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Latest Schmallenberg virus results show transmission even in a very cold winter and spring 2013.

10th May 2013

I would like to bring to the attention of sheep farmers and veterinary colleagues the results of recent active surveillance for Schmallenberg virus (SBV). Blood samples were taken in mid-March 2013 from six, yearling female sheep in a large commercial flock on the edge of Dartmoor, Devon prior to sale. The sheep had been managed outdoors in this location since birth. The animals were housed in a large, well-ventilated livestock shed for 48 hours prior to sample collection; around 50 ‘in-lamb’ ewes shared the same airspace but had no physical contact.

Samples were submitted to AHVLA for testing using quantitative RT-PCR (qRT-PCR) for SBV RNA and indirect ELISA (iELISA) for IgG antibody. Two of the animals were positive for viral RNA, one of which had an inconclusive iELISA result (60%) and one a strongly-positive result (>100%). Four of the individuals were qRT-PCR negative but antibody positive. A second sample was taken 16 days later (early April); six animals were seropositive with increased values indicating recent infection and all were qRT-PCR negative. The first suspected clinical case of Schmallenberg in this flock was diagnosed in early April by gross pathology examination of an aborted fetus.

These results have potentially important implications. First, the timing of seroconversion indicates infection while the animals were outdoors in early/mid-March when local temperatures did not exceed 8°C and few, if any, vectors (Culicoides species) are expected to be active (Sanders and others 2011). The diagnosis of a clinically-affected fetus in April also indicates infection in mid-winter. This raises the possibility that there may not be a truly vector-free period. Sanders and others (2011) reported that Culicoides species were caught in traps at a site in the same region as the flock as early as week 10 (early March) during 2008. In their study, Culicoides punctatus and C. pulicaris emerged earlier in the year than members of the C. obsoletus group, which are thought to be vectors of SBV (De Regge and others 2012; Rasmussen and others 2012); therefore, it may be that SBV is transmitted by a wider range of Culicoides species than has so far been determined. Alternatively, there is the possibility of non-vector transmission of the SBV virus, as has been suspected with bluetongue virus in cattle (Menzies and others 2008).  

The simultaneous positive results for antibody and antigen indicate that the viraemic period may be longer than the 2‒5 days previously reported (Hoffmann and others 2012) in some individuals as the iELISA used is reported to detect antibody from around day ten post infection (Breard and others 2013). However, the detection of viral RNA by qRT-PCR does not necessarily indicate infectious virus and the two positive qRT-PCR results were negative when re-tested. Nonetheless, simultaneous antigen and antibody testing is recommended, especially for livestock being transported to SBV-free regions.

Our findings suggest that further research to understand SBV transmission and implications for the epidemiology of this disease is required.

Acknowledgements

We would like to thank Rachael Collins, Veterinary Investigation Officer, Starcross for her discussions and advice regarding this interesting case.

Authors

Peers Davies, Janet Daly

Pro-Ovine Sheep Vets and School of Veterinary Medicine and Science, University of Nottingham, Sutton Bonington Campus,

Loughborough, Leicestershire LE15 5RD; Corresponding author: Peers Davies email at svxpld@nottingham.ac.uk

 
 

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