Header image: Jtrytin (Jarle Tryti Nordeide) (CC BY-SA 4.0, Wikimedia Commons)
Have you noticed ticks showing up earlier in spring, later in autumn, or in places they didn’t used to be before? No, you are not imagining things. A new study published in the Journal of the Royal Society Interface by researchers from the UK Centre for Ecology and Hydrology has built one of the most detailed computer models of how Ixodes ricinus (the tick responsible for most of Europe’s Lyme disease and tick-born encephalitis) responds to climate. And their results show ticks are thriving in wider parts of northern Europe than they used to.
Why ticks can be hard to predict
Ixodes ricinus has slightly complex life cycle. After hatching from the egg, the tick must find and feed on a host three separate times, once each as larva, nymph, and adult; the adults then go on to reproduce and die. Each blood feeding triggers the transformation into the next life stage, after which it undergoes a short ‘chilling out’ period before it carries on with life (and its next meal).
Additionally, ticks are not out and about all the time, waiting to jump on you (a common misconception). Instead, they ‘quest’, which is how it’s called when they climb up grass or plants and wait with their tiny front legs outstretched, waiting for an animal (or you) to brush past so they can grab on. Humidity regulates how long they’re questing about – if they dry out they must seek shelter to rehydrate. And when days shorten and temperatures drop, they can enter hibernation status until the environment improves. Even blood feeding times are variable, depending on whether it is a large or small host, and the activity in itself can prove deadly if they’re discovered.
Therefore, understanding how climate affects ticks needs a more nuanced model beyond heat equals more ticks; it needs to consider the variations in the interactions between tick biology and environmental conditions.
Translating ticks into math
To do this, the authors used a mathematical technique called stage-structured delay-differential equations. Instead of treating ticks as a constant process, the model explicitly tracks how long each stage takes to complete and considers how those times increase or shrink depending on the temperature and humidity the tick is experiencing. Then the team fed into the model some real climate data (temperature, humidity, and photoperiod) for different places across Europe, fine-tuning the tick biology variables based on decades of laboratory experiments.
Putting the model to the test
To know if the model matches what is actually happening in the real world, the researchers compared their predictions against 77 separately monitored tick populations from 20 different countries, including places like the UK, Spain, Finland, Italy and Portugal. The model successfully predicted when tick activity started, peaked, and ended across the wide range of climates, explaining an average of 55% of the year-to-year variation in nymph numbers. This is a very good result for ecological field data, which can be notoriously noisy.
Looking back at 45 years of historical climate data
The researchers ran the model using climate data from 1980 to 2024, to map how suitable Europe’s climate has become for ticks. A few highlights of their findings:
- Northward conditions have turned much more tick-friendly, even as far as 66° latitude (deep into Scandinavia).
- In Europe as a whole, areas with at least one month of active ticks grew by 4%, and the density of questing nymphs increased by 10%.
- Spring and autumn activities have extended significantly, indicating tick season is stretching.
- Unlike the north, southern Europe, especially areas with hotter, drier summers, are actually becoming less friendly to ticks, leading to local declines.
- When the authors compared moderate vs high host-density scenarios, the areas with meaningful tick activity grew by 21%, suggesting host changes (like the rising deer populations across Europe) could significantly amplify the effect climate has on tick populations.
What does this mean for tick-borne diseases?
Longer tick seasons mean longer exposure windows to Lyme disease and tick-borne encephalitis for people, pets, and livestock. The model that these researchers have created offers a genuinely practical tool to predict how tick-related risk is changing, so that awareness campaigns, surveillance, and livestock management can be planned accordingly.
A deeper understanding of tick and host ecology would provide data on host density estimates and species-specific life history variables, all of which will be necessary to improve the model. For example, information on deer-tick interactions is needed, as the rising deer populations are critical for tick dispersal and survival. This model shows us something valuable: even an animal with as complicated a life cycle as a tick can be predicted with accuracy once you take the details of its biology and its environment into account. This model could even help untangle how ticks are introduced to new areas in the first place, carried by migratory birds, livestock, or wild animals, a stepping stone towards mitigation and control.