Summer Buzz

Polyxeni Potter

Author affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA

Schmallenberg Virus among Female Lambs, Belgium, 2012

François Claine, Damien Coupeau, Laetitia Wiggers, Benoît Muylkens, and Nathalie Kirschvink

Author affiliations: Namur Research Institute for Life Sciences, University of Namur, Namur, Belgium

Abstract

Reemergence of Schmallenberg virus (SBV) occurred among lambs (n = 50) in a sheep flock in Belgium between mid-July and mid-October 2012. Bimonthly assessment by quantitative reverse transcription PCR and seroneutralization demonstrated that 100% of lambs were infected. Viremia duration may be longer in naturally infected than in experimentally infected animals.

During late summer and fall 2011, a nonspecific febrile syndrome characterized by hyperthermia, decreased milk production, and diarrhea occurred among lactating cows in Germany. A new virus, named Schmallenberg virus (SBV), was identified as the cause (1). This arbovirus of the genus Orthobunyavirus, family Bunyaviridae, affects domestic and wild ruminants and has been documented in Western European countries since 2011 (2). The most notable consequences of this new pathogen are caused by its ability to cross the placental barrier. Depending on the gestational age of the offspring, abortion, stillbirth, or severe congenital malformations, including arthrogryposis and defects of central nervous system, might occur (3,4). Transplacental infection of offspring that occurred during 2011 led to economic losses in animal husbandry of sheep, goats, and cattle during birthing periods occurring during November 2011 through spring 2012 (5,6).
Several vectors of SBV have been identified. Biting midges, small flying insects of the species Culicoides, were vectors for serotype 8 of bluetongue virus that emerged during 2006 in Europe (7), and they seem to play a key role in spreading SBV (8,9). Similar to distribution of serotype 8 of bluetongue virus in 2007 (10), SBV circulation occurred during 2012 in regions where viral circulation was limited or not yet detected in 2011 (11). However, few investigations of acute viral circulation in regions where most of the ruminant livestock were infected during 2011 have been performed. The high in-flock seroprevalence ranging from 70–100% in regions documenting SBV outbreaks (i.e., North Rhine-Westphalia in Germany, the Eastern part of Belgium, and the southern part of the Netherlands) is believed to limit reemergence of SBV (12).
The objective of this study was to assess whether SBV reemergence occurred in a sheep flock that had experienced an SBV infection outbreak during autumn 2011 and reached a seroconversion rate of 99.5% (13). Female lambs born in late autumn 2011 or early winter 2012 were followed bimonthly to assess natural SBV primary infection by using quantitative reverse transcription PCR (RT-qPCR) and seroneutralization (SN).

The study

Sheep of a flock that belonged to the University of Namur that included ≈400 ewes (Ile de France, Laitier Belge, French Texel, and crossbred) and ≈20 rams were investigated. During the lambing period of January 2012, 28 (17%) of 163 newborn lambs showed signs of congenital SBV infection that was confirmed by real-time RT-qPCR and SN assay (13). Two SBV strains were isolated in cell culture of brain tissue samples from congenitally infected lambs (14). Retrospective analysis of sentinels’ serum samples collected monthly in 2011 indicated that SBV infection of the flock occurred after September 15, 2011. SN assays were performed for all animals (n = 450) of the flock in January 2012 and revealed a seroprevalence of 99.5% (13).