Bluetongue virus – A Transcontinental Disease

If you’ve read my previous posts, you might have noticed that I’ve previously mentioned bluetongue virus, and this post is solely dedicated to that particular arbovirus. Bluetongue is an excellent example of a disease that will likely benefit from climate change, increasing in both frequency and range.

Part of the reason why the BTV-8 outbreak is so interesting, and indeed worrying, is that it wasn’t expected. Before 2006, most outbreaks had been limited to the Mediterranean Basin (Spain, Italy, southern France). This is due to the presence of Culicoides imicola, an Afro-Asiatic species of midge and vector of BTV. In fact, there hadn’t been a single case of BTV reported above 50°N (Szmaragd et al. 2010).

The first cases of BTV-8 were reported near Maastricht, the Netherlands, over 900km further north than previous outbreaks. By the end of the year, the disease would have spread to over 2000 farms and killed tens of thousands of livestock. It even reached the United Kingdom, infecting 125 farms by early 2008 (Szmaragd et al. 2010).

Map showing the distribution of Bluetongue virus serotypes. (Zientara et al. 2015)

While previous outbreaks can be attributed to Cu. imicola, the species was not found amongst the 100,000 Culicoides collected in the infected region (Elbers et al. 2007). This data revealed that species native to northern and central Europe, such as Cu. obsoletus, Cu. Pulicaris, Cu. chiopterus and Cu. dewulfi can transmit BTV (Gould & Higgs 2009).

Worryingly, it appears that they did not need a pre-adaptive phase in order to do so, which also raises the possibility that they can transmit other viral pathogens from the get-go. If true, it could result in diseases such as African horse sickness, Akabane virus and epizoonotic haemorrhagic virus being introduced into northern Europe (Elbers et al. 2007).

The BTV-8 outbreak demonstrates that the entire central and northern European region must now be considered at risk of BTV, and by extension, other tropical diseases transmittable by the Culicoides spp. Furthermore, evidence of overwintering of the BTV-8 strain demonstrates that the virus could persist in cooler regions and become endemic to northern Europe (Saegerman et al. 2008).

In general, bluetongue isn’t a disease that many people talk about. A lot of people talk about foot-and-mouth disease or bird flu, but bluetongue is a perfect example of a transcontinental virus, and one that could grow exponentially in the future.

In southern Europe, Cu. imicola peaks in late summer and autumn, however the northern vectors peak earlier in the year. This potentially opens the region up to continuous outbreaks starting from early summer and continuing into autumn (climatic conditions permitting). On top of this, outbreaks could last longer as Cu. imicola can be found alongside the Cu. obsoletus complex (Saegerman et al. 2008).

With BTV-8 being found in central and northern Europe, there is now a much higher chance of an encounter between two serotypes. This particularly true of the Mediterranean Basin which hosts BTV-1, 2, 4 and 16. The issue that this presents is that another serotype will considerably increase the potential for re-assortment (Saegerman et al. 2008).

To put this into perspective: The number of possible re-assortments for two serotypes of BTV is 1024. With three serotypes, this increases to 59,049 (Saegerman et al. 2008).

While the flight range of Culicoides is only 1 to 2km, they can be transported on the wind several hundred kilometres (Gloster et al. 2007) This means that there is quite a high chance that at some point, BTV-8 will come into contact with other serotypes (in fact, it already has in northern Spain).

Assessing the risk of the introduction of bluetongue-infected midges carried on the wind to the UK (Gloster et al. 2007)

And then there’s climate change. As the diagram above shows, higher temperatures increase the risk of transmission. The most efficient temperature for transmission is between 28°C – 30°C, but it can transmit at lower temperatures (Gloster et al. 2007). This means that as regional temperatures slowly increase, outbreaks could occur across Europe.

Regarding the UK, BTV is likely to become a persistent feature of the livestock industry, with the virus establishing itself within the country. Generally, outbreaks will become more common and widespread and the range of the Culicoides vector will increase (Brand & Keeling 2017). Alongside this, there is the threat that other viral pathogens could enter the country through the vector and once they do, they too could become endemic.

The 2006 outbreak is a story of a transcontinental disease. What was once considered a tropical virus, is now present across Europe. It is a warning that more devastating pathogens like African horse sickness can and may well reach northern Europe at some point.


References

Brand, S. P., & Keeling, M. J. (2017). The impact of temperature changes on vector-borne disease transmission: Culicoides midges and bluetongue virus. Journal of the Royal Society Interface14(128), 20160481.

Elbers, A. R. W., Mintiens, K., Staubach, C., Gerbier, G., Meroc, E., Ekker, H. M., … & Backx, A. (2007). Epidemiological analysis of the 2006 bluetongue virus serotype 8 epidemic in north-western Europe. Annexe2, 14.

Gloster, J., Mellor, P. S., Manning, A. J., Webster, H. N., & Hort, M. C. (2007). Assessing the risk of windborne spread of bluetongue in the 2006 outbreak of disease in northern Europe. The Veterinary Record160(2), 54.

Gould, E. A., & Higgs, S. (2009). Impact of climate change and other factors on emerging arbovirus diseases. Transactions of the Royal Society of Tropical Medicine and Hygiene103(2), 109-121.

Saegerman, C., Berkvens, D., & Mellor, P. S. (2008). Bluetongue epidemiology in the European Union. Emerging infectious diseases14(4), 539.

Szmaragd, C., Wilson, A. J., Carpenter, S., Wood, J. L., Mellor, P. S., & Gubbins, S. (2010). The spread of bluetongue virus serotype 8 in Great Britain and its control by vaccination. PloS one5(2), e9353.

Zientara, S., Sailleau, C., Bréard, E., Viarouge, C., Doceul, V., & Vitour, D. (2015). Benefits of PCR and decentralization of diagnosis in regional laboratories in the management of Bluetongue in France. Veterinaria italiana51(4), 393-399.

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