Surveillance of emerging diseases in cattle: Application to the Schmallenberg virus epidemic in the Netherlands

Abstract

Animal health surveillance is an essential component to protect animal health, facilitate trade, and protect public health. Reliable surveillance systems are able to rapidly identify outbreaks of emerging animal diseases in previously free areas to enable the implementation of control measures. In the late summer of 2011, Schmallenberg virus (SBV) emerged in ruminants in north-western Europe causing fever, drop in milk production and diarrhoea in adult cattle, as well as congenital malformations in new-born calves and lambs. SBV is a vector-borne virus, transmitted by the vector Culicoides biting midges. Due to the novelty of the virus, there was an immediate need for information regarding the epidemiological characteristics of SBV to inform surveillance and control strategies. This thesis showed that SBV infected a large fraction of the ruminant population in the Netherlands during the 2011/2012 epidemic. Grazing of cattle was identified as a herd-level risk factor for high within-herd prevalence and malformations in calves. However, the impact of the epidemic on cattle reproductive performance, milk production and mortality appeared to be limited. This provided confidence that control measures such as vaccination were unneeded. Also, syndromic surveillance for early detection of emerging diseases in cattle was assessed. It was concluded that a syndromic surveillance system based on milk production or reproductive performance data could have an added value, yet complementary to passive surveillance systems. Finally, this thesis underlined the need for a tailored surveillance strategy, adapted to the changing objectives during the different epidemiological phases of an emerging disease outbreak.