Synthetic Schmallenberg virus made in the lab
16 January 2013
Schmallenberg virus (SBV) is an emerging disease of livestock such as cattle, sheep and goats. Since its discovery in November 2011, the virus has spread very rapidly to many European countries. Although the virus causes only a mild disease in adult animals, infection of susceptible pregnant animals can be associated with outbreaks of birth defects, abortions and stillborn animals.
A team of researchers from the MRC Centre for Virus Research at the University of Glasgow used molecular biological methods to design and assemble the SBV genome completely in a test tube in a form that can be easily introduced and replicated in cultured cells. From these cells, the researchers recovered virus with identical infection properties to the naturally occurring SBV.
Using this approach, known as 'reverse genetics', allowed them to control the viral genome and identify a gene called NSs, which is involved in protecting the virus against the immune response of infected animals. The researchers made viruses missing the NSs gene and found that mice infected with this strain became less sick than those infected with viruses containing the NSs gene.
The researchers also discovered that SBV grows rapidly in the brain and spinal cord of aborted lambs and calves. The virus prefers to infect cells called neurons, which explains why it infects and damages the brain.
This results in muscular defects, such as abnormally flexed legs, that are often seen in stillborn animals when the virus is transmitted from an SBV-infected mother to the calves or lambs in the uterus during pregnancy.
It's thought that SBV is transmitted by midges, which helps to explain how quickly it spreads. Professor Massimo Palmarini, who led the study, explains "Viruses transmitted by insects (known as 'arboviruses') are increasingly becoming a problem throughout the world while years ago they were limited mainly to the tropical areas of the globe.
"The spread of arboviruses is probably the result of several factors, including increase in travelling and commercial exchanges, climate and ecological changes, and increased livestock production. This study will help us to understand how Schmallenberg virus works, but it can also serve as an example for other related viruses that may emerge in the future."
The study, published online last week in the journal 'PLoS Pathogens', was funded by the Medical Research Council and the Wellcome Trust.
Image: The brain of a calf infected with SBV who was born with porencephaly. Credit: M Varela et al (2013).
Varela M et al. Schmallenberg virus pathogenesis, tropism and interaction with the innate immune system of the host. PLoS Pathog 2013;9(1):e1003133.