Genetic evidence that culling increases badger movement: implications for the spread of bovine tuberculosis

Abstract

The Eurasian badger (Meles meles) has been implicated in the transmission of bovine tuberculosis (TB, caused by Mycobacterium bovis) to cattle. However, evidence suggests that attempts to reduce the spread of TB among cattle in Britain by culling badgers have mixed effects. A large-scale field experiment (the randomized badger culling trial, RBCT) showed that widespread proactive badger culling reduced the incidence of TB in cattle within culled areas but that TB incidence increased in adjoining areas. Additionally, localized reactive badger culling increased the incidence of TB in cattle. It has been suggested that culling-induced perturbation of badger social structure may increase individual movements and elevate the risk of disease transmission between badgers and cattle. Field studies support this hypothesis, by demonstrating increases in badger group ranges and the prevalence of TB infection in badgers following culling. However, more evidence on the effect of culling on badger movements is needed in order to predict the epidemiological consequences of this control strategy. Here, analysis of the genetic signatures of badger populations in the RBCT revealed increased dispersal following culling. While standard tests provided evidence for greater dispersal after culling, a novel method indicated that this was due to medium- and long-distance dispersal, in addition to previously reported increases in home-range size. Our results also indicated that, on average, badgers infected with M. bovis moved significantly farther than did uninfected badgers. A disease control strategy that included culling would need to take account of the potentially negative epidemiological consequences of increased badger dispersal.

Bovine Tuberculosis in Cattle and Badgers in Localized Culling Areas

“…Detailed analyses indicate that this rising prevalence was related to repeated culling (which was asynchronous across triplets) rather than reflecting a simple year-on-year increase (Woodroffe et al, 2006b). This rising prevalence is likely to have been caused by increased mixing among badgers as a result of culling-induced disruption of social and territorial organization (Woodroffe et al, 2006a;Pope et al, 2007). Since localized culling was, by definition, restricted to comparatively small areas of land, recolonization appears to have been rapid (Woodroffe et al, 2006a), and local badger densities were reduced only slight-ly (Woodroffe et al, 2008).…”

“…Although M. bovis infections are clustered in badgers, the edges of these clusters are not sharply defined (Delahay et al, 2000;Woodroffe et al, 2005c); so, even if every infected animal, or every member of an infected social group, could be identified and removed, it is likely that some animals immigrating into the cleared area would be infected, especially as infected badgers appear to disperse further than do uninfected animals (Pope et al, 2007). Imperfect detection of infection in badgers and imperfect badger removal elevate the chances of increased contact rates leading to increased transmission, constraining the ability of localized culling to reduce TB risks to cattle.…”

“…Widespread badger culling is known to be associated with such increases in prevalence in badgers (Woodroffe et al, 2006b), so it is reasonable to expect similar effects of localized culling. Badgers range more widely when their territorial organization has been disrupted by culling (Woodroffe et al, 2006a;Pope et al, 2007); this behavioral change could increase transmission among badgers, thus elevating local M. bovis prevalence. This increased ranging behavior could also increase contact between infectious badgers and susceptible cattle herds, even if M. bovis prevalence were unaffected.…”

Using genetic methods to investigate dispersal in two badger (Meles meles) populations with different ecological characteristics

“…Furthermore, dispersal can be an important consideration in the development of effective management and conservation measures. For example, host dispersal has a particularly important role in disease transmission, as it is a major factor influencing disease spread, and a key variable in epidemiological models (Coyne et al, 1989;Pope et al, 2007). Dispersal may also affect the impact of local extinction as it influences the probability of re-colonization.…”

“…In addition, in certain countries, badgers have been declining in numbers and are of conservation concern (for example, The Netherlands; Mulder, 1996). In the United Kingdom and Ireland, badgers have been implicated in the transmission of bovine tuberculosis to cattle and the species is a major management concern (for example, Pope et al, 2007).…”