What Ticks Carry Lyme Disease and Which Ones Don’t

Only ticks in the genus Ixodes transmit the Lyme disease bacterium to people in the United States. That means just two species matter for human risk: the blacklegged tick (Ixodes scapularis) in the eastern half of the country, and the western blacklegged tick (Ixodes pacificus) along the Pacific coast. Every other common tick you might pull off your skin after a hike, including the American dog tick, the lone star tick, and the recently arrived Asian longhorned tick, has been shown in laboratory experiments to be incapable of transmitting the Lyme spirochete. The reasons for this are more interesting than most people expect, and understanding them changes how you should think about tick bites depending on where you live.

The Two Ticks That Transmit Lyme Disease to Humans

The blacklegged tick, sometimes still called the deer tick, is the primary vector across the Northeast, mid-Atlantic, upper Midwest, and parts of the Southeast. It feeds on a wide range of hosts at different life stages: larval and nymphal ticks commonly feed on small mammals, while adults prefer white-tailed deer. When a larval tick feeds on an infected mouse, it picks up Borrelia burgdorferi, the spirochete that causes Lyme disease. The bacterium survives the molt to the nymphal stage and can then be passed to the next host the nymph feeds on, including you. This ability to retain the infection across life stages is central to what makes a tick a competent vector.

On the West Coast, the western blacklegged tick fills that same role. Ixodes pacificus was not recognized as a species of medical concern until the 1980s, when researchers linked it to transmission of Lyme spirochetes in California, Oregon, and Washington.¹ In parts of northwestern California, both nymphs and adult western blacklegged ticks are active enough throughout the year that Lyme transmission risk is essentially year-round, though human cases peak from May through July, driven mostly by nymphal-stage bites.²

Nymphs are the life stage responsible for most human infections. They are roughly the size of a poppy seed, easy to miss on the skin, and feed for several days. Adults are larger and more likely to be noticed and removed before transmitting the spirochete. This is why tick checks after spending time outdoors matter more than most people realize: finding and removing a tick within the first day substantially reduces your risk.

Common Ticks That Cannot Transmit Lyme Disease

Three tick species bite people frequently in the U.S. but do not serve as Lyme disease vectors. It is worth knowing which ones they are, because a lot of unnecessary anxiety follows bites from ticks that simply cannot give you Lyme.

The American dog tick (Dermacentor variabilis) is the bulky brown tick you often find on dogs and on people in grassy or brushy areas across the eastern two-thirds of the country. In controlled laboratory experiments, dog tick larvae could pick up B. burgdorferi from infected mice, but they generally failed to retain the infection through their molt to the nymphal stage, and nymphal dog ticks did not transmit the spirochete to new hosts.³ The bacterium gets in but cannot persist, so the tick is a dead end for Lyme. (Dog ticks can carry other pathogens, including the bacterium that causes Rocky Mountain spotted fever, but Lyme is not among them.)

The lone star tick (Amblyomma americanum) is the aggressive biter common across the Southeast and increasingly found farther north. It is named for the white spot on the adult female’s back. In the same set of experiments, lone star tick larvae were completely unable to acquire the Lyme spirochete at all, making them even less of a Lyme concern than dog ticks.⁴ Among the tick species tested, only I. scapularis proved to be vector competent for B. burgdorferi.

The Asian longhorned tick (Haemaphysalis longicornis) is a newer arrival, first confirmed in the eastern U.S. in 2017 and now established in multiple states. Its rapid spread raised alarm, but laboratory work has been reassuring on the Lyme front: longhorned tick larvae that fed on infected mice did acquire spirochetes, but the infection was lost during the molt. Out of 520 nymphs tested after molting, not a single one remained infected, putting the upper limit on infection prevalence below one percent.⁵ Other researchers have confirmed that this tick most likely does not contribute to Lyme transmission.⁶ The longhorned tick can vector other pathogens, but Lyme disease does not appear to be one of them.

