During summer and fall 2011, farmers and veterinarians notified dairy cattle with clinical signs such as fever, decreased milk production and diarrhoea, especially in the eastern regions of the Netherlands and in Northwest Germany. A novel orthobunyavirus, provisionally named Schmallenberg virus (SBV), was identified as causative agent. Shortly after, an epizootic outbreak of congenital malformations in new-born lambs and calves started in North-western Europe. On December 15th 2011, brain tissue samples from malformed lambs in the Netherlands tested positive in a real-time quantitative reverse transcription PCR (RT-qPCR) provided by Friedrich Loeffler Institut (FLI).
Problem definition: The impact of the SBV epidemic on small ruminants is uncertain, knowledge on impact is necessary for animal health policy makers to decide whether a control program is necessary. A risk factor study may provide tools for intervention and control.
Objectives of the project: The objectives of this study were to identify and quantify flock-level risk factors for malformations in new-born lambs caused by SBV, as well as to describe the effects of SBV infection on mortality rates, and reproductive performance in sheep flocks in the Netherlands.
Results and opportunities: Animal-level seroprevalence will be compared between case and control flocks. Risk factor distribution will be compared between case and control flocks. Incidence of malformations in newborn lambs at herd level will be compared between case ad control herds. The within-herd impact of SBV infection on milk production and mortality rates will be investigated.
Methodology and time table: A case-control study design was used. A sample size of 100 case flocks and 100 control flocks was chosen in order to detect a risk factor exposure odds ratio (OR) of 2-3, with 95% confidence and 80% power. Out of all sheep flocks that had notified malformations in new-born lambs from November 25th until April 30th (n=349), 115 flocks were classified as suitable case flock. Inclusion criteria were (i) birth of malformed lambs caused by SBV, based on results from post-mortem examination at the Animal Health Service (AHS), and (ii) a minimal flock size of 50 breeding ewes. If two flocks were located in the same four digit postal code area, one of the flocks was randomly selected. The first 100 flock owners that agreed to participate were included. After the start of data collection, seven case flock owners decided to withdraw from the study because of various reasons. In total, 93 case flocks met the inclusion criteria and agreed to participate. Control flocks were invited to participate through a publication in a professional magazine for sheep owners, e-mail, and the Animal Health Service website. In total, 84 control flock owners that applied to participate met the inclusion criteria.