Why ticks are becoming an even bigger danger

By Lara Williams

As someone who spends a lot of time outdoors, I’ve come across my fair share of natural hazards: lightning, poisonous snakes, unstable terrain. Nothing, though, fills me with more dread than the thought of a tick.

Depending on what life stage they’re at, they range from about the size of a poppy seed to an apple seed, though the adults can swell much larger as they feed on their host’s blood. While these arachnids are munching away, they can pass on a variety of bacterial and viral diseases. That host, by the way, could be you.

Climate change is making these tiny hazards to public health more threatening. As temperatures warm, ticks are spreading endemic diseases more readily, as well as introducing new ones. Both the public and health officials ought to be armed with as much knowledge as possible to combat the danger.

A recent study by researchers at the University of Stirling modeled the effects of climate change on populations of Ixodes ricinus, also known as the Castor Bean Tick, in Scotland. That’s one of the species which spreads the best-known tick-related disease: Lyme borreliosis, or Lyme disease. They found that in a worst case scenario, defined as global warming of 4C (7.2F), tick numbers would increase by 99 per cent by 2080. Limiting final warming to just 1C would still lead to an increase of around 26 per cent.

The largest absolute increases would be found in woodland habitats, which host the highest densities of ticks currently, while the largest proportional increase would be in montane habitats — mountainous areas above the tree line — which are currently considered mostly too cold for the ectoparasites. Only the highest peaks would be expected to remain tick-free in Scotland in 2080 under the high-warming scenario.

Modeling has its limitations — disease risk and tick populations also rely on factors other than climate, such as increased deer populations, habitat availability and human behavior. Yet it’s a key aid for governments and public health officials preparing for future risks, and more advanced modeling of climate impacts on vector-borne diseases should be a priority in the coming years.

But we don’t have to look ahead 56 years to see the climate crisis’s impact on ticks — it’s already helping them and their associated diseases spread.

The Castor Bean Tick, for instance, has been found in more northern latitudes and higher altitudes in Europe, a development that’s been linked to warmer temperatures, and cases of Lyme disease have also been steadily increasing. Across the Atlantic, Lyme disease instances in the US have tripled over the past three decades. While many cases can be treated successfully, between 5 per cent and 10 per cent of patients are left with chronic symptoms.

Emphasis is placed on Lyme disease for good reason – it’s the most abundant tick-borne disease in Europe and North America, but it can be challenging to diagnose, and there’s currently no vaccine available for humans although you can vaccinate your dog easily enough. There was once a shot available for people, but it was pulled by GSK Plc in 2002 after just three years due to plummeting sales. A new vaccine, produced by Pfizer Inc. and Valneva SE, has just entered its final clinical trial, which is welcome news.

But the risk doesn’t end with Lyme disease: We’re going to have to become accustomed to invasive ticks and non-endemic diseases, too.

Take tick-borne encephalitis (TBE), a disease which is endemic across parts of Europe but wasn’t a concern in the UK before 2019. Since then, the virus has been detected in ticks in England, and there were two confirmed cases in 2022 likely contracted in Britain, as well as two probable cases in 2019 and 2020.

Though the risk remains very low, the UK Health Security Agency has enhanced its surveillance of the virus and has recommended changes to testing in hospitals. Other countries that hadn’t previously seen endemic TBE have also recorded cases, including the Netherlands and Japan.

Meanwhile, southern Europe is facing the threat of an invasive tick: Hyalomma marginatum. Widely distributed across North Africa, Asia and the warmest parts of the Mediterranean basin, it’s increasingly been spotted in more northerly latitudes, including France, the UK and Sweden. Imported via livestock and migratory birds, the risk is that a warming planet enables the bugs to establish populations in new countries.

These giant ectoparasites — twice the size of the Castor Bean Tick — are troubling as they’re an important vector of Crimean-Congo haemorrhagic fever virus, which doesn’t sound like something you want to catch. A man recently died after contracting the virus from a tick in Toledo, Spain. With no specific antiviral drug or approved vaccine available, education of both the general public and healthcare professionals is the most effective control measure.

As the climate crisis brings new threats to our doorstep, we all play a part in being vigilant. That means checking yourself for ticks when you’ve been outdoors (there are some top tips for avoiding playing host to an ectoparasite here.) It means funding for strong surveillance, research and public awareness campaigns. It also means ensuring doctors are looped in on the risks.

Even where treatments are available, there’s anecdotal evidence that it’s difficult to get diagnosed. With Lyme disease, for example, there’s been a heavy focus on the classic bullseye rash – but a third of infections wouldn’t present with such a symptom. An independent review by the EPPI Centre, based at University College, London, cites “low-levels of knowledge among clinicians” as one of the barriers to diagnosis.

These knowledge gaps become wider when diseases enter non-endemic regions. A study of 20 European countries showed that while awareness amongst the public towards TBE and a TBE vaccine was 74 per cent and 56 per cent in endemic countries, it was only 30 per cent and 12 per cent in non-endemic countries.

Medical advances are hopefully on their way — I’m particularly excited about the idea of an anti-tick vaccine — but in the meantime, the best we can do is not get bitten. In the countryside this weekend, I’ll be spraying on insect repellent, wearing long, pale trousers and checking for bloodsuckers — before they get the chance to bite me.