Why Only Ixodes Ticks Can Do It

The common thread among ticks that fail as Lyme vectors is their inability to maintain the spirochete through the molt from one life stage to the next. This process, called transstadial passage, is the bottleneck. A larval tick feeds on an infected host, picks up bacteria, detaches, and then spends weeks or months off any host before molting into a nymph. During that transition, the spirochete has to survive inside the tick’s gut, then eventually migrate through the tick’s body to reach the salivary glands when the nymph takes its next blood meal.

In blacklegged ticks, B. burgdorferi manages this journey through a remarkably specific set of interactions. Researchers using live imaging have described a two-phase dissemination process inside the feeding tick. First, spirochetes in the midgut adhere to the gut lining cells as they change shape during feeding, forming networks that move toward the outer surface of the gut wall. Then, in a second phase, these bacteria become motile, punch through the gut’s basement membrane, enter the tick’s body cavity, and migrate into the salivary glands where they can be injected into the next host.⁷ The spirochete-tick interaction is thought to be highly specific to Ixodes ticks.⁸

In non-Ixodes species, something about the tick’s internal environment prevents the spirochete from surviving the molt or completing that migration. The bacteria may get killed, fail to adhere properly, or simply not receive the molecular signals they need. This is why exposure to these other ticks, even ones visibly engorged with blood, is not a Lyme risk. The tick’s biology acts as a firewall.

The Mouse-Lizard Split and Why Geography Matters

Even within the range of blacklegged ticks, your risk of contracting Lyme disease varies enormously depending on what animals the local tick population feeds on. The white-footed mouse (Peromyscus leucopus) is the primary reservoir for B. burgdorferi in the United States.⁹ These mice tolerate the infection without becoming visibly sick, and they pass it to ticks with remarkable efficiency. In one classic study, close to 100 percent of ticks that fed as larvae on mice that had been infected two to three weeks earlier picked up the spirochete.¹⁰

This is a big part of why Lyme disease is common in the northeastern U.S. and the upper Midwest: blacklegged ticks in those regions feed heavily on white-footed mice and other small mammals that maintain the spirochete. In the southeastern U.S., the picture is different. Even though Ixodes scapularis exists in the South, the ticks there attach largely to lizards, which do not sustain the infection. The result is that the enzootic cycle, the chain of transmission that keeps the spirochete circulating in nature, is much weaker in the South.¹¹

On the West Coast, a similar dynamic plays out with the western fence lizard. Larval and nymphal western blacklegged ticks often feed on these lizards, which are not competent reservoirs for the Lyme spirochete. Researchers who experimentally removed lizards from study plots found that only about five percent of larval ticks switched to other hosts.¹² The lizards essentially serve as a buffer, diverting tick meals away from animals that would amplify the infection. This is one reason Lyme disease rates in the West are far lower than in the Northeast, even though a competent vector tick is present.

Ticks That Maintain Lyme in Nature but Rarely Bite People

The story gets more nuanced when you look beyond the two main human-biting vectors. Several other Ixodes species play roles in keeping B. burgdorferi circulating among wildlife but rarely encounter people. In the southeastern U.S., Ixodes affinis and Ixodes minor are more important than the blacklegged tick for maintaining the spirochete in wildlife.¹³ These ticks feed on rodents, birds, and other small animals, keeping the bacterium alive in the environment, but they almost never bite humans. So they sustain the pathogen without creating much human disease.

A similar situation has been documented in the West. In Colorado, Ixodes spinipalpis maintains a cycle of B. burgdorferi with wood rats (Neotoma mexicana), but this tick lives primarily inside wood rat nests in semiarid habitats. The risk of human contact is minimal, making this an epidemiologically “silent” focus of Lyme spirochetes.¹⁴ These hidden enzootic cycles are one reason researchers caution that the distribution of B. burgdorferi in nature is broader than human case maps suggest. The spirochete is out there in places where people rarely get sick, simply because the ticks shuttling it around in those ecosystems are not the ones that bite us.

