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 American dog tick or lone star tick, spread the Lyme bacteria. Knowing which ticks carry the disease, where they live, and when they’re most active can help you assess your actual risk after a bite.
The Two Ticks That Carry Lyme
The blacklegged tick is the primary carrier of Lyme disease across the eastern and midwestern United States. It transmits the main Lyme-causing bacterium as well as a less common species called Borrelia mayonii, which has so far been found only in the upper Midwest (Minnesota, Wisconsin, Indiana, and Michigan). This single tick species is responsible for the vast majority of Lyme cases in the country.
The western blacklegged tick fills the same role along the Pacific coast, particularly in northern California. It carries the same Lyme bacterium but is found in far smaller numbers and lower infection rates than its eastern relative. Lyme cases on the West Coast are significantly less common as a result.
Other ticks you might encounter, like the American dog tick, the brown dog tick, or the lone star tick, do not transmit the Lyme bacterium. They can carry other diseases, but if you’re specifically worried about Lyme, only the two blacklegged species matter.
How Many Ticks Are Actually Infected
Not every blacklegged tick carries Lyme. But in high-risk regions, the numbers are striking. A 2025 Dartmouth study published in Parasites & Vectors found that 50% of adult blacklegged ticks in the Northeast carry the Lyme bacterium. Among nymphs (the younger, smaller life stage), up to 25% were infected.
Those are averages across a large region. Infection rates vary by county and even by individual property. In some pockets of New England and the upper Midwest, rates run even higher. In areas at the edges of the tick’s expanding range, rates tend to be lower. Still, the takeaway is straightforward: if you find an adult blacklegged tick attached to your skin in the Northeast, there’s roughly a coin-flip chance it’s carrying Lyme.
Where These Ticks Live
Blacklegged ticks are widely distributed across the eastern United States, from Maine down through the Mid-Atlantic and into the Southeast, and westward through the upper Midwest. The highest concentration of Lyme cases clusters in the Northeast (Connecticut through Maine), the Mid-Atlantic (New York, New Jersey, Pennsylvania), and the upper Midwest (Minnesota, Wisconsin). These regions combine dense tick populations with high infection rates in those ticks.
Western blacklegged ticks are found along the Pacific coast, with the heaviest populations in northern California. Oregon and Washington also have populations, but Lyme transmission there remains relatively uncommon compared to the eastern hotspots.
The range of the blacklegged tick has been expanding steadily. Counties reporting blacklegged tick populations have increased significantly over the past two decades, driven by warming temperatures, changes in land use, and growing deer populations. Areas that were once considered low-risk, particularly in the Southeast and southern Midwest, are seeing ticks establish themselves for the first time.
Nymphs vs. Adults: Which Stage Is Riskier
Both nymph and adult blacklegged ticks can transmit Lyme, but nymphs cause the majority of human infections. This seems counterintuitive since adults carry the bacteria at higher rates (50% versus 25%). The reason is size. Nymphs are roughly the size of a poppy seed, making them extremely difficult to spot on your skin. Adults are larger and more likely to be noticed and removed before they’ve fed long enough to transmit the bacterium.
Nymphs are most active from mid-May through mid-August, which lines up with the peak of Lyme season. People spend more time outdoors in shorts and short sleeves during these months, and the tiny nymphs latch on undetected. Adult blacklegged ticks are most active in the fall and early spring, when cooler weather and longer clothing make them slightly easier to manage. That said, ticks can be active any time the ground temperature is above 45 degrees Fahrenheit, so mild winter days still carry some risk.
How to Identify a Blacklegged Tick
Telling a blacklegged tick from other common species is important because it changes whether Lyme is even a possibility. The key features to look for on an adult female blacklegged tick are an orange-red body surrounding a dark black plate (called a scutum) on the upper back. The legs are uniformly dark. The overall body is teardrop-shaped and, when unfed, quite flat and small, roughly the size of a sesame seed.
The American dog tick, which people frequently confuse with a blacklegged tick, looks noticeably different. Dog ticks have a patterned, mottled shield on their back with white or cream-colored markings. They also tend to be larger. Their mouthparts are shorter and stubbier compared to the blacklegged tick’s longer, more prominent mouthparts. If you see an ornate, patterned shield on the tick’s back, it’s almost certainly not a blacklegged tick, and Lyme transmission is not a concern.
Nymphs are harder to identify visually because of their tiny size. At the poppy-seed stage, distinguishing species with the naked eye is difficult. If you find a very small tick on your body during the summer months in a Lyme-endemic area, treating it as a potential blacklegged nymph is the practical approach.
How Long a Tick Must Be Attached
Finding a blacklegged tick on your skin doesn’t automatically mean infection. The Lyme bacterium lives in the tick’s gut and needs time to migrate to the salivary glands before it can enter your bloodstream. In most cases, an infected tick must be attached and feeding for more than 24 hours before transmission occurs.
This window is why daily tick checks are so effective. If you remove a tick within the first day of attachment, the odds of Lyme transmission are very low. You can estimate how long a tick has been feeding by its appearance. A tick that’s flat and small just crawled on recently. A tick that’s visibly swollen and engorged has likely been feeding for a day or more, and the risk of transmission increases.
To remove a tick, use fine-tipped tweezers, grasp as close to the skin as possible, and pull straight up with steady pressure. Avoid twisting, crushing, or using folk remedies like petroleum jelly or a hot match, which can cause the tick to regurgitate bacteria into the bite. After removal, clean the area with rubbing alcohol or soap and water. Save the tick in a sealed bag or photograph it. Knowing the species helps your doctor assess the actual risk.
Borrelia mayonii: A Newer Concern
Since 2016, a second bacterium capable of causing Lyme disease has been identified in the upper Midwest. Borrelia mayonii has been detected in about 3% of blacklegged ticks tested in parts of Wisconsin, and patients with confirmed infections reported tick exposure in Minnesota and Wisconsin. One case involved a patient from North Dakota who had been in tick habitat in Minnesota.
Borrelia mayonii causes symptoms similar to classic Lyme but with some differences: patients are more likely to experience nausea, vomiting, and a more diffuse rash rather than the classic bull’s-eye pattern. It also shows up on standard blood tests at higher levels, which can actually make it easier to detect. For now, this bacterium appears geographically restricted to the upper Midwest and is carried by the same blacklegged tick responsible for classic Lyme. No additional precautions beyond standard tick prevention are needed.