Watching the research that surrounds the emergence of a novel virus is fascinating. I’m always amazed at how quickly scientists basically dissect the disease and the organism to find out what is going on.
Today’s post is going to look at Schmallenberg virus – a novel virus affecting livestock that was first identified only in November 2011.
The story actually begins before November 2011. It’s thought that the virus first started affecting livestock in Europe in summer 2011: cattle in Germany and the Netherlands started to show a few non-specific signs of disease (fever, diarrhoea, a drop in milk production). The animals generally got better again after a few days and herds were generally only affected for a few weeks. Samples were collected and tests were run to find out what was going on but the tests ruled out known viruses and it remained a mystery…
… until November. It was then that the virus was isolated by scientists who named it after the town, Schmallenberg, that the first positive sample came from. (I’m not sure how happy residents of Schmallenberg will be to have a virus named after their town!)
Since its first identification scientists have learnt a lot: SBV is very closely related to a subgroup of viruses in the Orthobunyavirus group. Other viruses in this subgroup are commonly transmitted from mammal to mammal by insects like midges and mosquitoes so it has been suggested that SBV may also be transmitted this way. This would fit in with the fact that the initial cases were seen in August and September – prime insect months.
The reason we (at least in the UK) started to hear of new cases at the beginning of 2012 is probably not because animals are still getting infected (in winter there are usually no midges or mosquitos around). We are seeing the long term consequences of animals infected by the virus when they were in their early pregnancy. Farmers are seeing an increase in the numbers of miscarriages and stillbirths of deformed young, especially in sheep, although cattle and goats have also been affected by the virus.
Since scientists first isolated SBV it has been grown in the lab and a very small number of cattle have been experimentally infected with it, resulting in a similar picture of non-specific symptoms as was seen in cattle in the summer of 2011. There is still a lot of work to be done in this area – we don’t yet know if animals can pass the virus directly to each other and we don’t know what is going to happen in 2012. Exposed animals may prove to have some immunity but what about those animals who were unexposed but are close to exposed ones?
SBV is currently not thought to be zoonotic: the European Centre for Disease Prevention and control states that it is “unlikely that this virus will cause disease in humans, but it cannot be excluded at this stage“. It actually must be quite hard to prove that a virus doesn’t cause disease in humans. You can do cell culture work and see if the virus affects particular cell lines, but barring injecting a large number of people with the virus, you basically have to say well x number of people have been exposed and no one has caught any disease (and you would have greater confidence the larger x is). (Well that’s how I understand it anyway – please correct me below if I am wrong.)
Countries across Europe are trying to get a handle on the disease and plan for the months when midges and mosquitoes will be around in 2012. In the UK, as of 30th March 2012 it had been detected 235 farms, mostly sheep farms. However the number of cattle herds affected may increase as those cattle infected in their pregnancy in late summer are now starting to calve. SBV is not currently a notifiable disease (although Defra does ask farmers to notify their vets if they suspect it) but I don’t know if this will change as the months go on.
I think it’s amazing how much scientists have learnt in the space of about 9 months since vets first saw the signs of infected cattle and I’ll be keeping a close eye out for the next set of results to be published.
Places to go for more info:
Scenarios for the future spread of Schmallenberg virus (Veterinary Record, 2012 170 245-246 doi: 10.1136/vr.e1598 – BEHIND A PAYWALL)
Lamb picture made available by Evelyn Simak under a Creative Commons Attribution-Share Alike 2.0 licence
Cow picture made available by CRV Arnhem under a CC-BY 3.0 licence