The interpretation of serial Johne's disease milk antibody results is affected by test characteristics, pattern of test results and parallel bovine tuberculosis testing

Evaluation of the diagnostic accuracy of the serological test for paratuberculosis in cattle according to tuberculosis status

BACKGROUND

Enzyme-linked immunosorbent assays (ELISAs) are the most widely used diagnostic tools in bovine paratuberculosis (bPTB) control. However, their diagnostic accuracy may be compromised by bovine tuberculosis (bTB) infection, as both diseases share diagnostic targets.

METHODS

The bPTB and bTB infection status of 228 animals was determined using microbiological tissue culture as a reference test. The diagnostic performance (sensitivity, specificity, likelihood ratios and predictive values) of the bPTB-ELISA on blood serum samples, taking into account the bPTB animal-level prevalence of the area and the bTB status of the animals, was evaluated.

RESULTS

A sensitivity of 40.7% (95% confidence interval [CI]: 27.5%-53.9%) and a specificity of 94.7% (95% CI: 91.4%-98.0%) were obtained for bPTB-ELISA in all animals. A bPTB-ELISA-positive animal would have a post-test probability of 70% or more of being infected in areas with a bPTB prevalence of 23% or more. A negative bPTB-ELISA result, in areas with a bPTB prevalence of 41% or less, would rule out the disease with more than 70% certainty. In bTB-positive animals, sensitivity increased (94.4% [95% CI: 81.4%-100%] vs. 25.1% [95% CI: 11.8%-38.4%]) and specificity decreased (82.6% [95% CI: 71.8%-93.4%] vs. 99.4% [95% CI: 98.0%-99.9%]). The bPTB-ELISA is a good tool to rule out bPTB co-infection in bTB-positive animals, while in bTB-negative animals, it allows confirmation of disease with more than 70% probability if disease prevalence is 6% or more.

LIMITATIONS

The observed differences could be enhanced by the effect of frequent application of the intradermal tuberculin test, which was unknown in the animals studied.

CONCLUSIONS

These results provide useful guidance for the application and interpretation of ELISA as a tool for bPTB disease control.

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

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.