Protective Effect of Oral BCG and Inactivated Mycobacterium bovis Vaccines in European Badgers (Meles meles) Experimentally Infected With M. bovis

Abstract

In Europe, badgers (Meles meles) are recognized as major tuberculosis (TB) reservoir hosts with the potential to transmit infection to associated cattle herds. Recent studies in Spain have demonstrated that vaccination with a heat-inactivated Mycobacterium bovis vaccine (HIMB) successfully protects captive wild boar and red deer against progressive disease. The aim of this study was to evaluate the efficacy of two oral vaccines against TB in a badger model: the live-attenuated M. bovis bacillus Calmette-Guérin BCG vaccine (Danish strain) and a HIMB vaccine. Twenty-four badgers were separated in three treatment groups: oral vaccinated with live BCG (10⁸ CFU, n = 5), oral vaccinated with HIMB (10⁷ CFU, n = 7), and unvaccinated controls (n = 12). All badgers were experimentally infected with M. bovis (10³ CFU) by the endobronchial route targeting the right middle lung lobe. Throughout the study, clinical, immunological, pathological, and bacteriological parameters of infection were measured. Both vaccines conferred protection against experimental TB in badger, as measured by a reduction of the severity and lesion volumes. Based on these data, HIMB vaccination appears to be a promising TB oral vaccine candidate for badgers in endemic countries.

Keywords: BCG vaccine; Mycobacterium bovis heat-inactivated (HIMB) vaccine; badger; efficacy; tuberculosis.

Figure 1

P22 ELISA results throughout the Mycobacterium bovis experimental infection in BCG and heat-inactivated M. bovis (HIMB) vaccinated animals and controls. (A) Mean of E% showed in each group at different time points using all animals (n = 24) included in the study design. (B) E% displayed by four animals (n = 4) that tested positive to P22 ELISA before the challenge. Animal 1 belongs to the control group, animal 2 to BCG group and animals 3 and 4 to the HIMB group. (C) Mean of E% showed in each group at different time points when only animals that were negative (n = 20) to the P22 ELISA before the challenge were analyzed. (D) Percentage (%) of positive animals to the P22 ELISA in different groups using only the animals that showed negative results (n = 20) to the ELISA prior the challenge. PV, Post-vaccination; PI, post-infection.

Figure 2

Cellular assays (Whole blood IGRA ELISA, IGRA ELISPOT, and PBMC IGRA ELISA) results throughout the Mycobacterium bovis experimental infection in BCG and heat-inactivated M. bovis (HIMB) vaccinated animals and controls. All studied badgers (TB pre+ and pre- badgers; n = 24) are included in the graphic. PV, Post-vaccination; PI, post-infection.

Figure 3

Gross and microscopic lesions in a control badger. (A) TB lesions 2 mm in diameter in the right middle lung lobe. (B) Miliar TB lesions in the mediastinum. (C) Right middle lung lobe: TB granuloma (type 4) in which central mineralization can be observed. Haematoxylin-eosin stain, bar = 200 microns. Inset: Acid fast bacilli (arrow). Ziehl-Neelsen stain, bar = 5 microns. (D) Mediastinum: TB granulomas type 2 in which central coagulative necrosis is observed. Haematoxylin-eosin stain, bar = 200 microns.

Figure 4

Magnetic Resonance Image (MRI) tuberculosis (TB) lesion volume expressed in volume (mm³) in vaccinated and control badgers. Examples of lungs “not infected or with low level infection” (A), “noticeably infected” (B), and “strongly infected” (C) with lesions inside the yellow circle. Lower lesion volumes are observed in BCG and heat-inactivated M. bovis (HIMB) vaccinated groups compared to the control group (D). Boxes represented the 25 and 75% percentiles; the horizontal lines inside boxes indicate the TB lesion volume (mm³) median values. All studied badgers (TB pre+ and pre- badgers; n = 24) are included in the graphic.