Aim: To assess the effect of vaccination against paratuberculosis (Johne's disease) on reactivity to diagnostic tests for bovine tuberculosis (Tb) in deer exposed to natural challenge with Mycobacterium avium subsp. paratuberculosis (Map), and to investigate Map infection as a factor in Tb test cross-reactivity at the herd level.
Methods: In Study 1, yearling deer (n=180 vaccinates and n=181 controls) were randomly selected from three commercial deer herds participating in a trial of a commercial vaccine against paratuberculosis. The deer were subjected to the comparative cervical skin test (CCT) for Tb at 44 weeks post-vaccination. Interpretation as a mid-cervical tuberculin skin test (MCT) was also recorded. Serum from deer positive to the CCT was collected 3-4 weeks after tuberculin injection and tested using the ELISA Tb test (ETB), with both standard and the modified-ETB interpretations. In Study 2, 102 herds were categorised as infected or uninfected with Map based on results of pooled faecal culture, and positive to MCT if one or more deer gave a positive reaction to the MCT following testing in 2005. Information on other potential risk factors for MCT reactivity was collected using a standardised questionnaire conducted on each farm. The data were analysed using a multivariable logistic regression model.
Results: In Study 1, 79/180 (44%) vaccinates and 42/181 (23%) controls were positive to the MCT (p<0.001). Two vaccinates (1.1%) that were CCT-positive were both positive to the standard ETB and negative to the modified ETB. One of three CCT-positive controls was negative to the standard ETB, and the other two were positive; both controls were modified ETB-positive. In Study 2, significantly more MCT-positive (41/58; 71%) than MCT-negative (18/44; 41%) herds were infected with Map (p=0.003). The OR for a positive MCT for herds infected with Map was 3.1 (95% CI=1.3-7.5), compared with uninfected herds. Herd size was positively associated with a positive MCT result (p=0.004).
Conclusions: Infection with Map and vaccination increased the risk of non-specificity of the MCT in deer herds. The CCT and ancillary testing of CCT-positive animals using the modified ETB are effective tools to address the reduced specificity of the MCT. However, where use of these tests is not permitted, non-specificity related to infection and vaccination will be more difficult to resolve.