Schmallenberg virus (SBV)

What are the clinical signs of the disease?

The severity of Schmallenberg disease varies among different species and ages of animal.

Acute clinical signs of the Schmallenberg virus are visible in adult cattle, however, adult sheep do not generally show signs of clinical disease.

The biggest problem with the virus is that it crosses the placenta and affects growing foetuses.

Clinical signs of Schmallenberg include:

• Abortions
• Stillbirths
• Foetal abnormalities
• Reduced milk yield
• Fever
• Loss of appetite
• Loss of body condition
• Diarrhoea

Source: National Animal Disease Information Service

Impact on pregnancy and foetuses

The greatest impact of Schmallenberg virus is that it crosses the placenta, affecting growing foetuses if the animal is pregnant at the time of infection.

If ruminants become infected during pregnancy, it can lead to abortion or malformations in the foetus. The outcome is likely determined by the stage of pregnancy.

The most susceptible stages of pregnancy for foetal deformities are days 62–180 in cattle and 25–50 in sheep. It is reported that early infection before this results in early embryonic death and therefore abortion or a high barren rate at scanning.

Malformations include:

• Bent limbs and fixed joints
• Twisted neck or spine
• Domed appearance to the skull
• Brain deformities
• Damage to the spinal cord

These malformations are particularly problematic for block-breeding systems.

In early lambing flocks (January/February), there are high reported losses and malformations from the disease. This is because infection occurs during the vulnerable gestation period (peak midge levels in August/September). However, cases have also been reported in March-lambing flocks.

The length of gestation is also important. Foetal malformations from Schmallenberg disease are observed first in lambs; malformations in calves are observed once spring calving starts.

Spring-calving herds tend to be affected more than those calving at other times of the year because infection occurs during the vulnerable gestation period.

Affected pregnancies in cattle also tend to be more spaced out than in sheep as cattle are not usually batch-mated as tightly.

During spring, we urge you to remain especially vigilant for signs of disease; report any suspicions to APHA or your vet.

It is important to remember that not all birth defects are due to Schmallenberg virus. Intra-uterine infection with bluetongue virus (BTV) may cause similar brain lesions in calves and lambs. BTV is a notifiable disease, and suspected BTV cases must be reported immediately.

Lambing and calving difficulties

Malformations of lambs and calves caused by exposure to the virus in pregnancy may lead to lambing or calving difficulties.

Excessive force must not be used during lambing or calving as this may risk injury to both the ewe and lamb or cow and calf; contact your veterinary surgeon to discuss options for safe delivery.

Lambs or calves delivered alive with severe deformities must be euthanised for welfare reasons.

Diagnosis and testing

The APHA is continuing to offer free-of-charge testing in 2024 on samples from lambs and calves born with congenital malformations or musculoskeletal deformities.

A fresh brain sample (brain stem is preferred) will be tested free of charge if submitted to APHA.

Please contact your local Veterinary Investigation Centre for more information and to discuss appropriate sampling or the submission of a foetus with placenta.

Where is the Schmallenberg virus found?

Schmallenberg virus (SBV) was first identified in Europe in 2011.

There was an outbreak in the UK in 2012/13 when the virus was introduced via wind-blown midges. The outbreak started in south-eastern counties and then spread inland and north.

In 2012/13, the Animal and Plant Health Agency (APHA) confirmed Schmallenberg virus infection on 656 holdings, although subsequent serology testing indicated exposure in all areas of the UK below the Scottish borders.

How is Schmallenberg transmitted and spread?

Schmallenberg virus can affect all ruminant species and has been particularly evident in cattle and sheep populations.

Midges are key to the transmission of the Schmallenberg virus between animals. These midge vectors are infected with Schmallenberg virus after ingesting blood from infected animals. Without the midges, the disease cannot spread directly from animal to animal.

Spread of the virus is closely linked to midge activity, which typically peaks during periods of high temperature and rainfall in late summer/early autumn (August/September). As midges are believed to be the major route of transmission, it is expected that significant spread is less likely during winter, when midges are usually much less active due to the cold weather and frosts.

The Schmallenberg virus is not transmitted to humans through contact with animals, wool or the consumption of milk. There is no public health or food safety risk associated with Schmallenberg.

What can I do to prevent and control Schmallenberg?

A vaccine for Schmallenberg is not currently (February 2024) commercially available within Great Britain; however, there are a number of preventative strategies that you can implement.

• Reducing exposure of sheep and cattle to infected midges during the susceptible period (late summer/early autumn) may reduce the impact of infection. Sheep in the second month of pregnancy are particularly susceptible to the effects of the virus, leading to abortions and malformations of offspring.
• Shifting mating until later in the season (for sheep flocks) is probably the most practical solution for minimising Schmallenberg risk; however, midge numbers may remain high until October in years with a late onset of frosts.
• Serology may be used to assess previous exposure and therefore the likely susceptibility of a flock.
• Purchased stock may introduce infection into a farm, but as the period of viraemia – how long the virus is present in the blood – is short, the risk is likely to be low.

Find more actions to reduce the impact of Schmallenberg virus in sheep on the Sheep Veterinary Society website.

What should I do if I suspect Schmallenberg?

Schmallenberg virus is not a notifiable disease in the UK, and no restrictions are placed on infected premises. However, farmers and vets should remain vigilant and report any suspicious cases to the APHA for free-of-charge testing as part of enhanced surveillance. 

Farmers are advised to contact their vet if they encounter symptoms and Schmallenberg virus infection is suspected.

Bluetongue (BTV) intrauterine infections can cause similar lesions in the brains of calves and lambs to Schmallenberg. There are currently positive cases of BTV in England (February 2024). Therefore, all suspected cases of Schmallenberg need further investigation.

Further information