Tick Bite Workup: Geography Drives the Differential, and Timing Determines the Test

Don’t order Lyme titers the day after a tick bite. They’ll be negative. And don’t order them at all if the patient was bitten in Oklahoma.

Tick-borne illness is one of primary care's most geography-dependent diagnoses. The tick species, the pathogen it carries, the region where the bite occurred, and the timing of your lab work all determine whether you'll catch the diagnosis or miss it entirely. This post maps out which diseases to suspect based on location, when to test, what tests to order, and—critically—when to treat empirically before labs come back.

Step One: Identify the Tick and the Geography

Tick Species	Common Name	Primary Geography	Diseases Transmitted
Ixodes scapularis	Blacklegged tick / Deer tick	Northeast, upper Midwest, mid-Atlantic (CT, MA, NY, NJ, PA, MN, WI)	Lyme disease, Anaplasmosis, Babesiosis, Powassan virus, B. miyamotoi relapsing fever
Ixodes pacificus	Western blacklegged tick	Pacific Coast (CA, OR, WA)	Lyme disease (lower incidence than East Coast), Anaplasmosis
Amblyomma americanum	Lone Star tick	Southeast, south-central US (TX, OK, AR, MO, TN, NC, VA, FL)	Ehrlichiosis, STARI (Southern Tick-Associated Rash Illness), Heartland virus, Bourbon virus, Alpha-gal syndrome
Dermacentor variabilis	American Dog tick	East of Rocky Mountains, Pacific Coast	Rocky Mountain Spotted Fever (RMSF), Tularemia
Dermacentor andersoni	Rocky Mountain Wood tick	Rocky Mountain states (MT, WY, CO, ID)	RMSF, Colorado Tick Fever, Tularemia
Amblyomma maculatum	Gulf Coast tick	Gulf Coast states, expanding northward	Rickettsia parkeri rickettsiosis (milder cousin of RMSF, often with eschar at bite site)

Geography Is Your Differential

A febrile patient with a tick bite in Connecticut = think Lyme, Anaplasmosis, Babesiosis. A febrile patient with a tick bite in North Carolina = think RMSF, Ehrlichiosis, STARI. A febrile patient with a tick bite in Oklahoma or Arkansas = RMSF and Ehrlichiosis dominate; Lyme disease is exceedingly rare. Always ask where the bite occurred, not just where the patient lives. Travel history changes everything.

The Major Tick-Borne Diseases: Lab Workup and Timing

Lyme Disease (Borrelia burgdorferi)

Stage	Timing	Clinical Presentation	Lab Approach
Early Localized	3–30 days post-bite	Erythema migrans (EM) — expanding bull's-eye rash ≥5 cm. May have flu-like symptoms.	DO NOT order serology. Diagnose CLINICALLY. EM is pathognomonic. Serology is negative in up to 60% of early localized disease. Treat empirically: doxycycline 100 mg BID × 10–21 days.
Early Disseminated	Weeks to months	Multiple EM lesions, facial palsy (CN VII), carditis (heart block), meningitis, radiculopathy	Two-tier testing: EIA/ELISA (screening) → if positive/equivocal, confirmatory Western blot (IgM if <30 days of symptoms, IgM + IgG if >30 days). OR the newer modified two-tier (MTTT): EIA #1 → EIA #2 (no Western blot). Serology is ~70–80% sensitive at this stage.
Late Disseminated	Months to years	Lyme arthritis (large joint, especially knee), late neurologic Lyme (encephalopathy, polyneuropathy)	IgG serology is almost always positive at this stage (>95% sensitivity). IgM alone is NOT sufficient for late disease—IgM positivity without IgG in late Lyme is likely a false positive.

The Timing Trap

Lyme serology takes 2–6 weeks to become positive after infection. Ordering Lyme titers the day after a tick bite—or even the day the EM rash appears—will be negative and gives false reassurance. In early localized Lyme (EM rash), the diagnosis is clinical and the treatment is empiric. Serology is for disseminated disease when the clinical picture is compatible but the EM rash was missed or absent.

Prophylaxis After Tick Bite (The 72-Hour Window)

Single-dose doxycycline 200 mg PO can be offered for Lyme prophylaxis if ALL of the following are met:

• The tick is identified as Ixodes scapularis (deer tick)
• The bite occurred in a Lyme-endemic area
• The tick was attached for ≥36 hours (engorged appearance)
• Prophylaxis can be given within 72 hours of tick removal
• No contraindication to doxycycline (pregnancy, age <8—though recent evidence supports doxycycline in shorter courses for children)

If any criterion is not met, observe without prophylaxis. Monitor for EM rash for 30 days.

