About This Page
This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- RMSF is a clinical diagnosis; start doxycycline immediately in adults and children — do not wait for serology
- Lyme erythema migrans is diagnosed clinically and treated with doxycycline, amoxicillin, or cefuroxime; serology may be negative early
- Ehrlichiosis/anaplasmosis: fever, headache, leukopenia, thrombocytopenia, transaminitis; treat with doxycycline
- Babesiosis: malaria-like hemolytic anemia with Maltese cross on smear; treat atovaquone plus azithromycin unless severe
- Single-dose doxycycline prophylaxis can be used after high-risk Ixodes tick bite meeting timing and endemicity criteria
- Step 2 CK: doxycycline is first-line for suspected RMSF even in children and pregnancy risk must be balanced against fatal delay
Overview
Tick-borne diseases are seasonal vector-borne infections with overlapping nonspecific early symptoms. Geography, tick exposure, rash morphology, cytopenias, hemolysis, and organ involvement help distinguish them. Lyme disease is caused by Borrelia burgdorferi and transmitted by Ixodes ticks. RMSF is caused by Rickettsia rickettsii and can be rapidly fatal. Ehrlichiosis/anaplasmosis cause systemic febrile illness with cytopenias. Babesiosis infects red blood cells and causes hemolysis, particularly severe in asplenia or immunosuppression.
Epidemiology
Lyme disease is concentrated in the Northeast, mid-Atlantic, upper Midwest, and parts of the Pacific Coast. RMSF and other spotted fever rickettsioses are reported across much of the United States, with notable burden in the Southeast and south-central regions. Ehrlichiosis is associated with lone star ticks, while anaplasmosis and babesiosis share Ixodes geography with Lyme. Coinfection can occur after Ixodes exposure.
Clinical Features
Symptoms
Lyme early localized: expanding erythema migrans, fatigue, fever, headache, myalgias, arthralgias
Lyme disseminated: facial nerve palsy, meningitis, radiculopathy, AV block, migratory arthritis
RMSF: fever, severe headache, myalgias, GI symptoms; rash may start wrists/ankles then palms/soles and trunk
Ehrlichiosis/anaplasmosis: fever, headache, malaise, myalgias, confusion; rash uncommon in adults
Babesiosis: fever, chills, sweats, fatigue, dark urine, jaundice from hemolysis
Severe tick-borne illness: respiratory failure, shock, renal failure, meningoencephalitis
Signs
Erythema migrans: expanding annular lesion, not always classic bullseye
RMSF petechial rash involving palms/soles is late and ominous
Facial palsy, meningismus, or irregular bradycardia/AV block in Lyme disease
Hepatosplenomegaly or jaundice in babesiosis
Hypotension, altered mental status, purpura, or respiratory distress indicates severe disease
Investigations
First-line
Clinical diagnosis and exposure historyDo not delay doxycycline for suspected RMSF, ehrlichiosis, or anaplasmosis while awaiting tests
CBC and CMPThrombocytopenia, leukopenia, hyponatremia, and transaminitis support rickettsial/ehrlichial disease; anemia supports babesiosis
Lyme two-tier serologyUseful after several weeks; often negative in early erythema migrans, which is diagnosed clinically
Second-line
Peripheral blood smearBabesiosis shows intraerythrocytic parasites and sometimes Maltese cross tetrads; also assesses parasitemia
PCR testingHelpful early for ehrlichiosis, anaplasmosis, babesiosis; sensitivity varies and treatment should not wait
ECGFor suspected Lyme carditis; look for PR prolongation or high-grade AV block
Specialist
Lumbar punctureIf meningitis/encephalitis suspected; Lyme meningitis shows lymphocytic pleocytosis
Hospital monitoringSevere RMSF, high-grade AV block, severe babesiosis, asplenia, pregnancy, or immunocompromise
1
Lyme disease
- Erythema migrans: doxycycline, amoxicillin, or cefuroxime; do not wait for serology
- Facial palsy without meningitis can often be treated orally; meningitis or radiculopathy may use IV ceftriaxone or oral doxycycline depending presentation
- Lyme carditis: hospitalize if PR >=300 ms, high-grade AV block, syncope, or myocarditis; IV ceftriaxone initially if severe
- Lyme arthritis: oral doxycycline/amoxicillin/cefuroxime; persistent arthritis may require repeat therapy or rheumatology evaluation
2
RMSF, ehrlichiosis, anaplasmosis
- Doxycycline is first-line for adults and children of all ages
- Start immediately when suspected; early serology is often negative
- Treat RMSF at least 5-7 days and until clinically improved and afebrile for several days
- Chloramphenicol is less effective and rarely used; sulfonamides may worsen RMSF outcomes
3
Babesiosis
- Mild/moderate: atovaquone plus azithromycin
- Severe disease: clindamycin plus quinine, consider exchange transfusion for high parasitemia, severe hemolysis, or organ failure
- Higher risk: asplenia, older age, immunocompromise, HIV, malignancy, rituximab exposure
4
Tick bite prophylaxis and prevention
- Single-dose doxycycline for Lyme prophylaxis if Ixodes tick, endemic area, attached >=36 h, prophylaxis within 72 h, and no contraindication
- Use permethrin-treated clothing, DEET/picaridin repellents, tick checks, and prompt tick removal
Complications
- RMSF mortality: Risk rises sharply when doxycycline is delayed beyond day 5
- Lyme carditis: High-grade AV block may require temporary pacing
- Neuroborreliosis: Meningitis, radiculopathy, facial palsy
- Severe babesiosis: Hemolytic anemia, ARDS, renal failure, DIC, death, especially in asplenia
- Post-treatment symptoms: Fatigue or pain can persist after Lyme treatment but prolonged antibiotics are not indicated
USMLE Step 2 CK Exam Tips
- 1Fever + headache + rash involving wrists/ankles/palms/soles after tick exposure = RMSF; give doxycycline now
- 2Never wait for RMSF serology before treatment; early tests are often negative
- 3Erythema migrans is a clinical diagnosis; early Lyme serology can be negative
- 4Lyme carditis = fluctuating AV block in young patient from endemic area; admit if PR >=300 ms or high-grade block
- 5Ehrlichiosis = fever + leukopenia + thrombocytopenia + transaminitis; doxycycline
- 6Babesiosis = hemolytic anemia + Maltese cross; atovaquone plus azithromycin
- 7Asplenic patient with babesiosis can deteriorate rapidly and may need exchange transfusion
practicetest your knowledge on tick-borne diseases (lyme, rocky mountain spotted fever, ehrlichiosis, babesiosis)Apply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — infectious diseases and beyond.
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