Abstract
Bovine tuberculosis (bTB), mainly caused by Mycobacterium bovis, can affect domestic and wild animals as well as humans. Identifying the major transmission mechanisms in an area is necessary for disease control and management. In this study, we aimed to evaluate the involvement of different types of contact in M. bovis transmission between cattle farms of south-western France between 2007 and 2015. We analyzed an empirical contact network of cattle farms as nodes, with known infection status and molecular types (16 circulated during the study period of which 14 affected only cattle and two both badgers and cattle). Edges were based on cattle trade data (T-edges) and on spatial neighborhood relationships between farms, either direct (P-edges) or badger-mediated, when two farms neighbored the same badger home range (B-edges), or two distinct but neighboring badger home ranges (D-edges). Edge types were aggregated so that the contact network contained only unique edges labeled by one or several edge types. The association between the contact network structure and bTB infection status was assessed using a non-parametric test, each molecular type being considered a marker of an independent epidemic. Using a logistic regression model, we estimated the contribution of each edge type to the probability for an edge originating from an infected farm to end at another infected farm. A total number of 1946 cattle farms were included in the study and were linked by 54,243 edges. Within this contact network, infected farms (whatever the molecular type) always belonged to the same component, suggesting the contact network may have supported bTB spread among those farms. A significant association between the pattern of bTB-infected farms and the structure of the contact network was observed when all the molecular types were simultaneously considered. The logistic regression model showed a significant association between M. bovis infection in direct neighbors of infected farms and the connection by T-, B- and D-edges, with odds-ratios of 7.4, 1.9, and 10.4, respectively. These results indicate a multifactorial M. bovis transmission between cattle farms of the studied area, with varying implication levels of the trade, pasture and badger networks according to the molecular type.
Highlights
Since its discovery by Theobald Smith in the late 1800’s [1] Mycobacterium bovis, the main agent of bovine tuberculosis has been found in a wide variety of domestic and wild animal hosts, as well as in humans [2, 3]
Herd skin-testing was performed each year in the study area in communes where infected farms had been detected the previous year, as well as in the neighboring ones, using either single intradermal comparative tuberculin tests (SICTT) or single intradermal tuberculin tests (SITT), both performed in Number of farms
Within the full contact network, the most frequent edge type was the combination of B- and distinct but neighboring badger home ranges (D-edges), followed by single D, T, and B-edges
Summary
Since its discovery by Theobald Smith in the late 1800’s [1] Mycobacterium bovis, the main agent of bovine tuberculosis (bTB) has been found in a wide variety of domestic and wild animal hosts, as well as in humans [2, 3]. A third category of risk factors involves wildlife interactions, especially with badgers, wild boars and deer For the latter two species, the sharing of feed or water on pastures appears to be a risk factor of M. bovis indirect transmission [23, 32, 33]. The transmission between badgers and cattle seems a bit more complex, with uncertain direct contacts on pastures [34,35,36] and/or inside farm buildings [37] This interspecies transmission could occur on pastures through the shedding of the mycobacteria in urine and feces of infected badgers [24], and in respiratory tract secretions and feces of infected cattle [6, 29]
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