Rocky Mountain Spotted Fever (Rickettsia rickettsii)

Treat First, Test Later

RMSF has a case fatality rate of 20–25% if untreated and can kill within days. Do NOT wait for lab confirmation to treat. If the clinical picture is compatible (fever + headache + myalgia ± rash in a tick-exposed patient in an endemic area), start doxycycline immediately—adults AND children of any age. The rash (petechial, starting on wrists/ankles, spreading centrally) may not appear until day 3–5 or may be absent in 10–15% of cases. Delaying treatment until the rash appears or until labs confirm increases mortality dramatically.

• Diagnosis: IFA (Indirect Fluorescent Antibody) for R. rickettsii IgG—paired sera (acute + convalescent 2–4 weeks later). A 4-fold rise confirms the diagnosis. The acute sample is almost always negative. Diagnosis is typically retrospective.
• CBC pattern: Thrombocytopenia, leukopenia (or normal WBC), elevated LDH, hyponatremia, elevated transaminases
• Geography: Despite the name, most cases occur in the southeast and south-central US (NC, TN, OK, AR, MO), not the Rocky Mountains. Also seen in tribal lands of the Southwest (Arizona).
• Treatment: Doxycycline 100 mg BID (adults) or 2.2 mg/kg BID (children) for 5–7 days or until afebrile ≥3 days.

Anaplasmosis (Anaplasma phagocytophilum)

• Vector: Ixodes scapularis (same tick as Lyme—co-infection is common)
• Geography: Northeast, upper Midwest (overlaps Lyme belt)
• Presentation: Fever, headache, myalgia, malaise. NO rash (or rash is rare). 1–2 weeks after bite.
• Lab pattern: Leukopenia + thrombocytopenia + elevated transaminases—this triad in a febrile patient from an endemic area is Anaplasmosis until proven otherwise.
• Diagnosis: Peripheral blood smear showing morulae (intracytoplasmic inclusions in neutrophils)—seen in 20–80% of cases. PCR (most sensitive in first week). IFA serology (paired sera; same timing issues as RMSF).
• Treatment: Doxycycline. Empiric treatment for the leukopenia/thrombocytopenia/transaminitis triad while awaiting confirmation.

Ehrlichiosis (Ehrlichia chaffeensis)

• Vector: Amblyomma americanum (Lone Star tick)
• Geography: Southeast, south-central US
• Presentation: Clinically identical to Anaplasmosis. Fever, headache, myalgia. Rash more common than Anaplasmosis (~30%) but still not reliable.
• Lab pattern: Same triad: leukopenia + thrombocytopenia + elevated transaminases
• Diagnosis: Morulae in monocytes (not neutrophils—that's the distinction from Anaplasmosis). PCR, IFA serology.
• Treatment: Doxycycline.

Babesiosis (Babesia microti)

• Vector: Ixodes scapularis (co-infection with Lyme and Anaplasmosis possible)
• Geography: Northeast and upper Midwest (concentrated in southern New England and NY)
• Presentation: Fever, fatigue, hemolytic anemia. Can be severe in asplenic, immunocompromised, or elderly patients.
• Lab pattern: Hemolytic anemia (elevated LDH, low haptoglobin, elevated indirect bilirubin, reticulocytosis) + thrombocytopenia
• Diagnosis: Thin blood smear showing intraerythrocytic ring forms (look like malaria—pathology must be alerted to consider Babesia). PCR. IFA serology.
• Treatment: Atovaquone + azithromycin (mild/moderate). Clindamycin + quinine (severe). NOT doxycycline—Babesia is a parasite, not a bacterium.

The Co-Infection Problem

Same Tick, Multiple Pathogens

Ixodes scapularis can carry Lyme, Anaplasmosis, and Babesiosis simultaneously. A single tick bite can transmit more than one pathogen. Suspect co-infection when a Lyme patient doesn't respond to doxycycline as expected, has persistent high fevers (uncommon in Lyme alone), has hemolytic anemia (Babesiosis), or has the leukopenia/thrombocytopenia/transaminitis triad (Anaplasmosis). When treating Lyme in the northeast, have a low threshold to check CBC, smear, and liver enzymes for co-infection.

Alpha-Gal Syndrome: The Lone Star Tick Surprise

The Lone Star tick (Amblyomma americanum) can sensitize patients to alpha-gal, a carbohydrate found in mammalian meat (beef, pork, lamb). This causes a delayed IgE-mediated allergic reaction 3–6 hours after eating red meat.

