Effects of vaccination against paratuberculosis on tuberculosis in goats: diagnostic interferences and cross-protection

Abstract

Background: Most countries carrying out campaigns of bovine tuberculosis (TB) eradication impose a ban on the use of mycobacterial vaccines in cattle. However, vaccination against paratuberculosis (PTB) in goats is often allowed even when its effect on TB diagnosis has not been fully evaluated. To address this issue, goat kids previously vaccinated against PTB were experimentally infected with TB.

Results: Evaluation of interferon-γ (IFN-γ) secretion induced by avian and bovine tuberculins (PPD) showed a predominant avian PPD-biased response in the vaccinated group from week 4 post-vaccination onward. Although 60% of the animals were bovine reactors at week 14, avian PPD-biased responses returned at week 16. After challenge with M. caprae, the IFN-γ responses radically changed to show predominant bovine PPD-biased responses from week 18 onward. In addition, cross-reactions with bovine PPD that had been observed in the vaccinated group at week 14 were reduced when using the M. tuberculosis complex-specific antigens ESAT-6/CFP-10 and Rv3615c as new DIVA (differentiation of infected and vaccinated animals) reagents, which further maintained sensitivity post-challenge. Ninety percent of the animals reacted positively to the tuberculin cervical comparative intradermal test performed at 12 weeks post-infection. Furthermore, post-mortem analysis showed reductions in tuberculous lesions and bacterial burden in some vaccinated animals, particularly expressed in terms of the degree of extrapulmonary dissemination of TB infection.

Conclusions: Our results suggest a degree of interference of PTB vaccination with current TB diagnostics that can be fully mitigated when using new DIVA reagents. A partial protective effect associated with vaccination was also observed in some vaccinated animals.

Figure 1

Kinetics of IFN-γ released after stimulation of whole blood with tuberculins in the vaccinated group. Avian tuberculin (PPD-A, ▲), bovine tuberculin (PPD-B, ■). Results are expressed as mean ΔOD₄₅₀ (mean of OD₄₅₀ of antigen-stimulated values with unstimulated values subtracted) ± 95% confidence interval.

Figure 2

Post-mortem analysis as measured by gross pathology and bacterial burden. Results are plotted for individual goats. (A) Total volume of visible lesions (VL) in lungs as log₁₀ cm³. (B) Total volume of VL in respiratory lymph nodes (LN) as log₁₀ mm³. (C) Bacterial load as log₁₀ cfu/g. (●) vaccinated goats, (■) unvaccinated goats. Horizontal lines indicate median values. Significance determined by unpaired t-test: *p < 0.05.

Figure 3

Protective efficacy measured by post-mortem results, and cross-sectional comparison of IFN-γ results with vaccination outcome. (A) Correlation between pathological and bacteriological parameters of all goats (n = 20). Dashed line (y = −x + 4.16) indicates the threshold defined to divide vaccine responders (VR) and vaccine non-responders (VNR). Symbols represent vaccine cross-protection outcome: (■) unvaccinated controls (UC), (○) VR goats, (●) VNR goats. (B) Specific IFN-γ responses at week 28 of the experiment as determined by ELISA after stimulation of whole blood with PPD-B, ESAT-6/CFP-10, or Rv3615c. Results are represented in relation to vaccine cross-protective outcome and are expressed as mean ΔOD₄₅₀ ± 95% confidence interval. Significances determined by non-parametric Kruskal-Wallis test: *p < 0.05.