Schmallenberg Virus: What are the facts?

Schmallenberg virus (SBV) was first encountered in October, 2011 in Northern Europe.1 The virus was initially isolated from a sample from the town of Schmallenberg, which is how it got its name. The virus is associated with clinical disease in ruminants, both domesticated and wild, and can cause a wide variety of clinical signs. It is transmitted by biting midges (Culicoides spp.), much like bluetongue virus, and can cause extreme problems in infected animals.

What is Schmallenberg Virus?

SBV is a single-stranded RNA virus with a protein envelope which helps it to infect mammal cells. These protein interfere with the cell’s interferon production which decreases the immune response to the virus and increases its virulence.2 It is part of the Simbu serogroup of Orthobunyaviruses, which is a large and well-known group of viruses known for causing significant diseases in humans, livestock and crops across the world.3

What are the signs of Schmallenberg infection?

The most significant clinical effects of Schmallenberg virus are malformations of newborns.
The most significant clinical effects of Schmallenberg virus are malformations of newborns.

When adult animals are infected, the symptoms of Schmallenberg tend to be quite mild. Cattle sometimes show decreased milk yield, high temperatures and occasionally diarrhoea, and sheep rarely show and clinical signs. There is some evidence to suggest that adult goats show decreased milk yield, but this is not common.4

The primary and most devastating effect of the virus is malformation and abnormalities in newborns of infected animals. When a pregnant female becomes infected, the virus can transfer to the foetus through the placenta. This can cause abortion, stillbirth, and congenital defects. Even in pregnancies that go to term, the likelihood of survival for these newborns is extremely low and there is a significant risk to the mother’s life as these deliveries can be difficult.5

The 2011-2013 Outbreak

The outbreak began in the autumn of 2011 in Northern Europe and spread quickly across Europe. By May of 2013, twenty-two countries had reported cases of Schmallenberg virus (SBV) and 75% of all herd and flocks of cattle, sheep and goats were affected in some way.6 Although many farms only had a few cases of newborn deformity, in some, 50-60% of lamb mortality was due to SBV.

How does Schmallenberg virus spread?

The virus is transmitted via insect vectors. These are predominantly Culicoides midges. Direct transmission between animals is unlikely, and the midges do not transmit the virus between each other. The virus is transferred when a midge sucks the blood of an infected animal, and then goes on to feed from an un-infected animal.7

While these midges are very small, usually between 1-3mm, and cannot fly very far, they are able to travel long distances with help from wind currents. They are also highly seasonal, as their reproduction relies on fresh water and mild temperatures. Extreme cold and lack of moisture cause immediate declines in populations, so spread of the virus tends to occur during late summer and early autumn.

How can the risk of SBV be managed?

There is no treatment currently available for SBV, but there is a vaccine against it.8 Infected adult animals recover quickly and develop a strong immune response to SBV, preventing deleterious effects upon re-infection. Youngstock can be exposed intentionally through vaccines, or by keeping them outside during likely periods of transmission.

Pregnancy is the most crucial time for preventing infection, so in some areas, it could be practicable to keep pregnant animals indoors during high-risk times if they are unvaccinated. While insecticides can be effective in reducing midge populations, this is rarely economically or practically viable, as the lifecycle of Culicodes is very short.

Is Schmallenberg virus still a threat?

In short, yes. While the incidence of SBV in populations across Europe may be low at the moment, there is still the potential for the disease to spread. In 2016, there was a second smaller epidemic of SBV across Europe and subsequent economical losses.

As a virus becomes less of an immediate threat, producers tend to reduce the amount of effort placed on management and prevention. This then may open a herd or flock up to infection. As herd immunity reduces in a flock, the likelihood and prevalence of SBV increases.2

As such, it is vital to maintain vigilance and management strategies for such dangerous diseases. Veterinary professionals can advise on how to protect your flock or herd and on whether vaccination is a good idea.

References

  1. Liu, D. Molecular Detection of Animal Viral Pathogens. (CRC Press, 2016).
  2. Endalew, A. D., et al. Schmallenberg Disease—A Newly Emerged Culicoides-Borne Viral Disease of Ruminants. Viruses 11, (2019).
  3. Elliott, R. M. Orthobunyaviruses: recent genetic and structural insights. Nat. Rev. Microbiol. 12, 673–685 (2014).
  4. Helmer, C. et al. Survey of Schmallenberg virus (SBV) infection in German goat flocks. Epidemiol. Infect. 141, 2335–2345 (2013).
  5. Stokes, J. E. et al. Survey to determine the farm-level impact of Schmallenberg virus during the 2016–2017 United Kingdom lambing season. Vet. Rec. 183, 690–690 (2018).
  6. Afonso, A. et al. The Schmallenberg virus epidemic in Europe—2011–2013. Prev. Vet. Med. 116, 391–403 (2014).
  7. Sick, F., et al. Biting Midges—Underestimated Vectors for Arboviruses of Public Health and Veterinary Importance. Viruses 11, (2019).
  8. Wernike, K. & Beer, M. Schmallenberg Virus: To Vaccinate, or Not to Vaccinate? Vaccines 8, (2020).