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The objective of this study was to quantify and compare the relative impacts of contiguity between cattle herds and the introduction of potentially infected animals on the risk of infection of a herd.

Herds in France and Sweden were tested using ELISA kits in tank milk between 2011 and 2012. The effect of animal movements was assessed using a network parameter called in-degree (ID), which corresponds to the number of herds from which a herd receives animals directly. Contiguity between herds was estimated by the local cattle density within a 5-km radius circle around each herd. A logistic regression model was used to assess the risk of a herd being ELISA positive associated with an increase in local density and ID.

The risk of a herd testing positive increases proportionally with increasing local density and ID. Thirty-four and 16% of positive herds are attributable to local density and animal movement, respectively, highlighting the strong impact of local density on the risk of infection.

Control measures to be implemented in herds should take into account the local density because, unlike animal movements, it is not controllable. Thus, in herds located in areas with high densities, control measures based on animal movements may not be sufficient, and the use of vaccination may be an appropriate strategy.

Location of all the dairy cattle herds in Finistère. The blue and grey dots represent respectively herds with and without antibodies against Cb in tank milk. The blue circle corresponds to the area in which the herds have a higher risk of testing positive.