• Presentation: Urticaria, angioedema, GI symptoms, or anaphylaxis occurring 3–6 hours after eating mammalian meat. The delay makes the connection non-obvious.
• Diagnosis: Alpha-gal IgE (specific IgE to galactose-alpha-1,3-galactose). Total tryptase if anaphylaxis suspected.
• Geography: Southeast US (Lone Star territory), expanding northward
• Management: Avoidance of mammalian meat and products (some patients also react to dairy, gelatin, or medications in gelatin capsules). EpiPen prescription for those with history of anaphylaxis.

STARI: The Lyme Look-Alike of the South

Southern Tick-Associated Rash Illness (STARI) presents with an EM-like expanding rash after a Lone Star tick bite in the southeastern US. It is NOT Lyme disease—B. burgdorferi is not transmitted by Lone Star ticks. The causative agent remains unidentified. STARI is milder than Lyme, does not cause the disseminated complications (carditis, arthritis, neurologic disease), and is generally treated empirically with doxycycline. The clinical pearl: an EM-like rash in Georgia or Texas is STARI, not Lyme. Don't order Lyme serology in non-endemic areas.

Timing Summary: When to Order What

Scenario	Lab Approach	Timing
Asymptomatic tick bite, tick just removed	No labs. Observe for 30 days. Prophylaxis if criteria met (see above).	N/A
EM rash present	No serology needed. Diagnose clinically. Treat with doxycycline.	Day of diagnosis
Fever + tick exposure, endemic for RMSF/Ehrlichiosis	CBC, CMP, start doxycycline empirically. Draw acute serology (IFA) but don't wait for results. Convalescent titers in 2–4 weeks.	Treat immediately. Labs are retrospective.
Suspected disseminated Lyme (facial palsy, carditis, arthritis)	Two-tier serology (EIA/ELISA → Western blot or MTTT). Sensitivity ~70–95% depending on stage.	≥2–4 weeks after infection for IgM; ≥4–6 weeks for IgG
Suspected Anaplasmosis/Ehrlichiosis	CBC (leukopenia + thrombocytopenia + elevated LFTs), blood smear for morulae, PCR (first week), IFA (paired sera)	PCR best in first week of illness. Serology requires convalescent sample.
Suspected Babesiosis	Thin blood smear, PCR, CBC (hemolytic anemia pattern), IFA serology	Smear and PCR at presentation. Serology takes weeks.
Suspected alpha-gal	Alpha-gal specific IgE	Anytime after sensitization (weeks to months post-bite)

The Pitfalls

• Ordering Lyme serology too early: The #1 error. Serology is negative in the first 1–3 weeks. EM rash is a clinical diagnosis. A negative titer at day 5 rules out nothing.
• Ordering Lyme serology in non-endemic areas: Lyme is concentrated in the Northeast and upper Midwest. A positive Lyme titer in a patient bitten in Alabama is almost certainly a false positive. Geography first, labs second.
• Waiting for labs to treat RMSF: This kills people. Doxycycline is the treatment for all suspected rickettsial disease, and it should be started on clinical suspicion alone. The labs confirm the diagnosis after the fact.
• Forgetting co-infection: A Lyme patient in Connecticut who isn't improving on doxycycline may have concurrent Babesiosis (which doesn't respond to doxycycline). Check the smear.
• IgM-only positivity in late Lyme: If symptoms have been present for >30 days and IgG is negative but IgM is positive, this is most likely a false positive IgM, not late Lyme disease.
• Checking “Lyme titers” as a screening test for chronic fatigue: Lyme serology is not a screening tool for nonspecific symptoms. It has a 5% false positive rate. In low-probability patients, a positive result is more likely false than true (Bayesian reasoning).
• Doxycycline in children: The AAP and CDC now endorse doxycycline for children of ALL ages for tick-borne illness. Short courses (≤21 days) do not cause dental staining. Do not withhold doxycycline from a child with suspected RMSF.
• Missing the Lone Star tick expanding range: Amblyomma americanum is moving northward due to climate change. Ehrlichiosis and alpha-gal are appearing in areas that were previously non-endemic. Stay current on your local tick maps.

Bottom Line

Tick-borne illness workup starts with two questions: where was the patient bitten, and when? Geography determines the differential. Timing determines the test. EM rash = treat without labs. RMSF suspicion = treat before labs. Lyme serology is useless in the first 2 weeks and useless outside endemic areas. The leukopenia/thrombocytopenia/transaminitis triad points to Anaplasmosis or Ehrlichiosis. Hemolytic anemia after an Ixodes bite = Babesiosis. And doxycycline is safe for children. Know your ticks, know your geography, and when in doubt, give the doxycycline.

Stay sharp out there.