Badgers and Disease Ecology

Traditionally one of the UK’s most charismatic (and certainly distinctive) mammals, the badger has received mixed attention in recent times as a result of their involvement in disease transmission. To celebrate National Badger Day, Dr Andy Robertson describes the long-term study of these animals in Woodchester Park. Dr Robertson is a NERC Knowledge Exchange Fellow at the University of Exeter’s Environment and Sustainability Institute, and also works closely with the Animal and Plant Health Agency on a range of applied research relating to badgers and bovine TB.

The European badger (Meles meles) is member of the Mustelid (weasel) family, but within that family it is rather unusual. While most mustelids are nimble carnivores, badgers are somewhat bulky omnivores, eating practically any food item they can lay their paws on, including invertebrates, carrion, fruits, small mammals, birds eggs, crops and even kitchen scraps. Badgers are also unusual among mustelids as they often live in social groups, sometimes exceeding 20 individuals. The sociality of badgers varies across their geographic range, but some of the most social badgers are found in regions of the UK and Ireland. Their unconventional diet and social habits have made the European badger the subject of an enormous amount of scientific research.

Despite their interesting ecology and behaviour, badgers are arguably most noteworthy (and newsworthy) because of their involvement in the transmission of bovine tuberculosis (bTB) to cattle. Bovine TB is caused by the bacterium Mycobacterium bovis (closely related to the bacterium that causes TB in humans) and badgers were first identified as a potential reservoir when M. bovis was detected in a dead badger in southern England in the 1970s. This discovery led to increased research on badger ecology and the epidemiology of bTB, which included the establishment of a long-term study in Woodchester Park, Gloucestershire, currently run by APHA (the Animal and Plant Health Agency). Much of the important work conducted in this study is summarised in our recent Synthesis in Journal of Animal Ecology.

Importantly, the badgers in the Woodchester Park study are known individuals, making it possible to monitor them throughout their lives. As part of the study badgers are captured, marked, examined and released. Every captured animal is tattooed with a unique code, and a wide range of data and samples are collected. Trapping takes place four times each year across approximately 20 social groups of badgers. The study is now in its 41^st year and during which time there have been more than 15,000 captures of over 3,000 different individuals. The prize for most captures, or “biggest peanut enthusiast” (the bait used to capture badgers) goes to “T047” which was caught an impressive 42 times over an 11 year period.

This unique study has yielded many valuable insights into the epidemiology of bTB in wild badgers, but has also provided opportunities for the collection of samples and data to explore other aspects of badger ecology. For example, my own PhD research involved collecting badger whiskers, which were then analysed using stable isotope analyses to investigate differences in foraging niche. Even within small social groups individual badgers had significantly different isotope signatures reflecting different foraging niches (utilising different prey or habitats) that were consistent over multiple years.

One key disease-related finding has been that the prevalence of the bTB (% of infected badgers) does not simply increase as the density of badgers increases, which is often the case in disease studies. Instead movement is key, with increased levels of movement among badger social groups corresponding with higher levels of disease. Combing individual-level life-history, behaviour and disease data has also shown that diseased badgers behave differently from their TB free counterparts. For example, ranging more widely than uninfected badgers and being more likely to occupy outlier setts (burrows) away from their social group’s main residence.

Several recent studies have involved fitting ‘proximity collars’ to badgers which record instances where badgers encounter other collared individuals. Fitting collars to cattle has demonstrated that, despite ample opportunity, badgers rarely encounter cattle on pastures, which suggests that direct “nose-to-nose” transmission is unlikely. Quantifying regular contacts among collared badgers allows the creation of social networks, and helped show that social behaviour is closely related to bTB infection in the population.

As with other long term wildlife studies, it is this wealth of individual data that is the real strength of the Woodchester Park study. Recent developments in animal tracking technology have also contributed greatly in a species that can be hard to directly observe. Combining this existing knowledge and new approaches offers a real opportunity to understand how differences between individuals influence the spread of disease in wildlife populations.