The Lone Star Tick and the Lyme Lookalike

One of the most common sources of confusion is the lone star tick. It cannot transmit B. burgdorferi, as established above, but its bite can cause a condition that looks strikingly similar to early Lyme disease. Southern tick-associated rash illness, or STARI, produces a circular rash that can mimic the classic expanding “bull’s-eye” of Lyme, and it occurs primarily in southeastern and south-central states where true Lyme disease is relatively rare.¹⁵

STARI is thought to be caused by a different spirochete, Borrelia lonestari, though the evidence for this remains incomplete and the organism has been difficult to culture and study. The condition is generally considered less severe than Lyme disease, but the visual overlap has led to plenty of misdiagnosis in both directions: people in the South panicking that they have Lyme after a lone star tick bite, and clinicians in Lyme-endemic areas occasionally mistaking a STARI rash for Lyme when a patient has traveled south. If you develop a circular rash after a tick bite and you are outside the range of blacklegged ticks, STARI is the more likely explanation, and telling your doctor which tick species bit you (if you saved it or can describe it) helps enormously with diagnosis.

The lone star tick is also now well known for a different reason: bites from this species can trigger alpha-gal syndrome, an allergy to red meat and other mammalian products. This has nothing to do with Lyme disease, but it adds to the lone star tick’s reputation as a medically significant pest even though it is not a Lyme vector.

Another Spirochete in the Same Ticks

Blacklegged ticks do not only carry B. burgdorferi. They also transmit Borrelia miyamotoi, a related but distinct spirochete that causes a relapsing fever-like illness rather than classic Lyme disease.¹⁶ The illness it causes tends to feature recurrent fevers, headache, and muscle pain, but typically not the erythema migrans rash associated with Lyme. Because it is vectored by the same ticks and often found in the same geographic areas as Lyme, co-infection is possible, and clinicians in endemic areas are increasingly testing for it when tick-borne illness is suspected.

Blacklegged ticks can also carry the parasites and bacteria responsible for babesiosis, anaplasmosis, and Powassan virus disease. A single bite from a single tick can, in theory, deliver more than one pathogen simultaneously. This is why a tick bite from a blacklegged tick in an endemic area deserves medical attention if symptoms develop, even if a Lyme test comes back negative: the tick may have delivered something else.

Climate Change Is Reshaping the Map

The range of blacklegged ticks is not static. Climate modeling work has projected that warming temperatures could push I. scapularis populations northward by roughly 200 kilometers by the 2020s and as much as 1,000 kilometers by the 2080s, with tick abundance nearly doubling at the current northern boundary of the species’ range.¹⁷ Other analyses have predicted that the range of B. burgdorferi itself could expand 250 to 500 kilometers northward by 2050.¹⁸

In the northeastern U.S., where Lyme cases are already highest, warming annual temperatures have been the strongest climate-related predictor of rising incidence. One study projected a significant increase in cases in the Northeast by 2050 under a high-emissions scenario, though with substantial uncertainty around the exact numbers.¹⁹ The practical meaning for people living at the current edges of Lyme country, whether that is southern Canada, northern New England, or the upper Midwest, is that areas historically considered low-risk may not stay that way. Ticks that used to die off during cold winters are increasingly surviving and establishing populations in places they could not have thrived a few decades ago.

How to Tell What Bit You

Identifying the tick that bit you is the single most useful piece of information for assessing your Lyme risk, and it is something most people skip. Adult blacklegged ticks are small and reddish-brown with a dark dorsal shield, roughly the size of a sesame seed. Nymphs are much smaller, closer to the size of a poppy seed, and are the stage most often responsible for Lyme transmission. American dog ticks are noticeably larger, with prominent white or cream-colored markings on the back. Lone star ticks have a distinctive white dot on the female’s back and are aggressive biters. Asian longhorned ticks are small, reddish-brown, and can be tricky to distinguish from blacklegged ticks without magnification.

If you save a tick in a sealed bag or piece of tape, many county extension offices and some state health departments will identify it for free. Some private services will also test the tick for pathogens, though the clinical utility of tick testing is debated since a positive result does not necessarily mean you will develop disease, and a negative result does not guarantee you are safe. What matters most is knowing the species: if it is not an Ixodes tick, Lyme disease is essentially off the table, and your worry (and possibly your doctor’s antibiotic prescription) can be redirected to the pathogens that tick actually carries.