The Occurrence of Mycobacterium avium Subspecies paratuberculosis Positive Milk Antibody ELISA Results in Dairy Cattle under Varying Time Periods after Skin Testing for Bovine Tuberculosis

Simple Summary Milk from dairy cows is screened for antibodies to the bacterium causing Johne's disease, which causes diarrhoea. The bacterium causing bovine tuberculosis has similarities in its structure with the bacterium causing Johne's disease. Bovine tuberculosis is diagnosed by injecting avian and bovine tuberculin in the skin of the neck and comparing the reactions three days later. Potentially, tuberculosis testing may interfere with Johne's disease testing. This study aimed to clarify the relationship between the time interval between tuberculosis and Johne's disease testing and the number of Johne's disease-positive cows. Data were collected from 51 herds, totalling 46,738 cow observations. Analysis showed that Johne's disease testing in each 14 day interval increased the probability of detecting Johne's disease-positive cows compared to Johne's disease testing over 85 days after tuberculosis testing. The probability was 2.5-fold higher in the first 14 day interval after tuberculosis testing, increasing each two-week period to 4.0-fold higher at 57-70 days before dropping again. A previous history of tuberculosis within the herd increased the probability by 1.2 fold. This was less important compared to the timing of Johne's disease testing after tuberculosis testing. Enzyme-linked immunosorbent assays (ELISA) are used to screen cows for Mycobacterium avium subspecies paratuberculosis (MAP) infections, informing Johne's disease (JD) management practices in dairy herds. The causative agent of bovine tuberculosis (bTB), Mycobacterium bovis, and MAP share multiple antigens. Moreover, Mycobacterium avium subspecies avium is used in the single intradermal cervical comparative tests (SICCT) that are routinely used in early detection of cows infected with bTB. Although these are different types of immune responses, potentially the SICCT may interfere with the levels of MAP antibodies. This study aimed to clarify the relationship between the SICCT-MAP milk ELISA testing interval and apparent prevalence of JD risk statuses. Data from 51 herds were used, totalling 46,738 cow observations. The Poisson models showed that MAP milk ELISA testing at 14 day intervals post-SICCT statistically significantly increased the odds of detecting JD-positive cows compared to JD testing 85+ days post-SICCT. The odds ratio (OR) started at 2.5 in the first 14 day interval post-SICCT, increasing each two-week period to an OR of 4.0 at 57-70 days, to subsequently drop. Additionally, a herd history of bTB increased the odds of detecting JD-positive cows (OR = 1.2); this was relatively limited compared to the magnitude of the post-SICCT effect.