Biting bugs are set to benefit from climate change — here’s why that’s a problem
In the UK, getting an insect bite is usually more of an annoyance than a health risk. But could that change as our climate gets warmer? We talked to Professor Matthew Baylis, Oxenhale Chair of Veterinary Epidemiology and Director of the Pandemic Institute at the University of Liverpool, to find how climate change is affecting a class of diseases known as vector-borne diseases.
Hi Matthew. What exactly is a vector-borne disease?
It’s an infectious disease which is spread by insects or ticks and occasionally some other organisms. So good examples would be malaria, which is spread by mosquitoes, or Lyme disease, which is spread by ticks. There are hundreds, thousands of vector-borne diseases and they’re equally important to the health of humans, animals and plants.
What do these diseases have to do with climate change?
Insects and ticks are very strongly affected by the climate because unlike us, they can’t control their temperature or water content very effectively. As a result of climate change, we’re seeing many species changing their distribution — so species which live in slightly warmer conditions than we previously had in the UK are now finding that our climate is more suitable. And some of these species are disease vectors.
We are also seeing a change in the ability of vectors to transmit diseases. One of the key factors is how long the vector lives, and therefore how many hosts it can feed on after becoming infectious. If a mosquito only feeds once more in its life after becoming infected, it’s only got one chance to transmit malaria. If it feeds five times, that’s potentially five more hosts infected. Temperature also has complex effects on their feeding behaviour and how soon they can pass on an infection.
What could this mean for us in the UK?
In Europe, we see some species of mosquito which have gradually moved along the auto route system across the continent, often congregated around service stations where there are humans providing a good source of food. It seems likely that these insects sometimes get into a car or lorry or container and move on to the next service station. I think through that route we’ve begun to see some of these species being found in the UK. Usually, the Channel acts as a barrier that stops many insects expanding into the UK. But we are seeing more species starting to invade from France, and this may be connected to rapid transport through the Euro tunnel.
For example, the Asian tiger mosquito became established in southern Europe after being unintentionally introduced from Asia in the late 1970s, and has spread further into northern Europe since. Then in just the last four or five years we’ve begun to trap it in South East England in very low numbers.
It’s a very important species because it’s a good vector of some really important human diseases like dengue, Zika and chikungunya. It also transmits heartworm, which is a parasitic disease of dogs. I think it will be quite some time before we’re at any significant risk of that species transmitting these diseases, but I do think that time is coming.
We’ve also seen another species appearing that’s a very good vector of West Nile disease, which is a viral disease that circulates among birds but can cause fatalities in humans and horses. We haven’t had West Nile disease in the UK, but the vector is now quite well established in a large part of South East England, and it seems very possible to me that we would begin to see cases in the UK, as we’ve seen in northern Europe.
The vectors and diseases which we’re seeing a little south of us in France would be a clear risk to the UK as climate change continues, but we can’t discount invasions of diseases and vectors arriving from much further afield via aeroplanes or ships.
We see ticks coming into the UK on migratory birds, too — for example, from Africa, which is much further than a mosquito could travel — and they can also come in on our pets. We have seen new tick species in the UK in the last few years, important vectors of many animal diseases which I think can be attributed to the movement of pets over to Europe and then back to the UK.
You’ve mentioned that vector-borne diseases affect plants and animals too — does that mean they could affect our food supply as well as our health?
Yes — so vector-borne diseases can cause devastating epidemics in livestock, but I do think the greatest food security threats from vector-borne diseases are in plants. Insects like aphids and whitefly transmit a number of bacteria and viruses to a whole range of plants that can cause significant risk. We’ve already seen devastation of olive trees by bacterial disease in southern Europe for example, and big problems in the citrus industries in the United States.
Some of these are more connected to trade and travel moving pathogens around rather than climate change, but they do show the risks we’re taking — especially with trees that are tens or even hundreds of years old that can’t easily be replaced.
