Climate change may drive an increase in UK tick numbers

The number of ticks bite cases may be stable or steadily rising, although that might just be because we’re becoming better at spotting and diagnosing cases. But, to quote myself about 5 minutes ago, the last thing we want to do is become complacent, particularly when another, omnipresent threat is rearing its ugly head: climate change. The shift to warmer drier summers is threatening to cause an up-tick in tick numbers here in the UK. Fortunately, or unfortunately, this is a phenomenon occurring all around the world, so other countries can provide valuable information about what we might expect climate change to do to tick numbers and distribution. One such country is Canada, as the Public Health Agency of Canada’s Nick Ogden and Catherine Bouchard explain.

Catherine - When I was a PhD student 15 years ago, I used to harvest ticks in the environment or collect ticks from rodents and deer. And back then I would collect about 2000 ticks over a 12 month sampling. Nowadays, if I return to the same study region, I will harvest 2000 ticks within a two month period. And since, well, I've been studying these ticks for 15 years now. They are still quite fascinating. Last summer I did a lab experiment with a University of Montreal colleague, involving live ticks. And ticks were calm, sort of on pause and kept at the high humidity level. But whenever I would blow on top of the vial, they would reactivate like little zombies, I call them. Driven by the CO2, the vibration and the humidity of my breath. And for me, that image explains it all. They are so resilient.

Will - Yes. You're painting a quite striking and concerning picture with that, the fact that it's increased so much and these things are so resilient to bring you in. Nick, could you talk us then through what is going on in terms of their relationship with the climate? What is the climate modelling saying?

Nick - Around about 2003, we started to work on developing a model of the lifecycle of the tick, really to understand what is the impact potentially of climate on the survival of tick populations and we're talking here about the Ixodes scapularis, or the blacklegged tick, which is our version of the Ixodes ricinus tick that you have in the UK, which transmits lyme disease. What we hypothesised was that it was another significant effect of temperature, which is on the development rates of the ticks from each life stage. So an engorged lava through to nymph, engorged nymph to adult, engorged adult female to producing eggs and then eggs developing to larvae. All of those stages are dependent on temperature or how long they last depends on temperature. The warmer it is, the shorter the life cycle becomes. And we hypothesised that Canada, for the most part, was too cold because it meant that the life cycle was too long. But with climate change, that would change, more and more parts of Canada would have a climate that would allow the ticks to complete their life cycle.

Will - If this is the case, then, if an increase in temperature is kind of speeding up all aspects of the life cycle of the tick, does this increase the distribution in which ticks might be found?

Nick - It does, and we are seeing a number of things happening at the same time. Firstly, the ticks are spreading northwards, so there are greater areas of Canada where the tick populations have set up and where people are now getting bitten by ticks. We're also seeing the abundance of the ticks increasing behind that kind of advancing frontline as it were. And the proportion of ticks infected with the agents of lyme disease, the bacteria and Borrelia burgdorferi, the proportion of ticks infected is increasing as well.

Will - Catherine, does this also increase the variety of species that we'll encounter?

Catherine - Yes, we do have more ticks now, but we do have more species in more regions. So on our side, in North America, three main tick species bite humans - the black legged tick, the American dog tick and the lone star tick. So Canada does watch for the establishment of the lone star tick population while the other two species that I mentioned are already present and occurring. We're also on the lookout for the Asian longhorn tick, which was first found in the US in 2017 and it was likely introduced through various pathways including importation. So yeah, more species are expected.

Will - And I suppose it goes without saying then does an increased number of species kind of lead to an increased number of diseases that we should be concerned about too?

Catherine - Exactly. Emerging tick borne diseases pose an increasing public health challenge. So in Canada we've witnessed this first hand with Lyme disease emerging more significantly 20 years ago, but followed recently by anaplasmosis with a large outbreak in 2021. They're both transmitted by black legged ticks. Both of them are bacteria, but the different tick species spread different diseases. And so when I mentioned the black legged tick, it does transmit Lyme disease and anaplasmosis, human babesiosis, miyamotoi relapsing fever and also powassan encephalitis. The American dog tick now can spread Rocky Mountain spotted fever. And then lastly, the one we're on the lookout for, the lone star ticks, can transmit the human alkalosis and the southern tick associated with rash illness. So we're monitoring those different tick species, but also testing the specimen and trying to detect the incoming pathogens.

Will - And Nick, to bring you back in, obviously a lot of this, in fact almost entirely the show so far, has been quite rightly centred on the disease and health aspects of ticks and tick bites. But are there other impacts that we should be considering too in terms of, say, economics?

Nick - Yes, we did a study recently to assess the potential economic impact of emerging Lyme disease in Canada. So by the 2050s, somewhere between half a billion a year or 2 billion a year, depending on what's the kind of final incidents that we see when the ticks have moved in, it's quite a substantial potential economic impact.

Will - So to put this all together then, given what you've known and what you've learned in the case of Canada, what might we be able to expect in the UK when it comes to the future of ticks?

Catherine - Well, from our experience for sure, the ecological and climate changes that we've seen mostly driven by human activities did lead to an increase in species diversity distribution, like we mentioned. But in your case, you do have the Ixodes ricinus, which is long established in the UK. And that tick is really like a cousin for our Canadian black legged tick. It is a vector for diseases like tick-borne encephalitis and Lyme disease. And from the literature we can see and it is documented that it is still expanding its range, a bit going even northward. And similarly, the are ticks called Hyalomma, sometimes they can be vectors for haemorrhagic fever virus and they are becoming more occurrence in Europe, across Europe. So this is something that has been monitored and is being monitored actually by different European tick experts, but the common underlying trend being more ticks, northward, I think this is something that we share. More tick species in more regions.