cultured for the presence of Mycobacterium avium subsp. paratuberculosis, 12% of these were contaminated. The percentage of non-contaminated samples with positive culture results was 17.3%, with a substantial difference between vaccinated (1.7%) and non-vaccinated cattle (20.2%). Of the positive cultures, the number of colonies varied from 1-10 (22% of cultures), 11-100 (22%), to more than 100 (55%). The percentage of ELISA-positive, non-vaccinated cattle tested culture-positive was positively correlated with the magnitude of the S/P ratio. This percentage varied from 12% (S/P ratio 0.3-0.5) to 58% (S/P ratio > 1.1), a result that might have implications for interpretation of the test. In this study, the percentage of ELISA-positive cattle with positive faecal culture results was limited and these individuals were mostly moderate to heavy shedders.