The origin of Schmallenberg Virus and the need for more surveillance.

Last week we finally got the answer to where Schmallenberg virus came from. At least genetically speaking that is (we still don't know from what geographical region it was nor whether it had been in Europe this whole time). But we do now have some clues.

It has come to light that this previously unheard-of microbe is a mixture of two previously known and closely related viruses: Sathuperi and Shamonda viruses. Writing in Archives of Virology earlier this month, a Japanese group (Yanase et al, from the Japanese National Institute of Animal Health) delved into the depths of bunyavirus genetics and uncovered Schmallenberg's closest cousins by sequencing a number of other viruses from Africa and Australiasia. Sadly this paper is not open access.

I wrote about Schmallenberg virus soon after it was discovered earlier this year. This was the virus that popped up in sheep and cows last summer then by the time the next Spring came we quickly realised it's aftermath: it had induced a number of malformations in their young - who at the time of infection were in the womb. These often times had fatal consequences. 



Back then we had no idea where this virus had come from as it's genome sequenced really looked like nothing we had seen previously. The original paper only used one small part of the viruses genome to trace it's ancestry due to the low amount of sequence data for these viruses. Although it bore a distant relation to known viruses there were some significant gaps in our knowledge of genomes from this group of viruses.

A bunyavirus (from ViralZone). Note the three segments of genome.

You see Schmallenberg is a Bunyavirus, a group of single-stranded, negative-sensed RNA virus. But the special thing about these guys is that they are segmented. Just like influenza is. And we all know what flu likes to do with it's segmented genome: it likes to reassort and swap bits and pieces of it's self around. A bit like virus sex. Well Bunyaviruses do this as well and it turns out so did the direct ancestor to Schmallenberg.

The three segmented genome. Schmallenberg had the S and L of Shamonda and the M or Sathepuri. (From Viralzone)

When the Japanese group compared the Schmallenberg genome to those of the other viruses that they had just sequenced, the true ancestry of this deadly virus emerged. It was strikingly clear that it's entire genome did not share the same genetic history. Two of it's three segments were very closely related to the Shamonda viruses while the last segment seemed to have a different story to tell: it was more closely related to another, distinct virus called Sathepuri virus.

All this indicates is that at some point in time, two different viruses (Shamonda and Sathepuri) infected the same cell - maybe in an insect, maybe in a mammal - out came a entirely, never-before-seen virus. This virus somehow made it's way to North-West Europe and started infecting various biting insects and farm animals. We can't yet say where this occurred, nor can we say when but what we can say is that it definitely happened. We also can't be sure of what genetic changes the virus had to make in order to function as this kind of chimera and for it to spread into a new geographic niche.

The situation from Influenza (From Virology Blog). Just the same 

The one issue with this work is that of undersampling. We know so little about the genomic diversity of this group of virus and currently have very little data to compare Schmallenberg to. What we need is to sequence a whole range of isolates from across all continents in order to truly answer the question of where this virus originated. 


And even then, this information will be of little use and may even be used to point the finger of blame. We need to hope that Schmallenberg doesn't come back again in the next couple of years and then if it does, we are ready for it this time. The only way this will happen is with increased global recognition and surveillance of these viruses. 

Reference:


Yanase, T., Kato, T., Aizawa, M., Shuto, Y., Shirafuji, H., Yamakawa, M., & Tsuda, T. (2012). Genetic reassortment between Sathuperi and Shamonda viruses of the genus Orthobunyavirus in nature: implications for their genetic relationship to Schmallenberg virus Archives of Virology DOI: 10.1007/s00705-012-1341-8