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WHAT stopped bluetongue taking off in the UK in 2008, despite having arrived in south-eastern England the year before and having already become well established, and continuing to spread, in continental Europe? In an article on pp 384-387 of this issue, researchers from the Met Office and the Institute for Animal Health examine this question and seem to come up with a pretty clear answer.
In the article, the researchers - from the same team that successfully predicted the arrival of bluetongue in the UK via infected midges borne on the wind from the Continent in 2007 (VR, March 31, 2007, vol 160, pp 422-426) - describe how they used meteorological and epidemiological data to assess the number of occasions on which the disease might have arrived by this route in each of the years 2006, 2007 and 2008. For the disease to reach the UK in this way, a number of criteria must be met simultaneously. There must be a substantial source of infected midges on the Continent near to the UK, suitable conditions for the transmission of midges and susceptible livestock at their new location. In 2006, when bluetongue virus serotype 8 (BTV-8) first appeared in northern Europe, there were few occasions when this might have happened. In 2007, however, the three criteria were met on a number of occasions and the disease actually arrived as predicted.
In 2008, there was a risk of infection from two possible sources. First, the virus could have overwintered in the UK in the vicinity of the outbreaks that had occurred during the previous autumn. Secondly, if the disease again became established in areas of the Continent near the UK, there could be further windborne introductions of infected midges. In the event, the disease did become established on the near Continent, and the researchers found that temperatures were favourable for virus replication and midge activity, and that suitable winds were present on many occasions to blow any airborne midges towards the UK. However, there were no outbreaks of bluetongue in the UK in 2008, and the authors attribute this to a dearth of susceptible animals as a result of the voluntary vaccination programme that was gradually 'rolled out' from the beginning of May when vaccine started to become available.
The question now must be, can the apparent success in preventing bluetongue in 2008 be maintained in 2009? There is the danger that success can lead to complacency, but it is vital that the momentum achieved during last year's vaccination campaign is not lost. This, clearly, is less of an issue in Scotland, where a compulsory programme of vaccination against bluetongue has been under way since last November, but it is certainly an issue in England and Wales, where vaccination remains voluntary and concern remains about whether the necessary coverage will be achieved. Even in 2008, when efforts to promote the uptake of vaccination by farmers were buoyed by the relative novelty of the prospect of the disease and Defra's advance purchase of vaccine to help kick-start the campaign, reports suggested that uptake was patchy, with good coverage being achieved in south-eastern England but uptake tailing off elsewhere. This could partly be a result of the way vaccination was rolled out in England and Wales, but it could also be due to a misguided belief that vaccination might not be necessary. Vaccination remains the only effective way of preventing bluetongue and such misconceptions need to be dispelled. Things will not be helped this year by Defra's decision not to underwrite the purchase of additional vaccine for livestock keepers in 2009 beyond the surplus stocks left over from 2008, but the hope must remain that farmers will take heed of the 'Don't hesitate, vaccinate' message of the Joint campaign Against Bluetongue and make sure they vaccinate their animals in good time.
The article on pp 384-387 is mainly concerned with windborne spread of bluetongue, but it draws attention, too, to the risks associated with movement and imports of livestock from countries or areas affected by the disease. Although primarily concerned with BTV-8, it also raises concerns about threats to the UK if BTV-1 re-emerges in Brittany in France this year or if BTV-6 reappears in the Netherlands. It makes clear that the UK is far from out of the woods as far as bluetongue is concerned, and that the disease will continue to present new challenges for some time to come.