BCG vaccination reduces risk of tuberculosis infection in vaccinated badgers and unvaccinated badger cubs

Abstract

Wildlife is a global source of endemic and emerging infectious diseases. The control of tuberculosis (TB) in cattle in Britain and Ireland is hindered by persistent infection in wild badgers (Meles meles). Vaccination with Bacillus Calmette-Guérin (BCG) has been shown to reduce the severity and progression of experimentally induced TB in captive badgers. Analysis of data from a four-year clinical field study, conducted at the social group level, suggested a similar, direct protective effect of BCG in a wild badger population. Here we present new evidence from the same study identifying both a direct beneficial effect of vaccination in individual badgers and an indirect protective effect in unvaccinated cubs. We show that intramuscular injection of BCG reduced by 76% (Odds ratio = 0.24, 95% confidence interval (CI) 0.11-0.52) the risk of free-living vaccinated individuals testing positive to a diagnostic test combination to detect progressive infection. A more sensitive panel of tests for the detection of infection per se identified a reduction of 54% (Odds ratio = 0.46, 95% CI 0.26-0.88) in the risk of a positive result following vaccination. In addition, we show the risk of unvaccinated badger cubs, but not adults, testing positive to an even more sensitive panel of diagnostic tests decreased significantly as the proportion of vaccinated individuals in their social group increased (Odds ratio = 0.08, 95% CI 0.01-0.76; P = 0.03). When more than a third of their social group had been vaccinated, the risk to unvaccinated cubs was reduced by 79% (Odds ratio = 0.21, 95% CI 0.05-0.81; P = 0.02).

Figure 1. Configuration of badger social group territories in the first year of the study.

Territories have been derived from bait marking data and show allocation of vaccine treatment (shaded areas) and control (open areas). Circles indicate additional main setts located after the 2006 bait marking for which the territorial boundaries could not be delineated until the following year, but which were determined to represent discrete social groups in 2006 and allocated a treatment accordingly. Additional badger groups may have been present in non-surveyed areas adjacent to the mapped territories, particularly around the edge of the study area.

Figure 2. Decreasing risk of an unvaccinated badger cub testing positive to a triple diagnostic test for M. bovis infection as the proportion of vaccinated badgers in its social group increases.

The number of cubs within each category is shown.