We return to the theme of ‘Silent Killers’, disease risks that, whilst applicable to all livestock, can pose a disproportionate threat to rare breeds because of their potential impact on small populations.

Schmallenberg Virus was first identified in November 2011 as the cause of a disease outbreak in adult cattle reported in both the Netherlands and Germany. Symptoms included mild to moderate fever, reduced milk yield, loss of appetite, loss of body condition and diarrhoea. Testing for common causes of diarrhoea proved negative but from December 2011, abortions and stillbirths associated with foetal abnormalities, affecting mainly sheep, but also cattle and goats, were identified in the Netherlands, Germany and Belgium. A new virus was identified and named Schmallenberg Virus (SBC) after the town in Germany where the first identification took place.

SBV belongs to a group of viruses occurring in Asia, Africa and Australia but not identified in Europe prior to 2011. The virus is spread by biting midges of the culicoides type and does not spread directly from animal to animal. Its spread is closely linked to the number of midges, which typically peak in late summer/early autumn and drop sharply once frosts begin. SBV first appeared in the UK in 2012/13 when it is thought to have arrived with windblown midges in south eastern counties,
spreading inland and north.

At the end of 2013 the APHA had confirmed virus infection on 656 holdings, though subsequent serology testing indicated that all areas of the UK below the Scottish borders had been exposed, with between 25 and 75% of animals having antibodies, depending on location. Individual herd or flock losses of new-borns due to the virus vary, but on average SBV-affected flocks report an extra 3% lamb mortality, with some flocks experiencing 50 to 60% losses due to SBV.

Following the initial outbreak, sheep born in the southern counties in 2014/15 did not seroconvert to the virus at all, indicating that it did not circulate during those years, and as a consequence total herd immunity dropped. This created the conditions for a re-emergence of the virus and in 2016/17,
SBV clinical cases began being reported in large numbers in the UK.

Experts predict that the scenario of years with large outbreaks followed by years with little circulation of the virus is likely to continue. Experience with Akaban virus, very similar to SBV, in Japan has shown that large outbreaks tend to occur every three to five years, with risk higher in warm years with high midge numbers and in years where seroprevalence in the national herd is low.
Acute clinical disease in adult cattle presents as indicated when the virus first manifested in Europe, while adult sheep and goats generally do not show signs of clinical disease. The biggest problem, however, is that the virus crosses the placenta to affect the growing foetus in pregnant animals with the most susceptible stages of pregnancy for foetal deformities are days 62 to 180 in cattle and 25 to 50 in sheep. The virus damages foetal nerve tissue and results in brain and spinal cord abnormalities with secondary problems with the muscles and skeleton leading from the nerve damage. Deformities vary depending on when infection occurred during pregnancy and in sheep there may be only one lamb out of a multiple birth affected.

Although several vaccines were released after the initial SBV outbreak, they are not currently commercially available. Schmallenberg virus is not a notifiable disease, but it is recommended that any suspicious cases be reported to APHA for testing.

Source: NADIS: Author: Phil Scott BVM&S DVM&S DIPECBHM
CERTCHP DSHP FRCVS. Reviewed: Dr Rachel Tarlinton PhD
BVSc MRCVS 2018 and Richard Laven PhD BVetMed MRCVS
2017