Benefits of Badger Vaccination

A Summary of Carter et al. 2012 by Badgergate
BCG Vaccination Reduces Risk of Tuberculosis Infection in Vaccinated Badgers and Unvaccinated Badger Cubs
Stephen P. Carter, Mark A. Chambers, Stephen P. Rushton, Mark D. F. Shirley, Pia Schuchert, Stéphane Pietravalle, Alistair Murray, Fiona Rogers, George Gettinby, Graham C. Smith, Richard J. Delahay, R. Glyn Hewinson and Robbie A. MacDonald

Published: December 12th 2012

Main article: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0049833 Supporting information: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0049833#

Brief summary of study and key findings

This study investigated the direct effects of Bacille Calmette-Guérin (BCG) vaccination against tuberculosis (TB) on individual vaccinated wild badgers and its indirect effects on unvaccinated wild badgers.

TB in badgers is caused by Mycobacterium bovis (M. bovis), a bacterium similar to the one that causes TB in humans. A key constraint of using badger vaccination as a management tool for TB is the need to ensure that a sufficient proportion of individuals are vaccinated before they become infected.

Badger cubs do not emerge from their underground sett for the first two months of their life and could become infected during this period, i.e. before they can be caught and vaccinated above ground.

‘Herd immunity’ can be important in these instances – where non-immunised individuals in a given group still benefit indirectly if enough members are vaccinated (or have acquired resistance to the disease) due to reduced risk of infection within the group.

The importance of herd immunity through the use of vaccination has been documented previously in the successful global eradication of smallpox in humans.

Highly significant study findings included the following.

  • Vaccinating over one third of a social group resulted in a nearly 80% reduction in TB infection risk among unvaccinated cubs. In other words, you do not have to vaccinate all badgers in a social group or even the vast majority to achieve a major reduction in TB infection risk within the group.
  • Vaccination reduced the likelihood of vaccinated individuals getting TB even when other members of the group were at an advanced stage of the disease and likely to be highly infectious. In other words, BCG vaccination really does help protect vaccinated badgers against TB.

How the study worked

The study ran for 4 years between November 2005 and October 2009 in a mixture of 55km2 of woodland and agricultural land in Gloucestershire, southwest England. This site was deliberately chosen because of its high badger density (25.3 badgers/km2), the fact that TB was known to occur in the badger population, and because there had been no recent culling of badgers in the area.

Badgers live in territorial groups called social groups. The study’s first task was to identify the territory of each social group by bait marking. This involves placing bait (usually a mixture of peanuts, syrup and harmless plastic markers that can be eaten) at a badger sett for resident badgers to consume. (A sett generally refers to the system of tunnels and chambers shared by a social group, together with their entrances and immediate surrounds.) Different coloured markers are used for each main sett. These plastic markers show up in badger droppings and can be used to establish the boundaries of each group’s territory, including other setts used by the group in addition to the main sett. Once territories have been established, the social group of an individual was identified based on the location of the individual’s capture.

Once their territories had been identified, badger social groups were randomly divided into two groups: one, the ‘vaccinate group’, involving vaccination of 60% of all identified groups; and one where no vaccination would take place, to act as a ‘control group’ for statistical analysis, with 40% of all identified groups. This ratio of 60:40 has been found to be most effective when using statistics to determine the significance of any observed difference between two different ‘treatments’, which in this case were either being vaccinated against TB or not being vaccinated.

Before starting vaccination, badgers in the study area were captured in baited steel-mesh traps. On first capture each badger was anaesthetised, micro-chipped and tattooed on the abdomen with a unique 3-digit ID number, regardless of whether it was from a ‘vaccinate’ or ‘control’ treatment group.

Vaccination was then carried out on those badgers captured from the ‘vaccinate’ group. The vaccine was injected into the muscle in the lower back and was re-administered, when the same badger was recaptured, at a rate of one dose per calendar year for the duration of study (in line with the advised dose by Brock Vaccination, badger vaccination specialists: http://www.brockvaccination.co.uk).

Not every badger in the ‘vaccinate’ group was vaccinated as not all members of the group could be captured during the initial capture event. Badgers that originally came from the ‘vaccinate’ group but were found in ‘control’ groups during captures in subsequent years were repeat vaccinated as this was consistent with the group and treatment to which they were originally assigned at the start of the study. (This occurs because there is annual movement of some badgers, to other social groups, where they stay either temporarily or permanently. This is particularly common among males during the breeding season and is thought to occur to prevent in-breeding)

On each successive re-capture, the 3-digit ID, location, sex, age and tooth wear of each individual were recorded. Blood was also taken and processed on the same day. Samples of tracheal mucus, urine, faeces and swabs of any wounds/discharges/abscesses were also taken at each capture where possible. After all sampling and treatment, the badgers were returned and released at their point of capture.

