The blacklegged tick, also called the deer tick, is the tick that carries Lyme disease in the United States. Specifically, two closely related species are responsible: the blacklegged tick (Ixodes scapularis) across the eastern U.S. and the western blacklegged tick (Ixodes pacificus) along the Pacific coast. No other tick species in North America transmits the Lyme disease bacterium to humans.
The Blacklegged Tick in the Eastern U.S.
The blacklegged tick is widely distributed across the eastern United States and is by far the most common source of Lyme disease infections. You might hear it called the “deer tick” because adult females frequently feed on white-tailed deer, though the ticks pick up the Lyme bacterium from smaller animals like mice and chipmunks earlier in their life cycle.
Unfed female blacklegged ticks are easy to identify: they have an orange-red body surrounding a dark black plate (called a scutum) on their back. Males are smaller and almost entirely dark brown or black. Nymphs, the immature stage most likely to bite people unnoticed, are roughly the size of a poppy seed. Both nymphs and adult females transmit disease.
These ticks have expanded their range significantly since the 1980s. Reforestation, recovering deer populations, and warming temperatures have all helped them spread from coastal New England and the upper Midwest into the Ohio Valley, Virginia, and North Carolina. Areas that had few or no blacklegged ticks a generation ago now have established populations, and Lyme disease cases have followed.
The Western Blacklegged Tick
Along the Pacific coast, the western blacklegged tick fills the same role. It’s found primarily in northern California, Oregon, and Washington. Lyme disease rates on the West Coast are considerably lower than in the Northeast and upper Midwest, partly because the western blacklegged tick feeds on lizards during its nymph stage. Lizards don’t carry the Lyme bacterium effectively, so fewer ticks end up infected compared to their eastern relatives, which feed on mice and other small mammals that are excellent bacterial reservoirs.
What Percentage of Ticks Are Infected
Not every blacklegged tick carries Lyme disease. A large meta-analysis of tick surveillance data from 1990 to 2020 found that among ticks actively seeking hosts, about 10.5% of nymphs and roughly 32% of adults tested positive for the Lyme bacterium. In some high-risk regions like Ontario, adult infection rates reached nearly 35%. These numbers have been climbing over time, with infection prevalence in host-seeking ticks showing a statistically significant upward trend between 2007 and 2019.
So if you find a blacklegged tick on your body, the odds that it’s carrying Lyme bacteria range from about 1 in 10 to 1 in 3, depending on the life stage and where you live. That’s meaningful but not a guarantee of infection, especially if you remove it quickly.
How Transmission Works
The bacterium that causes Lyme disease, Borrelia burgdorferi, lives in the tick’s gut. When a tick feeds, it takes time for the bacteria to migrate from the gut to the salivary glands and into your bloodstream. This is why an infected tick generally needs to be attached for more than 24 hours before transmission occurs. Removing a tick within that first day dramatically reduces your risk.
A second bacterial species, Borrelia mayonii, was identified in 2016 in blacklegged ticks from Minnesota, Wisconsin, and North Dakota. It causes Lyme disease with slightly different symptoms, including nausea and higher concentrations of bacteria in the blood. Like its more common relative, B. mayonii is transmitted by the blacklegged tick.
Where Blacklegged Ticks Wait for You
Blacklegged ticks don’t jump or fly. They use a behavior called “questing,” where they climb to the tip of a low shrub, blade of grass, or leaf and extend their front legs, waiting to grab onto anything that brushes past. Adult ticks quest about knee-high on the tips of low-growing shrub branches. Nymphs tend to stay closer to the ground in leaf litter and on logs, which is one reason their bites so often go unnoticed.
The highest-risk habitats are the edges where forests meet open areas: trails, yards bordering woods, stone walls, and gardens near tree lines. Ticks thrive in humid, shaded environments. Open, sunny lawns are far less hospitable to them.
Ticks That Don’t Carry Lyme Disease
Several common tick species in the U.S. look similar to blacklegged ticks but do not transmit Lyme disease. The most important one to know is the lone star tick, recognizable by the single white dot on the female’s back. Lone star ticks are aggressive biters found throughout the southeastern and eastern U.S., and their range is expanding. They can cause a condition called southern tick-associated rash illness (STARI), which produces a circular rash that looks a lot like the Lyme disease rash. But STARI is caused by a different organism, and lone star ticks are not capable of transmitting Borrelia burgdorferi. They do carry other concerns, including ehrlichiosis and alpha-gal syndrome, a condition that triggers allergic reactions to red meat.
The American dog tick and the brown dog tick are also common across much of the country. Neither transmits Lyme disease. The American dog tick can carry Rocky Mountain spotted fever, while the brown dog tick rarely bites humans at all. If you find a tick and aren’t sure what species it is, many university extension programs and state health departments offer free tick identification services, sometimes with pathogen testing included.