Lyme disease is transmitted by blacklegged ticks, also called deer ticks. In the eastern United States, the species is Ixodes scapularis. Along the Pacific coast, its close relative Ixodes pacificus (the western blacklegged tick) fills the same role. No other tick species in North America transmits the Lyme disease bacterium.
The Blacklegged Tick and Where It Lives
The blacklegged tick is widely distributed across the eastern United States, with established populations documented from Maine down through the mid-Atlantic states and across the upper Midwest, particularly Minnesota and Wisconsin. These are the regions where Lyme disease cases concentrate. On the West Coast, the western blacklegged tick occupies forests and grasslands from southern Oregon through California, though it transmits Lyme at much lower rates.
Surveillance data from Canada illustrates the difference between these two species. In 2022, roughly 24% of eastern blacklegged ticks tested positive for the Lyme bacterium, compared to just 1% of western blacklegged ticks. That gap helps explain why Lyme disease is far more common in the Northeast and upper Midwest than on the Pacific coast.
Why Nymphs Are the Biggest Risk
Blacklegged ticks go through three feeding stages: larva, nymph, and adult. Nymphs are responsible for the majority of Lyme infections in humans, and the reason is simple: they’re nearly invisible. A nymph is about the size of a poppy seed, small enough to attach to your skin and feed for days without being noticed.
Nymphs are most active from mid-May through mid-August, and most Lyme disease cases are reported during exactly that window. Adult female ticks can also transmit the bacterium, but they’re roughly the size of a sesame seed, large enough that people tend to spot and remove them before transmission occurs. Adults are most active in two bursts: March through mid-May, and again from mid-August through November.
How Transmission Works
The bacterium behind Lyme disease, Borrelia burgdorferi, lives in the tick’s gut. When a tick feeds, it doesn’t inject bacteria immediately. The longer a tick stays attached, the higher the chance of transmission. This is why prompt tick checks after spending time outdoors matter so much. A tick you find and remove within the first several hours is far less likely to have passed along the bacterium than one that’s been feeding for a day or more.
A second, rarer bacterium called Borrelia mayonii can also cause Lyme disease in the United States, but the vast majority of cases trace back to Borrelia burgdorferi carried by blacklegged ticks.
Common Ticks That Do Not Carry Lyme
Several other tick species bite humans regularly in the United States, and people often wonder whether they carry Lyme. They don’t, but they can cause other problems.
- American dog tick: One of the most frequently encountered ticks in the eastern U.S., this species does not transmit Lyme disease. It is, however, the primary carrier of Rocky Mountain spotted fever, a less common but potentially more serious illness. It can also transmit tularemia.
- Lone star tick: Recognizable by the single white dot on the female’s back, lone star ticks are aggressive biters found across the southeastern and eastern states. They do not carry Lyme, but their bites are linked to ehrlichiosis, Rocky Mountain spotted fever, tularemia, STARI (a rash illness that can mimic early Lyme), and alpha-gal syndrome, an allergy to red meat that can develop after a bite.
If you’ve been bitten by an American dog tick or a lone star tick, Lyme disease is not a concern from that particular bite. Identifying the tick matters, and your local extension service or health department can often help if you save the tick.
Recognizing a Lyme Infection Early
The most recognizable early sign is a spreading rash at the bite site, which appears in over 70% of people who develop Lyme disease. The rash often expands over days, sometimes forming a bullseye pattern with a clear center, though it can also appear as a solid red oval. It typically shows up between 3 and 30 days after the bite.
Not everyone gets the rash, and not everyone remembers being bitten, especially by a poppy seed-sized nymph. Other early symptoms include fatigue, fever, headache, and joint or muscle aches. If you’ve spent time in an area where blacklegged ticks live and develop these symptoms during tick season, the possibility of Lyme is worth raising with a healthcare provider, even if you never saw a tick on your body.
Reducing Your Risk
Because blacklegged ticks live in wooded and brushy areas, your risk goes up any time you walk through leaf litter, tall grass, or forest undergrowth. Staying on cleared trails helps. Wearing light-colored clothing makes ticks easier to spot, and tucking pants into socks creates a physical barrier.
The single most effective habit is a thorough tick check after coming indoors. Pay close attention to areas where ticks tend to hide: behind the ears, along the hairline, in the armpits, behind the knees, and around the waist. Showering within two hours of coming inside can help wash off ticks that haven’t yet attached. If you find a tick, remove it with fine-tipped tweezers by grasping it as close to the skin as possible and pulling straight up with steady pressure.