Lyme disease is transmitted by black-legged ticks, also called deer ticks, belonging to the genus Ixodes. In the United States, two species are responsible: the blacklegged tick (Ixodes scapularis) in the eastern half of the country, and the western blacklegged tick (Ixodes pacificus) along the Pacific Coast. No other tick species in the U.S. transmits the Lyme-causing bacteria.
The Two Species That Carry Lyme in the U.S.
The blacklegged tick, Ixodes scapularis, is the primary vector. It’s widely distributed across the eastern United States and transmits infection in the northeastern, mid-Atlantic, and north-central states. These are the regions where the vast majority of U.S. Lyme disease cases originate.
The western blacklegged tick, Ixodes pacificus, fills the same role along the Pacific Coast but transmits Lyme at lower rates. Fewer of these ticks carry the bacteria compared to their eastern relatives, which is one reason Lyme cases are far less common in California, Oregon, and Washington than in states like Connecticut, Pennsylvania, or Wisconsin.
Both species carry the bacterium Borrelia burgdorferi, the primary cause of Lyme disease in the U.S. A second, rarer species called Borrelia mayonii has also been identified in the upper Midwest.
Ticks That Do Not Transmit Lyme
Two other common ticks in the U.S. frequently get confused with the blacklegged tick. The lone star tick, recognizable by the single white dot on the female’s back, is not believed to transmit Borrelia burgdorferi. It can cause other tick-borne illnesses and has been linked to alpha-gal syndrome (a red meat allergy), but Lyme disease is not among them. The American dog tick, the large brown tick most people encounter on pets, also does not carry Lyme.
If you’ve been bitten by a tick and are worried about Lyme, correctly identifying the tick species matters. Only Ixodes ticks pose a Lyme risk.
How to Identify a Blacklegged Tick
Blacklegged ticks are smaller than many people expect. Adult females are typically less than 1/8 inch long, brown to reddish-orange with a dark brown or black shield on their back, just behind the head. Adult males are slightly smaller and uniformly dark brown. Both have distinctive dark brown or black legs, which give the tick its common name.
Nymphs, the juvenile stage, are about the size of a poppy seed. This is the life stage responsible for most Lyme transmission, precisely because they’re so small that people don’t notice them. Many people with Lyme never recall being bitten. Larvae are even tinier, under 1 millimeter, and have six legs instead of the eight seen on nymphs and adults.
When a female feeds, she can swell dramatically, increasing to about 1/2 inch long and changing color to tan, gray, or dark brown. An engorged tick looks very different from an unfed one, which can make identification tricky if you find the tick late.
How Transmission Actually Happens
A blacklegged tick doesn’t transmit Lyme bacteria the moment it bites. The bacteria live in the tick’s gut, and it takes time for them to migrate to the salivary glands and enter your bloodstream. Current guidelines consider a bite high-risk for Lyme transmission only when the tick has been attached for 36 hours or more.
This is why prompt tick checks matter so much. If you find and remove a tick within the first day, the odds of infection are very low. Ticks that are still flat and small when you find them likely haven’t been feeding long enough to transmit the bacteria. An engorged tick that’s visibly swollen has been attached longer and poses more concern.
For bites considered high-risk, a single dose of the antibiotic doxycycline can be given within 72 hours of tick removal to help prevent infection. A bite qualifies as high-risk when all three of these criteria are met: the tick is an identified Ixodes species, the bite occurred in a highly endemic area, and the tick was attached for 36 hours or longer.
When and Where Risk Is Highest
Nymphs are most active from late spring through summer, roughly May through July, which is when Lyme transmission peaks. Adults are more active in fall and early spring. Because nymphs are so tiny and hard to spot, summer remains the most dangerous season for Lyme exposure.
Geographically, risk is concentrated in the Northeast (from Virginia up through Maine), the upper Midwest (Minnesota, Wisconsin), and to a lesser extent the Pacific Coast. The CDC tracks blacklegged tick populations at the county level. An area is considered to have an established population when six or more ticks of a single life stage are collected within a 12-month period. But absence of an “established” designation doesn’t mean ticks aren’t present; it may simply mean surveillance hasn’t documented them yet. Tick ranges have been expanding, so areas previously considered low-risk may not stay that way.
Lyme-Carrying Ticks Outside the U.S.
Lyme disease is not limited to North America. In Europe and Asia, a related tick species called Ixodes ricinus (the castor bean tick or sheep tick) is the main vector. It transmits several Borrelia species that circulate between ticks, small mammals, and ground-feeding birds. European Lyme disease can present somewhat differently than the North American version because different Borrelia species are involved, but the tick genus responsible is the same: Ixodes.
If you’re traveling in Europe, particularly in forested or grassy areas of central and northern Europe, the same precautions apply. Tick checks, prompt removal, and awareness of symptoms like the expanding red rash known as erythema migrans are your best tools regardless of which continent you’re on.