Ticks that carry Lyme disease live across a huge swath of the United States, throughout much of Europe, and in parts of Asia. In the U.S., they’ve been documented in nearly half of all counties, and their range is still expanding. But geography is only part of the answer. Knowing the specific landscapes, microclimates, and even backyard features where these ticks concentrate is what actually helps you avoid them.
The Two Tick Species in the U.S.
Two closely related ticks transmit Lyme disease in the United States. The black-legged tick covers the eastern half of the country, from the Atlantic coast westward to the edge of the Great Plains. It has been collected in 37 states and is now classified as established in 842 counties, roughly 27% of all counties in the continental U.S. The western black-legged tick handles the West Coast. In California alone, it has been reported in 56 of the state’s 58 counties.
These two species behave somewhat differently. Eastern black-legged ticks quest for hosts in leaf litter and on low vegetation in forests. Western black-legged ticks are commonly encountered in open grassland, chaparral, and along the edges of hiking trails, particularly in coastal counties and Sierra Nevada foothills. In California, the highest Lyme disease rates cluster in the northwestern counties of Trinity, Humboldt, and Mendocino, plus the northern Sierra county of Nevada and Santa Cruz on the central coast.
Highest-Risk Regions in the Eastern U.S.
Two historically separate hotspots, one in the Northeast and one in the upper Midwest, have been merging into a single enormous zone. The connection point is the Ohio River Valley, where tick populations from both directions have converged over the past two decades. This means a nearly continuous band of established tick populations now stretches from Maine through the mid-Atlantic states, across Pennsylvania and Ohio, and into Wisconsin and Minnesota.
The Northeast has seen especially dramatic expansion. Black-legged ticks pushed northward into upstate New York, Vermont, New Hampshire, and all of Maine. They spread westward across Pennsylvania, which now has established populations in all 67 counties. Ohio’s tick range closely follows the footprint of its deciduous forests. Meanwhile, the tick moved south and southwest into West Virginia, Virginia, and North Carolina.
The southeastern and south-central states tell a different story. While black-legged ticks do live in the South, the number of counties where they’re considered established has stayed relatively stable for over two decades. Lyme transmission rates in those areas remain much lower than in the Northeast and upper Midwest.
Lyme Ticks Outside the U.S.
In Europe, a different species carries Lyme disease across a vast range that includes Scandinavia, the British Isles, central Europe, France, Spain, Italy, the Balkans, eastern Europe, and even parts of North Africa. This tick also transmits several other infections, including tick-borne encephalitis. If you’re hiking in European forests or grasslands, the risk profile is similar to the eastern U.S., with the added concern of encephalitis in certain regions.
In parts of Asia, related tick species also transmit the Lyme bacterium, though the specific species and risk levels vary by country.
The Habitats Where Ticks Actually Live
Ticks that carry Lyme disease don’t survive just anywhere. They need humidity. The immature stages, which are the ones most likely to bite you unnoticed, are highly vulnerable to drying out. They depend on the leaf litter layer of temperate deciduous and mixed forests, where humidity regularly sits above 85% and often approaches 95 to 99%. Without that moist microenvironment, they desiccate and die.
This is why you’ll find the highest tick densities in and around forests with thick leaf litter, particularly where maple, oak, and other broadleaf trees drop leaves each fall. The ticks spend most of their time in or near the litter layer, climbing onto low vegetation or plant stems only when actively seeking a host. Open, sunny, dry areas like mowed lawns or paved surfaces are inhospitable to them.
Elevation Limits
Ticks thin out as you gain altitude, but they can survive higher than many people expect. In central European mountain ranges, infected ticks have been collected at elevations up to about 1,370 meters (roughly 4,500 feet). The practical takeaway: being in the mountains doesn’t guarantee you’re above the tick line, especially at lower and mid-elevation trails through forested terrain.
Urban Parks Are Not Tick-Free
If you assume ticks are only a wilderness problem, think again. Studies from urban parks in New York have documented high Lyme infection rates in ticks collected right in the city. Research in the UK found ticks present in over a third of surveyed transects in Richmond Park, London. Black-legged ticks in urban settings concentrate in unmaintained herbaceous vegetation, leaf litter accumulations, and the transition zones where maintained areas meet wooded edges. Adults tend to cluster along these edge zones, while immature ticks stay closer to the leaf layer.
Western black-legged ticks show up frequently in urban parks across southern California, typically in grassy areas. So whether you’re walking a trail in a national forest or cutting through a weedy park in a metro area, the risk can be real if the habitat is right.
Your Backyard’s Tick Hotspots
In residential areas, ticks don’t distribute evenly across your property. They cluster in specific microhabitats. The single most important zone is the border where your lawn meets woodland or brush. This transition edge is where ticks are most abundant, because it’s where their animal hosts travel and where humidity stays high enough for ticks to survive.
Stone walls are another major hotspot. They provide shelter for white-footed mice, the primary reservoir for the Lyme bacterium in the eastern U.S. After mice pick up ticks, the ticks drop off in places mice frequent: stone walls, woodpiles, rodent burrows, garden beds, and areas with accumulated brush or debris. Old furniture, trash piles, and unsealed gaps in stone walls all create habitat for rodents and, by extension, for ticks.
Tall grasses, weeds, and overgrown brush near your home are also prime tick territory. Research on white-footed mice in fragmented suburban landscapes found that mice captured just 30 to 40 meters into the forest edge rarely ventured into nearby yards. This suggests that maintaining a clear buffer zone between your lawn and the woods, and targeting tick management right at that boundary, can meaningfully reduce your exposure.
When Ticks Are Active
Black-legged ticks don’t disappear in winter the way mosquitoes do. Adults have a questing threshold of just 4°C (about 39°F). Any winter day that rises above that temperature can bring adult ticks out looking for a host. This catches people off guard: a mild January afternoon is enough to put you at risk in endemic areas.
The nymphal stage, which is responsible for most Lyme transmission because of its tiny size, peaks in late spring and summer, roughly May through July in most of the Northeast and upper Midwest. Adults are most active in fall and early spring. In California, western black-legged tick adults are active during the cooler, wetter months from fall through early spring, while nymphal activity is less well defined, particularly in central and southern parts of the state.
A Range That Keeps Growing
The geographic footprint of Lyme-carrying ticks has expanded substantially over the past two decades and shows no signs of stabilizing. The merging of the Northeast and upper Midwest populations into one continuous zone is a major shift. Pennsylvania went from partial tick coverage to established populations in every county. Maine, New Hampshire, and Vermont have all seen northward spread. Ohio’s forests are now colonized from the east.
Adding to the complexity, the Asian longhorned tick, an invasive species first detected in New Jersey in 2017, has since been documented in 17 mostly eastern states. While it’s not currently considered a primary Lyme vector in the U.S., its rapid spread illustrates how quickly tick ranges can change. The practical implication is straightforward: areas that were tick-free a decade ago may not be today, so checking current local tick data before assuming your region is safe is worth the effort.