A new invader is complicating the tick-borne disease playbook. Are you ready for the diagnostic curveball?
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Erythema migrans is not as reliable a marker of Lyme disease as was previously thought.
—Steve E. Schutzer, MD, Anna-Marie Wellins, DNP, and Raymond J. Dattwyler, MD, writing in NEJM
The lone star tick (Amblyomma americanum) was formerly found primarily in the Southeast, South Central, and Mid-Atlantic United States, but it is now increasingly found in the Northeast and Midwest, overlapping with the tick Ixodes scapularis, which is the vector for Lyme disease. [] []
As these species increasingly overlap, clinicians may face growing diagnostic uncertainty when evaluating erythema migrans–like rashes and other tick-borne presentations. Below, we break down what this expanding footprint means for differential diagnosis, empiric treatment decisions, and patient counseling in everyday practice.
Lone star tick range expansion and disease risks
“This tick species, in addition to being an aggressive human-biting tick … is involved in transmission of several disease agents, causing diseases such as tularemia, Ehrlichia, or ehrlichiosis … It is also associated with red meat allergy or alpha-gal syndrome. In addition to these disease agents, it is also involved in transmission of Southern tick-associated rash illness, or STARI, and a few viruses named the Heartland virus and Bourbon virus,” said Goudarz Molaei, MD, in an interview with NEJM. []
Researchers think the expansion of the lone star tick’s range is linked to environmental and land-use changes. []This tick species can expand rapidly in areas with a growing deer population. []
Unfortunately, it can be even harder to avoid than blacklegged ticks (Ixodes scapularis, also known as deer ticks), which spread Lyme disease, because it thrives in a wide range of open habitats, not just wooded areas.
A surveillance map of the lone star tick’s range is available on the CDC's website. []
Potential confusion between STARI and Lyme disease
Due to the recent range expansion, lone star tick–associated diseases may be found in areas where clinicians are less familiar with them. One of these diseases is STARI, which typically begins with a rash that resembles the erythema migrans of Lyme disease. [] The rash may be accompanied by fatigue, headache, muscle and joint pain, and fever.
According to a recent letter published in NEJM highlighting this diagnostic uncertainty, although Lyme disease–carrying Ixodes scapularis ticks were previously dominant on Long Island, the majority of ticks found in 2023 were actually lone star ticks. [] This pattern has also been observed in parts of Massachusetts and New Jersey.
According to the authors, this situation could lead to misdiagnosis of STARI as Lyme disease. However, they write, “patients presenting with an erythema migrans–like lesion may still be treated for Lyme disease because the risks of antibiotic treatment are probably lower than those associated with untreated Lyme disease.” []
The cause of STARI remains unknown, and it is unclear whether antibiotics help. The lone star tick has been shown not to transmit Lyme disease. [] Given this, management decisions often rely on clinical judgment rather than laboratory confirmation.
Takeaways for the clinic
For clinicians in the Northeast and Midwest, an erythema migrans–like rash can no longer be assumed to signal Lyme disease alone. Lone star ticks do not transmit Borrelia burgdorferi, but they are linked to STARI, ehrlichiosis, tularemia, Heartland virus, and alpha-gal syndrome. As their geographic range expands, clinicians should consider broader tick-borne differentials, review updated CDC surveillance maps, and counsel patients accordingly.
Current IDSA guidelines still support empiric treatment for classic erythema migrans in endemic areas given the risk of untreated Lyme disease.[][] However, awareness of STARI, whose etiology remains unclear and for which antibiotic benefit is uncertain, may help refine diagnostic reasoning, testing decisions, and follow-up planning.
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