Vaccination of calves with Mycobacterium bovis Bacillus Calmette-Guerin reduces the frequency and severity of lesions of bovine tuberculosis under a natural transmission setting in Ethiopia

Abstract

Bovine tuberculosis (bTB) is highly prevalent in intensive dairy farms of the urban "milk-sheds" in Ethiopia, and vaccination could be a cost-effective disease control strategy. In the present study, the efficacy of Bacillus Calmette-Guerin (BCG) to protect against bTB was assessed in Holstein-Friesian calves in a natural transmission setting. Twenty-three 2-week-old calves were subcutaneously vaccinated with BCG Danish SSI strain 1331, and matched 26 calves were injected with placebo. Six weeks later, calves were introduced into a herd of M. bovis-infected animals (reactors) and kept in contact with them for 1 year. In vitro and in vivo immunological tests were performed to assess immune responses post-vaccination and during exposure. Successful vaccine uptake was confirmed by tuberculin skin test and IFN-γ responses in vaccinated calves. The kinetics of IFN-γ responses to early secretory antigen target 6 and culture filtrate protein 10 (ESAT6 and CFP10, respectively) and tuberculin skin test responses post-exposure suggested that the animals were infected early after being placed in contact with the infected herd as immunological signs of infection were measurable between 2 and 4 months post-initial exposure. Protection was determined by comparing gross and microscopic pathology and bacteriological burden between vaccinated and control calves. BCG vaccination reduced the proportions of tissues with visible pathology in vaccinates compared to control calves by 49% (p < .001) with 56%, 43%, 72%, and 38% reductions in the proportion of lesioned tisues in head, thoracic, abdominal lymph nodes, and lungs, respectively (p-values .029-.0001). In addition, the lesions were less severe grossly and microscopically in vaccinated calves than in non-vaccinated calves (p < .05). The reduction in the overall incidence rates of bTB was 23%, 28%, and 33% on the basis of the absence of gross pathology, M. bovis culture positivity, and histopathology, respectively, in vaccinated animals. In conclusion, BCG vaccination reduced the frequency and severity of the pathology of bTB significantly, which is likely to reduce onwards transmission of the disease.

Keywords: BCG vaccination; bovine tuberculosis; natural transmission.

Figure 1

Means of change in skin thickness (mm) following the single intradermal comparative tuberculin test (SICTT) in vaccinated and control calves at sixth week post‐vaccination with BCG. Skin thickness as shown individually (a) and in group (b) was significantly higher in vaccinated calves than in control calves (p = .0025)

Figure 2

Single intradermal comparative tuberculin testing (SICTT) responses in vaccinated and control calves with visible (VL) and non‐visible lesions (NVL) The calves were tested with SICTT at the fourth (a), eighth (b) and 12th (c) month post‐exposure to infected herd. The means of change in skin thickness following SICTT were significantly greater (unpaired t test with Welch's correction) in control calves with VL than in control calves with NVL at eighth (p = .002) and 12th (p = .002) month post‐exposure to infected herd. However, although the mean of change in skin thickness after SICTT was greater in vaccinated calves with VL than in vaccinated calves with NVL at all the months tested, the differences were not significant at any of the months

Figure 3

Kinetics of IFN‐γ response to ESAT6/CFP10 peptide cocktail in vaccinated and control calves with visible lesions (VL) and non‐visible lesions (NVL) groups at various time points post‐exposure to infected herds for one year. The means of IFN‐γ response estimated by optical density (OD) measured at 450 nm were monitored for 12 months after the calves were exposed to the infected herd. The means of the IFN‐γ responses are compared (unpaired t test with Welch's correction) in control calves with VL and in control calves with NVL at the second (a; p = .011), fourth (b; p = .15), sixth (c; p = .0002), eighth (d; p = .001), 10^th (e; p = .0001) and 12^th (f; p = .001) month post‐exposure to the infected herd. However, although vaccinated calves with VL demonstrated relatively stronger IFN‐γ responses than vaccinated calves with NVL at the different months of exposure, the difference between these two groups was not significant at any of the months

Figure 4

Severity of gross pathology of bTB in different tissues of vaccinated and control calves. Mann–Whitney test was used to compare the severity of pathology between vaccinated and control calves in different tissues. The pathology was more severe (p = .04); in the thoracic lymh nodes of control calves than in the thoracic lymph nodes of vaccinated calves; total pathology was more severe (p = .03) in tissues of control calves than in tissues of vaccinated calves

Figure 5

Frequency of occurrence of the different stages (I–IV) granulomas within the head, neck (a), and thoracic lymph nodes (b) of vaccinated and control calves. The bars represent the percentage of granulomas within each developmental stage. A significant difference was observed in the frequencies of lesions in the head and neck lymph nodes of the vaccinated and control groups (panel a, p < 0.05), but not in the frequencies of the thoracic lymph nodes (b) between the vaccinated and control groups