Lyme disease in the UK: is it on the rise?

Historically, especially in the UK, Lyme disease has been regarded as rare, which is why many may overlook or write-off the initial symptoms. But the condition does appear to be on the rise, or at least it’s being diagnosed more frequently. Richard Birtles, from the University of Salford, has been looking at the possible reasons why…

Richard - It's not a straightforward answer because if you go into a wood, the number of ticks that are present in that wood, what determines that? Then you have to think about, well, what proportion of those ticks are infected and what determines that. For Lyme disease, we have a very complex ecology. So trying to implicate any specific factors as the main cause of increasing tick numbers, increasing cases of Lyme disease is very difficult.

Chris - So you think that the rise has been detected because I was looking at some of the reviews and they suggest really quite dramatic increases in case numbers. Up maybe 300% in a 10 year period, according to some papers that have been published. Do you think that's a real finding, it's not that people like me and you were talking about it, so then other doctors think about it or people think about it, and they take themselves off and get tested.

Richard - No, I think what you're saying is playing a part as well. We've certainly seen, as you suggest, the number of reported cases rising in England and Wales and in Scotland by five, six fold over the past 20 years or so. And I certainly think awareness in the medical circles, awareness from the public as well, but there is some evidence that tick numbers are on the rise. And certainly the places in the UK where you find ticks is increasing,

Chris - But where are the hot spots? Then you mentioned a couple of places, but where are we tending to see the most Lyme activity?

Richard - When we think historically about Lyme disease, we think about particular places that tend to be wild places, particularly areas that are heavily wooded or heavily grassed, rough pasture, or heathland. So those are the places that historically most cases have been reported. But what we're learning more about now is that we are seeing Lyme disease spread around the country more, and we're seeing urban cases of Lyme disease. So we think that there's a risk of being bitten by ticks in parks, for example, parks in the middle of London are known to support quite high tick numbers and similarly parts of central England where we're seeing a change of land use. So we're seeing a reversion to more woodland, aforestation, replanting of trees, less intensive farming seems to be bringing Lyme disease into those parts of the UK as well. Perhaps the most important driver of this is that these kinds of habitats are fantastic for deer. And we know that deer are fantastic vehicles for ticks. I, for example, have spent a very pleasurable afternoon checking for ticks on a headless legless roe deer in Kildow. And when we got to about 15,000, we gave up and went to the pub. So they carry huge numbers of ticks. And we know that in the UK, the deer population has exploded really. We've got more deer in the UK now than we have in the past thousand years or so. And the numbers are thought to have doubled really in the past 20 or 25 years. So you've got these articulated trucks, the supertankers carrying ticks around the UK. It's not surprising that the distribution of ticks in the UK is increasing and therefore where we're acquiring Lyme disease is changing.

Chris - How does this fit into the life cycle that we were hearing about that Justin was talking about, and also Tom was talking about earlier in the program, when you've got small ticks and big ticks, small animals and big animals, the big animals, presumably are the deer. How does the relationship work?

Richard - Well, it's very complicated because deer are not reservoirs for Borrelia, which is amazing. They carry these huge numbers of ticks, but they cannot transmit infection to the ticks. The infection is reservoirs in, as Justin said, I think, in small mammals, so voles and mice and shrews, and also in birds in foraging birds, and also game birds, pheasants, but also thrushes and blackbirds and things like that. So we've got this tension really between a higher abundance of deer, meaning an awful lot of ticks, but maybe not so many of those ticks infected because they're all feeding on deer, which can't transmit the infection to the ticks. So there's a very complex arrangement in that cycle that we need to explore more and understand more. If we are going to somehow be able to manipulate it, to reduce the risk of us acquiring Lyme disease.

Chris - I know what you mean about deer because I took until I was about in my forties before I even saw one properly in the wild. And now if I don't see one, when I'm driving around in the country lanes every day when I'm out and about something weird is happening. I mean, there must be enormous numbers of them. Does that mean then that we, if we've got a plague of deer, we really need to be eating more venison. And I mean the nicest possible way.

Richard - Well, yes, there's a lot of talk. We're concerned about deer from the perspective of Lyme disease, but also we're worried about things like road traffic accidents and crop destruction and destruction of commercial woodland that we know deer are party to. So there's a big discussion going on now about how we weather for the sake of Lyme disease. We look at trying to control and reduce deer numbers, but also for all of these other possible implications.

Chris - Just one final point, which is that in Australia, they've had a plague of mice or parts of the Eastern seaboard of the country. And this is attributed to various factors, including ideal warm conditions, rainfall, surging vegetation. What about climate change? We haven't discussed that yet. Do we foresee that that could also drive a widening of the range of the very small mammals that we heard about from Justin and Tom who can support the Lyme bacteria and therefore widen its geography across countries like the UK?

Richard - Well, I think in truth, the distribution of the small mammals that are important is fairly ubiquitous in the UK already and is driven really by habitat and land use. And we know that the tick that transmits Lyme disease in the UK, in Europe, you can find it right up to the Arctic Circle. So it's unlikely that new parts of the UK are going to be infiltrated by ticks simply as a result of climate change, but there are models, climate change models, looking at the impact of various scenarios of climate change on tick abundance in the UK and Lyme disease risk in UK. And these tend to suggest that first of all, ticks will be active for longer in the year. So there'll be more active in February, for example, and they'll continue to be active well into November and December. So that's a longer period of the year where we can get bitten. And also that the climate change will provoke higher densities of ticks. Ticks really need humidity. So more rainfall is likely to be good for them. So that's what the model suggests. Of course they need to be validated and we need to check for the degree of uncertainty in those, but that's what the models are suggesting.