Risk Factors for Malformations and Impact on Reproductive Performance and Mortality Rates of Schmallenberg Virus in Sheep Flocks in the Netherlands

Abstract

In Northwestern Europe, an epizootic outbreak of congenital malformations in newborn lambs due to infection with Schmallenberg virus (SBV) started at the end of 2011. The objectives of this study were to describe clinical symptoms of SBV infection, the effect of infection on mortality rates, and reproductive performance in sheep, as well as to identify and quantify flock level risk factors for SBV infections resulting in malformations in newborn lambs. A case-control study design was used, with 93 case flocks that had notified malformed lambs and 84 control flocks with no such lambs. Overall animal seroprevalence in case flocks was estimated at 82.0% (95% CI: 74.3–87.8), and was not significantly different from the prevalence in control flocks being 76.4% (95% CI: 67.2–83.6). The percentages of stillborn lambs or lambs that died before weaning, repeat breeders, and lambs with abnormal suckling behaviour were significantly higher in case flocks compared to control flocks. However, effect of SBV infection on mortality rates and reproductive performance seemed to be limited. Multivariable analysis showed that sheep flocks with an early start of the mating season, i.e. before August 2011 (OR = 33.1; 95% CI: 10.0–109.8) and in August 2011 (OR = 8.2; 95% CI: 2.7–24.6) had increased odds of malformations in newborn lambs caused by SBV compared to sheep flocks with a start of the mating season in October 2011. Other flock-level risk factors for malformations in newborn lambs were purchase of silage (OR 5.0; 95% CI: 1.7–15.0) and flocks with one or more dogs (OR = 3.3; 95% CI: 1.3–8.3). Delaying mating until October could be a potential preventive measure for naïve animals to reduce SBV induced losses. As duration of immunity after infection with SBV is expected to last for several years, future SBV induced congenital malformations are mainly expected in offspring of early mated seronegative animals.

Impact of Schmallenberg virus on British farms

“…Another important question that needs to be further assessed is the real impact of SBV on the farming community and the economy. It is known that SBV has caused significant economic losses that are principally due to international trade restrictions (EFSA 2014); however, only a few studies have been conducted to determine the real impact of SBV on productivity and livestock welfare in dairy farms (Afonso and others 2014, Veldhuis and others 2014, Wernike and others 2014) and sheep farms (Dominguez and others 2012, Luttikholt and others 2014, Saegerman and others 2013). These reports suggest that SBV probably causes abortion, short gestation and reduced reproductive performance, but these data remain too limited to accurately evaluate the impact of SBV.…”

Schmallenberg disease in sheep or goats: Past, present and future

“…The highest losses have been experienced in farms with increased numbers of stillborn, congenitally malformed lambs. In the Netherlands, in sheep flocks with early start of the breeding season, i.e., before September 2011, there were increased odds of finding malformations in newborn lambs caused by Schmallenberg disease compared to sheep flocks with a start of the mating season in October of the same year (Luttikholt et al, 2014). Furthermore, the emergence of SVB has a financial impact on international trade of live sheep and goats.…”

“…Postponing mating of females until later in a year, when Culicoides spp. are less abundant, could be an option (Luttikholt et al, 2014), but it should be taken into account that prices of products (milk, meat) from small ruminants lambing late every year are, in general, reduced. Vaccination before the mating season appears thus to be the most effective method of intervention.…”

Highs and Lows of Lambing Time: Sheep Farmers’ Perceptions of the First Outbreak of Schmallenberg Disease in South West England on Their Well-Being

“…However, by the end of December 2012, this description did not fit with our veterinary experiences in the South West of England. This was supported by data on disease outbreaks in the Netherlands, which identified a considerably higher number of deformed lambs on early lambing flocks compared to spring lambing flocks [28]. However, the impact of higher-than-normal levels of animal death and deformities on farmer well-being had not been explored.…”