Potential infection-control benefit of measures to mitigate the risk posed by Trojan dams in the Irish BVD eradication programme

In the epidemiology of Bovine Viral Diarrhoea (BVD), Trojan dams (animals that are not persistently infected (PI) with BVD (BVDv) virus but carrying PI foetuses) are a vehicle through which infection can be transmitted. We investigated the degree to which restricting movement of cattle from BVDv infected herds would prevent Trojan births in other herds (effectiveness) and the extent to which this would reduce other, non-Trojan, movements (proportionality). We focussed on Irish herds with BVD + animal(s) present during 2014 and/or 2015. The effect of restricting movements of female animals over 12 months of age from these herds was compared with data collected on Trojan dams that calved in 2015. Four different potential restriction lengths were considered, varying from the period when a BVD + animal was present in the herd, to extending this to 12 months after removal of the last BVD + animal. In terms of effectiveness, none of the four restriction measures evaluated was effective at preventing the movement of all Trojan dams. Between 18.3% and 37.3% of Trojan births in 2015 would have been prevented under the proposed measures, and all Trojan births would have been prevented in between 14.4% and 32.5% of herds with BVD + births. In terms of proportionality, between 4.4% and 15.4% of all females > 12 months of age that moved between herds during 2015 would have been prevented from moving, and between 3.5% and 10.1% of Irish herds with at least one such movement would have been affected. These results show how measures to control the movement of Trojan dams should be targeted in a way that fits the Irish context and reduces the spread of BVDv, without unduly impacting other trade.