Contact networks in a wildlife-livestock host community: identifying high-risk individuals in the transmission of bovine TB among badgers and cattle

Abstract

Background: The management of many pathogens, which are of concern to humans and their livestock, is complicated by the pathogens' ability to cross-infect multiple host species, including wildlife. This has major implications for the management of such diseases, since the dynamics of infection are dependent on the rates of both intra- and inter-specific transmission. However, the difficulty of studying transmission networks in free-living populations means that the relative opportunities for intra- versus inter-specific disease transmission have not previously been demonstrated empirically within any wildlife-livestock disease system.

Methodology/principal findings: Using recently-developed proximity data loggers, we quantify both intra- and inter-specific contacts in a wildlife-livestock disease system, using bovine tuberculosis (bTB) in badgers and cattle in the UK as our example. We assess the connectedness of individuals within the networks in order to identify whether there are certain 'high-risk' individuals or groups of individuals for disease transmission within and between species. Our results show that contact patterns in both badger and cattle populations vary widely, both between individuals and over time. We recorded only infrequent interactions between badger social groups, although all badgers fitted with data loggers were involved in these inter-group contacts. Contacts between badgers and cattle occurred more frequently than contacts between different badger groups. Moreover, these inter-specific contacts involved those individual cows, which were highly connected within the cattle herd.

Conclusions/significance: This work represents the first continuous time record of wildlife-host contacts for any free-living wildlife-livestock disease system. The results highlight the existence of specific individuals with relatively high contact rates in both livestock and wildlife populations, which have the potential to act as hubs in the spread of disease through complex contact networks. Targeting testing or preventive measures at high-contact groups and individuals within livestock populations would enhance the effectiveness and efficiency of disease management strategies.

Figure 1. Dalby Forest study site.

The main sett (den) and territories of the three badger social groups under study, as well as broad habitat categories, are shown. Territory estimates were derived from bait-marking returns (Valley & Cottages group) and radio-tracking (Farm). Both Farm and Valley groups overlap greatly with the grazing pasture of the study dairy herd (D, day pasture; N, night pasture).

Figure 2. Connectivity Index (CI) scores.

A) Badgers only (Cottages group included); B) Cattle only. Arrows mark individuals implicated in inter-species interactions.

Figure 3. Contact profiles over time, showing the duration of contacts in seconds.

A) inter-group contacts, B) inter-specific contacts (top: badger V58, adult female; bottom: badger V59, yearling male; weeks with no cattle collared are excluded). The data were summarised per week. Totals for the duration of contacts in the week starting 05/06/06 are shown. Note: the x-axis scale is not continuous, as weeks outside the sampling periods were omitted.