Wednesday, 23 January 2013

Lessons from Schmallenberg

Whilst I haven't spoken to one, being a UK sheep farmer at the moment can't be much fun. Schmallenberg virus (SBV) is robbing farmers of lambs as well as being an altogether disturbing and unpleasant experience. How long it remains an issue will only become apparent with time. It's worth looking at whether this outbreak of SBV represents a sign of what's to come.

The Hollywood films all seem to make a big issue of a virus 'going airborne'. That's unsurprising considering airbrone spread is probably one of, if not the, most efficient form of transmission when it comes to humans and their viruses, crammed together in offices and public transport. I'm not so sure it's the same for livestock viruses though, where the densities of animals may be locally high (i.e. within a farm), but with greater spaces between groups (i.e. between farms). Even though Foot and Mouth disease virus is airborne, and pig farms in particular can release sufficient amounts of virus into the air to infect other properties, the majority of the FMDV spread in 2001 in the UK was due to both direct contact between animals and indirect contact through so-called fomites. Arboviruses, on the other hand, are transmitted when insects find and bite animals, resulting in a form of transmission which is more heat seeking missile than carpet bomb. Both of the most recent two exotic livestock viruses to enter the UK (Bluetongue and Schmallenberg) have been spread by insects, more specifically, Culicoides midges. The fact that it's midge-borne viruses that have made it to the UK may be as a result of the midges being small enough to be blown in wind plumes, including across the channel from the continent where the viruses emerged. 

Wind plumes blowing across to the UK from the continent; the probable route of entry
into the UK for SBV (and a few years earlier Bluetongue virus).

Whilst wind may explain why midge-borne viruses reach the UK, it doesn't explain why it's midge borne viruses that have spread across the continent so fast. Why aren't mosquito-borne viruses going just as crazy? West Nile managed to spread from the US east coast to the west in just a few years.

Image from CDC.

Could it be insect vectors provide an efficient form of virus transmission providing the climate is right? It certainly seems so once it's introduced, although maybe not ideal for global spread when considering the relative immobility of livestock compared to humans. How Schmallenberg (or Bluetongue) arrived in the Europe to begin with is open to speculation; but the fact that it spread extremely fast across Northern Europe is undeniable. Within months of the initial observations in Germany, SBV had made its way to the UK and, within one summer, had more or less spread throughout the entire country.

The rapid spread of SBV; from covering North West Germany, Belgium and the Netherlands in the spring (top, 5th Jan 2012) to large swathes of Europe, including the UK, by the autumn (bottom, 26th October 2012).

And the future? Just like events such as the spread of West Nile virus in North America, we should perhaps regard it as a warning. Rift Valley fever seems to be the vogue virus in terms of a likely prospect for the future in Europe. Rift Valley, West Nile, as well as the majority of other candidates such as Chikungunya virus are all spread by mosquitoes; what about other Culicoides- borne viruses? First, African horse sickness; the most lethal infectious disease of horses, with up to 90% mortality, is a close relative of Bluetongue, as is epizootic haemorrhagic disease virus which, whilst still only on the fringes of Europe, already causes problems in the US. Human viruses? Oropouche virus (like SBV, an orthobunyavirus) can cause a dengue-like illness. Oropouche currently seems to be limited to South America; not many people would right now put a lot of money on it making its way to the UK. Before 2006 though, nobody thought Bluetongue virus would seriously enter Northern Europe, let alone go on to infect the vast majority of sheep and cattle in the region. Uncertain times are ahead. 

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