Schmallenberg: how to spot and report the disease

Schmallenberg virus (SBV) is a virus first identified in Europe in 2011. It was first detected in the UK in 2012, and then in 2016-2017 and from 2021 onwards. 

It is spread by biting midges, can infect sheep, cattle and goats and is notable for congenital malformations especially in lambs. Adult animals may show mild general signs if any at all. 

SBV is not notifiable in the UK, no restrictions are placed on premises where SBV is present and SBV is not known to be a risk to human health.

Individual herd or flock losses of neonates due to Schmallenberg vary but on average, in 2013 SBV affected flocks reported an extra 3% lamb mortality than those flocks not affected, which is relatively a low impact. However, some flocks experienced 50-60% losses due to SBV which is severe. 

The impact of the disease will be influenced by factors such as midge activity and the immunity status of herds and flocks.

Animals exposed to the disease seem to develop immunity, making it likely that disease will die out in areas where the disease has been present. However, immunity will fade with the turnover of livestock and disease can return. 

Clinical signs

There is evidence from Great Britain and Europe that many animals have been infected with SBV without any clinical signs being detected. Typically, the impact in most herds or flocks affected by SBV has been low, although some farms have reported more significant effects. 

Related viruses are known to stimulate a strong immune response, which then protects infected animals from subsequent ill effects. This means that they do not usually give birth to further deformed offspring. It is expected that in the long term SBV will behave in a similar manner, although monitoring will continue as SBV is a relatively new disease.

Adult animals

In adult cows acute infection results in diarrhoea, fever and reduction in milk yield, with a full and rapid recovery over several days. Affected herds typically see outbreaks of disease lasting two to three weeks, but the possibility of other patterns of disease occurring in a herd should be considered. This stage of the disease has not been noted in adult or growing sheep, although there is anecdotal evidence of milk drop in milking sheep in Netherlands.

New-born animals and foetuses

The second presentation of the disease is associated with abnormalities in animals born alive or dead at term or aborted following infection of the dam, affecting mainly sheep but also cattle and goats. Malformations can include bent limbs and fixed joints (arthrogryposis), twisted neck or spine, a domed appearance to the skull, short lower jaw and brain deformities (fluid-spaces in the brain, abnormally small brain parts (such as cerebellum and brainstem) and marked damage to the spinal cord. Some animals are born looking normal but have nervous signs such as blindness or a ‘dummy’ presentation - uncoordinated movement, recumbency, an inability to suck and sometimes convulsions. It is suspected that the degree of foetal deformity depends on when in pregnancy infection occurred.

How Schmallenberg is spread

Typically orthobunyaviruses are primarily spread by biting insects, such as midges, however the routes of SBV transmission have yet to be confirmed. The potential for direct transmission (i.e. from one animal to another) is thought unlikely. 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.

Vaccinations

There is a vaccine but due to lack of demand from within GB industry, it is not currently being manufactured. Livestock farmers should seek veterinary advice if they are considering vaccinating stock against SBV.

Human health implications

A Europe-wide risk assessment concluded that Schmallenberg virus is very unlikely to cause illness in people. No human cases have been detected in any country, and the most closely related viruses cause only animal disease.

Biosecurity

Biosecurity is about being aware of the ways disease can spread and taking every practical measure to minimise the risk of disease spreading. The advice details practical things you can do on your farm to help prevent the introduction and spread of Schmallenberg to and from your animals.