What impact could climate change have on vector-borne diseases on a global scale?
Vector-borne diseases are a problem worldwide, especially in tropical regions, but are becoming more of an issue in temperate areas. There are some parts of the world where it may actually become too hot for certain vector species and we’ll see a reduction in some diseases — but there will be many others that see invasions of new species.
In many cases, when a vector-borne disease expands into a new area it encounters a population that has never been exposed to it and has no immunity. And whether it’s humans or animals or plants, that can be particularly risky where there’s no prior immunity or experience of dealing it, and we can see particularly large outbreaks.
How easily can we predict how things will pan out in the coming decades?
We can predict with a degree of success where new diseases or new vectors might be introduced from and to, using information on where people are moving and where goods are being shipped. But we can also do predictions of what happens after a new disease arrives using mathematical modelling.
We do this in Liverpool with some vector-borne diseases, using data on how people and livestock move around the UK, and we’re able to develop an understanding of what helps that disease spread, what hinders that spread. And climate is a big part of that — so we can run models for some vector-borne diseases under today’s climate, but we can also turn the heat up to 2100 projections and see what we think will happen.
But it’s very hard to actually predict the scale of what might happen. My experience in epidemiology over the past 30 years is that whatever we predict is very rarely what actually happens — I often say that we are only one surprised epidemiologist away from an outbreak of dengue in London!
A really good example is the animal disease called bluetongue. It’s African in origin and spread by biting midges, and it used to occasionally spread into the hottest parts of southern Europe for epidemics of a year or two. I came into this field with the understanding that this was a disease that likes warmer climates, and that climate change might impact it over time, but that seemed like a very long way away. I published a paper in around 2006 saying I thought the UK wouldn’t be at any significant risk of bluetongue for at least another 20 years or so, and then to my horror, it appeared in the UK the following year!
Should we be worried? What can we do about these diseases?
My first message would be we don’t need to be highly worried. The disease risks we might face in the UK in the next few years may become similar to the risks that people face if they live in the south of France now — but most of us would still be very happy to go and live there! The processes driving this are very difficult — are we willing to change our freedom to travel and to trade to reduce the spread of vectors? I find that hard to believe.
As an island country, I do think we’re in a lucky position that we can strengthen biosecurity at our borders to prevent new vectors entering. So I do think that there’s a role for government to decide whether it wants to invest in that.
For example, it would be quite possible to spray cars and lorries with insecticides as they’re loaded onto the train to come under the Euro tunnel and remove any mosquitoes that may be stowing away.
Also, if we did start to get disease outbreaks, let’s say caused by some of the mosquitoes I’ve mentioned, then we’d be able to take some relatively simple steps to reduce that risk. Things like changing the way we store water in our gardens can have an impact on local mosquito populations. We’re not in that position yet and maybe we never will be, but there are options there to reduce vectors in and around our own houses.
What would your message be to world leaders meeting in Glasgow for COP 26 later this year?
My key messages would be to take this seriously. Because the world in 20, 30, 40 years’ time is not going to be same as the one we’re living in today. We’re going to be seeing new diseases and we’re going to have to deal with them. But there’s no need to panic — we have time and we have the technology, it’s really just a case of deciding that we’re going to start to do something.
Want to know more?
We heard from Matthew and other experts at Climate Change Bites, an online event ahead of COP26 that explored vector-borne diseases, health and food security under climate change through expert talks and panel discussions.
You can watch a recording of the event on YouTube: https://youtu.be/04Um2SAal08
Find out more and sign up via Eventbrite: https://www.eventbrite.co.uk/e/climate-change-bites-tickets-180020224337
If you’re a UK taxpayer, your contributions help fund the work of researchers tackling these questions, via UK Research and Innovation — the funding body that allocates government funds for research — and the nine research councils. Projects in this article are funded by the Engineering and Physical Sciences Research Council. You can read more about what we do here.
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