After badger BCG was first administered, a total of 793 badgers were capture for further study. Of these, 393 had not been vaccinated and could only be studied for the indirect effects of vaccination. Badgers that were likely to have been infected prior to vaccination were excluded from the remaining 400, leaving 252 vaccinated badgers for investigating the direct effects of vaccination. Animals first assigned to vaccinate treatment that were later found in control groups were still considered to be part of the vaccinate treatment analysis. (See Table S5 of the supplementary material.)

What the study showed

  1. Over the whole duration of the study, the prevalence of infection at a population-level indicated a reduction in infection from 53% in 2006 to 35% in 2009.
  2. Reductions in infection were observed in both the ‘vaccinate’ and the ‘control’ groups i.e. reduction in infection were observed in groups that had not been vaccinated.
  3. Vaccination led to significant reductions in infection risk to individual vaccinated badgers, even when other individuals from the same social group showed signs of advanced infection.

Direct vaccination results: 

  1. After studying 252 vaccinated individuals, it was found that vaccination was associated with a significantly lower risk of an individual badger testing positive for TB using both dual and triple diagnostic tests (see Notes below).
  2. Badgers were significantly more likely to test positive in all diagnostic tests for TB if there was at least one other infected badger captured in the same social group.
  3. Male badgers were 3 times more likely to test positive for one or more diagnostic tests for TB than females.

Indirect vaccination results:

  1. It was consistently found that badger social groups with the highest levels of vaccination showed the greatest reductions in risk to unvaccinated cubs (79% and 86% reduction for the highest levels of vaccination), i.e the probability of an unvaccinated cub testing positive to M.bovis was reduced by 79% when more than one third of the social group members had been vaccinated (see page 5 of original study & Table S6 of Supplementary material for further information).
  2. Adult male badgers were nearly four times more likely to show a positive result for TB than adult females.

Summary of key findings

These results give an insight into the beneficial effect of the BCG vaccination on wild badgers in their natural setting.

  • Vaccination led to significant reductions in infection risk to individual badgers, even when those vaccinated were in the presence of other individuals from the same social group that showed evidence of advanced infection. Reductions in infection risk within a social group were also observed within a relatively short period of time from the start of the initial vaccination year.
  • The findings also show that susceptible unvaccinated badger cubs that are unreachable below the ground were indirectly protected from disease transmission through a ‘‘herd immunity’’ effect.
  • Additionally, the fact that TB infection risk was lowered so significantly and so quickly in a high-density badger area such as Gloucestershire is a testament to the power of BCG vaccination as a tool for controlling TB in wild badgers.

Main Take Home Message

This study strongly suggests that vaccinating free-living wild badgers with BCG can significantly reduce the risk of individual badgers getting TB without having to vaccinate all badgers in the population. 

Limitations of the study and some unanswered questions

  • The authors recognise a need for a vaccination threshold model i.e. a way of working out exactly how many badgers need to be vaccinated before full herd immunity is achieved.
  • There is no explanation in the study as to why male badgers seem much more likely to test positive for M. bovis infection than females.
  • While we do not know for certain what effect reduced TB transmission in badgers would have on TB in cattle populations, it appears safe to assume that if badgers are responsible for some proportion of the TB incidence in cattle, then reducing TB in badgers will have a beneficial effect on cattle assuming there is no increase in TB due to other non-badger related factors.
Notes: 1. Diagnostic tests Four different tests were used alone or in different combinations for detecting M.bovis infection. These were bacterial culture for M. bovis, the Brock (TB) Stat-Pak test, the IGRA (PPDB-PPDA) and the IGRA (ESAT-6 & CFP10). In the absence of post-mortem, these tests represent the most sensitive means to detect M.bovis infection. A dual test is associated with more advanced/severe disease. This aims to identify a positive result to the Brock (TB) Stat-Pak test and a culture test. The former is evidence of more advanced infection, while the latter identifies active excretion. A triple test aims to identify a positive result to one or more of three tests when used together. A combined triple test using Stat-Pak, bacterial culture and IGRA ESAT-6 & CFP-10, a blood test to check the immune response to specific M.bovis antigens, was used to examine the direct effects of vaccination, while the combined test using Stat-Pak, culture and IGRA PPDB-PPDA was used to look at the indirect effects of vaccination.