Only two tick species in the United States transmit Lyme disease: the blacklegged tick (commonly called the deer tick) and the western blacklegged tick. No other ticks, including the common lone star tick and the American dog tick, carry the bacteria responsible for this infection. Knowing which ticks pose a real risk, what they look like, and how they transmit the disease can save you a lot of unnecessary worry after a bite.
The Two Tick Species That Carry Lyme
The blacklegged tick is the primary carrier across the eastern and midwestern United States. It’s found from Maine down through the Mid-Atlantic states, across the upper Midwest, and into parts of the Southeast. This is the tick responsible for the vast majority of the roughly 476,000 Lyme disease cases estimated each year in the U.S.
The western blacklegged tick fills the same role along the Pacific coast, particularly in northern California. It carries the same Lyme-causing bacteria but is found in far lower numbers, which is one reason Lyme disease rates on the West Coast are a fraction of what they are in the Northeast.
Both species transmit the corkscrew-shaped bacterium Borrelia burgdorferi. A second, rarer species of bacteria called Borrelia mayonii can also cause Lyme disease but has only been identified in a handful of upper Midwestern states.
How to Identify a Blacklegged Tick
Blacklegged ticks are smaller than the ticks most people picture. An adult female, the stage you’re most likely to notice on your skin, is roughly the size of a sesame seed before feeding. She has a distinctive orange-red body with a dark brown or black shield-shaped plate near the head. Males are smaller and almost entirely dark brown. Both are noticeably smaller and flatter than the brown, round American dog ticks commonly found on pets.
Nymphs, the juvenile stage active in late spring and summer, are only about the size of a poppy seed. They’re translucent tan and extremely easy to miss on skin, especially in areas like the back of the knee, the groin, or along the hairline. This tiny size is a big part of what makes them dangerous: people simply don’t notice them in time.
Nymphs vs. Adults: Which Stage Is Riskier?
Adult blacklegged ticks are actually more likely to be infected. A 2025 Dartmouth study published in Parasites & Vectors found that 50% of adult blacklegged ticks in the Northeast carried the Lyme bacterium, compared to up to 25% of nymphs. Despite that, nymphs are responsible for most human Lyme cases. The reason comes down to visibility and timing. Nymphs are active during the warmer months when people spend the most time outdoors, and their poppy-seed size means they often feed undetected for the full time needed to transmit the infection.
Adults are most active in fall and early spring, and their larger size makes them easier to spot and remove before transmission occurs.
How Transmission Actually Works
Finding a tick on your body doesn’t automatically mean you’ll get Lyme disease. The bacterium lives in the tick’s gut, and it takes time for the bacteria to migrate to the tick’s salivary glands and enter your bloodstream. An infected tick generally needs to be attached for more than 24 hours before it can transmit the infection. Removing a tick within that window greatly reduces your risk.
This is why daily tick checks matter more than almost any other preventive measure. After spending time in wooded or grassy areas, check your entire body carefully, paying special attention to warm, hidden spots: armpits, behind the ears, the belly button, and between the toes. Showering within two hours of coming indoors helps you find unattached ticks before they bite.
Where Ticks Pick Up the Bacteria
Ticks aren’t born with the Lyme bacterium. They acquire it by feeding on infected animals, and the most important source is the white-footed mouse. This small woodland rodent is the primary reservoir for Borrelia burgdorferi across the eastern U.S. In the Northeast, an estimated 80% of the white-footed mouse population carries the bacterium. The mice show no symptoms themselves, effectively serving as a silent supply line that keeps infection rates high in local tick populations.
Other animals like chipmunks and shrews can also carry the bacterium, while deer play an indirect role. Deer don’t efficiently transmit the bacteria to ticks, but they serve as key hosts for adult ticks to feed and reproduce on, sustaining tick populations overall.
Ticks That Don’t Carry Lyme
Several common tick species bite humans regularly but do not transmit Lyme disease. The lone star tick, recognizable by the single white dot on the female’s back, is widespread across the southeastern and eastern U.S. It can cause other problems, including a condition linked to red meat allergy, but it does not carry Borrelia burgdorferi. The American dog tick, the large brown tick most people find on their dogs, also does not transmit Lyme. Neither does the Rocky Mountain wood tick or the Gulf Coast tick.
If you’ve been bitten and aren’t sure what kind of tick it was, size and color are your best clues. Blacklegged ticks are distinctly smaller and darker-legged than dog ticks or lone star ticks. Several university extension programs offer free tick identification services where you can submit a photo or mail in the tick itself for species confirmation.