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WE wish to draw attention to the recent detection of Schmallenberg virus (SBV) in lambs in southern England.
Since mid-March we have seen a marked reduction in the number of suspect and confirmed cases from sheep farms. This was expected, as the lambing season for the majority of flocks is ending. However, the submission of some lambs since April (five to date, the last so far submitted to us on May 1), seemingly born at term with typical gross pathology for SBV and confirmed by PCR, emphasises the need for continued vigilance. Taking the current hypothesis of SBV infection and lambing periods into account (European Food Safety Authority 2012), which proposes a vulnerable period for fetal infection between days 28 and 50 of gestation, the latest cases in England, with parturition dates after April 20, 2012, were likely to have been infected with SBV between Christmas 2011 and the end of January 2012. This observation tests aspects of our current understanding of SBV epidemiology. By analogy with other orthobunyaviruses, the presence of active vectors is the most plausible explanation, indicating the existence of localised pockets of active infected vectors (perhaps inside animal housing) over winter months. It is also plausible that the susceptible period of infection for the fetus may start earlier in gestation than previously assumed.
While, in the absence of experimental evidence, direct horizontal transmission cannot be ruled out, the principal mode of transmission seems to be through Culicoides species midges, as for bluetongue virus. Accordingly, the risk of infection during winter is considered very low, but not impossible, as shown for bluetongue virus type 8, a case of which occurred in February 2008 in northern Germany (Hoffmann and others 2008). A recent study of Culicoides midges in England also indicated catches of female Culicoides obsoletus in every calendar month (Sanders and others 2011), albeit at very low levels during the winter period. These results may be of importance for the risk assessment regarding the re-emergence and further spread of SBV.
Although lambing is now coming to a close in many areas, we would wish to encourage the continued reporting of new suspect cases in lambs from flocks lambing out of season. Cases should be submitted in line with the case definition, which includes newborn lambs affected with arthrogryposis, via our network of regional laboratories, so that we may understand more of the epidemiology of the disease and ensure an accurate assessment of its impact.
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