There are roughly 900 species of ticks in the world. Most of them are irrelevant to Lyme disease. The ones that matter belong to the genus Ixodes — hard-bodied ticks with a two-to-three year life cycle that involves three separate blood meals on different hosts. It is during these blood meals that pathogens pass from animal to tick, and from tick to human.
Ixodes ticks — the Lyme disease vectors
Four Ixodes species are responsible for the vast majority of Lyme disease transmission worldwide. They occupy similar ecological niches on different continents — humid, wooded environments where wildlife is abundant. Same family, same behaviour, same risk — just different geography.
The primary Lyme vector across Europe — from the British Isles to western Russia, from Scandinavia to North Africa. Also called the sheep tick or deer tick. Found in forests, meadows, and increasingly in urban parks. Responsible for transmitting multiple Borrelia species including B. burgdorferi, B. afzelii, and B. garinii, which is why European Lyme presentations can differ from North American ones.
The main Lyme vector in the eastern and midwestern United States and Canada. Also called the deer tick. Responsible for an estimated 476,000 new Lyme cases in the US per year. Thrives in the northeast and upper midwest — states like Connecticut, New York, Wisconsin, and Minnesota — but its range has been steadily expanding northward as temperatures rise.
The Lyme vector on the US west coast — California, Oregon, Washington. Closely related to I. scapularis but tends to feed more often on lizards, which are resistant to Borrelia. This may partly explain the lower Lyme transmission rates in western states compared to the northeast, though the risk is still real and should not be dismissed.
The dominant Lyme vector across Russia, China, Japan, and parts of eastern Europe including the Baltic states, Finland, and parts of Poland. Named for the taiga — the boreal forest belt where it is most common. Transmits B. garinii and B. afzelii, and is also a significant vector of tick-borne encephalitis virus in regions where both are co-endemic.
European Lyme disease is caused by multiple Borrelia species — including B. afzelii and B. garinii in addition to B. burgdorferi. According to mainstream medical sources, North American Lyme is predominantly caused by B. burgdorferi. Whether this reflects the full picture — or simply the limits of what standard tests are designed to detect — is a different question. What is clear is that the diversity of European strains is one reason why US-developed tests routinely underperform in European patients.
Four stages — and why the nymph is the one to fear
Ixodes ticks take two to three years to complete their life cycle, passing through four distinct stages. At each stage after the larva, they need a blood meal from a new host to survive and develop. This multi-host cycle is what allows them to acquire pathogens from infected wildlife and pass them on to humans.
A tick does not transmit Borrelia instantly. In most cases, it needs to be attached and feeding for at least 24 hours before the bacteria can migrate from its gut into its saliva and enter the host. This is why checking yourself thoroughly after being in tick habitat — and removing attached ticks promptly — genuinely reduces your risk. Time matters.
Habitat — high risk zones and safer spaces
Ticks cannot fly or jump. They wait — sometimes for months — in a behaviour called questing: clinging to low vegetation with their front legs extended, ready to grab onto any warm-bodied host that brushes past. They need humidity to survive, which is why they cluster in specific environments. Knowing these environments lets you assess your risk before entering.
The boundary where trees meet open ground — the single highest-risk zone. Deer and rodents concentrate here, and so do ticks.
Fallen leaves provide humidity, warmth and shelter. Ticks overwinter in leaf litter and remain active underneath it even in cool temperatures.
Ticks quest from tall grass stems waiting for a host to pass. Unmowed meadow edges and overgrown trails are especially high risk.
Humid forest floor, abundant wildlife, low vegetation — optimal tick habitat. Risk is highest where the canopy is dense and ground cover lush.
Research has confirmed ticks in city parks, including Central Park. Anywhere wildlife can access — raccoons, deer, birds — ticks can follow.
Low risk. Ticks dehydrate quickly in direct sun. A well-maintained, regularly mowed lawn in full sun is largely safe.
The questing behaviour — how ticks find you
Ticks do not drop from trees. This is one of the most persistent myths about tick behaviour. They stay close to the ground — on grass blades, low shrubs, and the edges of vegetation — and extend their front legs outward, waiting for a host to make contact. You pick them up by brushing against vegetation, not from above. Staying on the centre of trails, avoiding brushing through undergrowth, and tucking trousers into socks are all behaviours that directly reduce the chance of a tick making contact.
When are ticks most active?
Tick activity is not uniform throughout the year, and different life stages peak at different times. Understanding this seasonality helps you calibrate your vigilance — though it is worth noting that in mild winters, adult ticks can remain active even below freezing.
Adult ticks become active early in spring — sometimes as soon as temperatures rise above freezing. Nymphs begin emerging from late spring onward. This is a significant risk period that many people underestimate.
The peak nymph season — and therefore the highest-risk period of the year. Nymphs are tiny, abundant, and active precisely when people are most outdoors. Most human Lyme infections occur during this window.
Adult ticks are active again in autumn as they seek their final blood meal before winter. Larger and more visible than nymphs, but still capable of transmitting Lyme and co-infections.
Ticks do not die in winter — they go dormant or remain active on mild days. A frost does not kill blacklegged ticks. On days above freezing, adult ticks may resume activity. Year-round vigilance is warranted in temperate climates.
Tick ranges are expanding. Ixodes scapularis is spreading northward into Canada at a measurable rate driven by warmer temperatures. Ixodes ricinus has expanded to higher elevations and latitudes across Europe over the past two decades. Regions that were historically low-risk are no longer reliably so. If you live in a temperate climate and spend time outdoors, the question is not really whether ticks are present — but how many, and of what species.
Sources & further reading
- ECDC — Ixodes ricinus factsheet, European Centre for Disease Prevention and Control
- CDC — Tick Lifecycles, Centers for Disease Control and Prevention
- Comparison of Lyme Disease in the United States and Europe — Emerging Infectious Diseases, CDC (2021)
- Ixodes ricinus complex of ticks — ScienceDirect Topics
- National Geographic — Ticks are taking over city parks (2019)
- ILADS — Lyme Disease Basics for Providers
Last updated: April 2026