Lyme disease does not cause cellulitis, which is an acute bacterial infection of the skin usually caused by Staphylococcus or Streptococcus bacteria. The primary skin manifestation of Lyme disease, known as erythema migrans (EM), is frequently mistaken for cellulitis, leading to misdiagnosis. This confusion arises because both conditions present with redness and warmth, but they are caused by different organisms and require distinct antibiotic treatments. Differentiating between the Lyme rash and true cellulitis is important to prevent complications from untreated Lyme disease or an inappropriate antibiotic regimen for cellulitis.
Erythema Migrans: The Primary Lyme Rash
Erythema migrans (EM) is the characteristic expanding skin lesion that occurs in approximately 70% to 80% of Lyme disease cases. This rash results from the Borrelia burgdorferi bacteria multiplying and migrating outward from the site of the tick bite. It typically appears 3 to 30 days after the tick has fed, with an average onset of about one week.
The rash begins as a reddened area that gradually expands over days or weeks, often reaching a diameter of 16 centimeters or more. While the classic “bull’s-eye” pattern with central clearing is widely known, this appearance is only present in a minority of cases, perhaps 20% in the United States. More commonly, the lesion is a uniformly red or oval patch with a well-demarcated, expanding edge. EM is usually asymptomatic, meaning it is rarely painful, tender, or intensely itchy.
Why Lyme Rashes Are Mistaken for Cellulitis
The diagnostic confusion stems from the fact that many erythema migrans rashes do not present with the textbook bull’s-eye pattern. When the rash is uniformly red and lacks central clearing, it can closely resemble the diffuse redness associated with cellulitis. Both conditions involve an area of skin that is red and warm to the touch, and both can be found on limbs or areas commonly exposed to tick bites.
Physicians or patients often incorrectly identify the EM rash as a spider bite, shingles, or cellulitis. This misidentification is common when the rash appears in an atypical location, such as the neck, or when the patient does not recall a tick bite. Cellulitis is a common diagnosis for unexplained spreading redness, and this frequent misdiagnosis can lead to the prescription of antibiotics that are ineffective against Borrelia burgdorferi.
Differentiating the Two Conditions
Differentiating between erythema migrans and true cellulitis requires a thorough clinical assessment and detailed patient history. Cellulitis is an infection of the deep dermis and subcutaneous tissue that presents with marked pain and tenderness to the touch. The affected area is often swollen and warm, and the redness tends to have poorly defined, irregular borders.
Patients with true cellulitis often present with systemic symptoms associated with a severe bacterial infection, such as a high fever, rigors, and general malaise. Cellulitis usually has a clear entry point for bacteria, such as a cut, scrape, or insect bite, and it typically progresses rapidly over a day or two. The redness in cellulitis is usually uniform, lacking the central clearing or concentric rings seen in a classic EM rash.
Erythema migrans, conversely, is usually painless and non-tender, with a rash that expands gradually over days or weeks. While a patient with Lyme disease may experience systemic symptoms, these are usually flu-like, including headache, fatigue, and muscle aches, rather than the acute, high-grade fevers common with severe cellulitis. The history of potential tick exposure, even if the bite was not noticed, is a strong indicator pointing toward Lyme disease.
Treatment Protocols and Urgency
Correct diagnosis is necessary because standard antibiotics for cellulitis do not work for Lyme disease. Cellulitis, caused by Staphylococcus or Streptococcus, is treated with antibiotics like cephalexin or clindamycin. If untreated, cellulitis can rapidly spread and potentially lead to severe complications, including sepsis.
Lyme disease, caused by the spirochete Borrelia burgdorferi, requires specific antibiotics, such as doxycycline, amoxicillin, or cefuroxime. Antibiotics commonly used for cellulitis, like cephalexin, are ineffective against the Lyme bacteria. Treating Lyme disease with an inappropriate drug results in a failure to cure the infection, allowing the bacteria to disseminate and cause later-stage complications involving the nervous system, joints, or heart. In cases where the diagnosis is uncertain, some practitioners may choose an antibiotic like doxycycline that is effective against both the Lyme bacteria and common cellulitis pathogens.