Source: Baylis, M. (2017). Potential impact of climate change on emerging vector-borne and other infections in the UK. Environmental Health, 16(1), 45-51.
You would have seen in my recent posts about the impacts of climate change on disease, however those have always concerned tropical regions. In the UK, we’ve always been relatively safe, due to our temperate climate that isn’t suitable for many diseases. This may not be true for the future though, and there is a noteworthy risk of tropical diseases either creeping north through Europe, or being brought in through travel.
There is undoubtedly some link between climate and disease. A review of 157 human and animal pathogens found that 66% had at least one climatic variable affecting disease occurrence (McIntyre et al. 2017). Another found that 26 (49%) out of 53 reportable infectious diseases in Europe were either directly or indirectly linked to climate (Lindgren et al. 2012). However, currently climate is probably not a leading factor affecting disease, as an analysis of 300 disease outbreaks only attributed 3% to climate and weather (Jones et al. 2008).
The main threat that the UK would face comes from vector-borne disease, notably mosquitoes, sand-flies and ticks. These types of diseases usually have geographical restrictions because of the impact on the insects, however this also means they are liable to spread if the climate becomes suitable for them. While arguments regarding the link between climate change and disease are rife, it’s worth noting that cases of vector-borne diseases have increased in Europe, as has the spread of vectors.
We’re going to start with the West Nile Virus (WNV) which is endemic in some southern and eastern European countries and spread by the Culux genus of mosquito. The number of incidences of WNV is increasing and the vectors are very sensitive to changes in temperature. In regards to the UK, Culux modestus has been discovered in southern England, having not been reported since the 1940s. It coincides that is period was the warmest recorded in England, until the present. Therefore, it wouldn’t be unrealistic to suspect that we may see WNV cases in the UK.
The important characteristic that enables mosquitoes to act as vectors is that they need to be able to live long enough for the pathogen to develop. For this reason, native species of mosquito may be a greater risk because they’re better suited to our climate. One excellent example is bluetongue, a midge-borne viral diseases affecting ruminants. It’s found in Mediterranean countries where it’s spread by an invasive midge species from Africa and wasn’t believed to be transmitted by indigenous midges. However, during 2006-2009, it spread across farms in northern Europe, including the UK.
This opens up the possibility of diseases being able to move across regions by bouncing between different species. And on that note, I’d like to introduce Ochleratatus detritus, a UK native mosquito that feeds on humans and is a good vector for flaviviruses, the viral group that includes WNV, dengue and yellow fever.
We next move onto sandflies which can spread cutaneous and visceral leishmaniasis in both people (low incidence) and dogs (high incidence). While present in central and northern France and southern Germany, they aren’t currently found in the UK. There is the possibility that infected dogs could bring the disease in through travel, therefore introducing it to the UK. Leishmaniasis causes sores to develop and while cutaneous leishmaniasis do heal over time (but can leave scarring), severe visceral leishmaniasis is typically fatal if left untreated.
Lastly, we’re covering ticks, notably sheep ticks (Ixodes ricinus). There’s two emerging diseases that are worth writing about, Lyme disease and tick-borne encephalitis (TBE), the emergence of which is a major concern. Lyme disease is already present in the UK and is increasing in incidence, possibly due to the influence of climate changes on the tick vector. It’s also been reported to be spreading through Sweden, likely linked to milder winters. TBE occurs as far north as Scandinavia, indicating that it could occur in the UK. However, currently there haven’t been reports of it. Lastly, there is also Crimean Congo Haemorrhagic Fever (CCHF) which is endemic to eastern and south-eastern Europe and is also emerging.
When could this happen by?
Modelling suggests that by 2030, the UK will be climatically suitable for P. vivax malarial transmission for 2 months of the year in southern England, and 4 months per year in southeastern England. By 2080, even southern Scotland will be climatically suitable for 2 months of the year. Though it may become endemic, it is unlikely to pose a major health problem. However those in low-lying salt marshes should take precautions.
The figure to the left indicates how many months P. vivax malaria could spread through the UK (Vardoulakis & Heaviside, 2012).
Many parts of the UK may become climatically suitable for dengue, chikungunya and Zika viruses via Ae. albopictus in the future. Worryingly in 2016, Ae. albopictus eggs were found in southern England. In the US, Ae. albopictus was introduced in used car tyres and is spreading rapidly (Vardoulakis & Heaviside, 2012).
Risk of transmission will increase with temperature, likely peaking in summers where droughts will create breeding sites from small pools left in river beds and water butts. As people will spend more time outdoors and windows will be left open, there will also be a higher exposure to mosquitoes.
With ticks, it’s more complicated because of the conflicting forces of higher temperatures and drier summers. Warmer temperatures, notably winters, will accelerate development and eliminate diapause (the period through winter where ticks don’t feed). However, drier summers will limit tick host-seeking activity and increase mortality.
Jones, K. E., Patel, N. G., Levy, M. A., Storeygard, A., Balk, D., Gittleman, J. L., & Daszak, P. (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990-993.
Lindgren, E., Andersson, Y., Suk, J. E., Sudre, B., & Semenza, J. C. (2012). Monitoring EU emerging infectious disease risk due to climate change. Science, 336(6080), 418-419.
McIntyre, K. M., Setzkorn, C., Hepworth, P. J., Morand, S., Morse, A. P., & Baylis, M. (2017). Systematic assessment of the climate sensitivity of important human and domestic animals pathogens in Europe. Scientific reports, 7(1), 1-10.
Vardoulakis, S., & Heaviside, C. (2012). Health effects of climate change in the UK 2012. London: Health Protection Agency, 10, 1600-0668.