What are the major differences between Lyme disease and Ehrlichiosis?

Major Differences Between Lyme Disease and Ehrlichiosis

While both Lyme disease and ehrlichiosis are tick-borne infections that can present with fever and nonspecific symptoms, they differ fundamentally in their causative organisms, cell tropism, clinical presentations, geographic distribution, and treatment considerations—most critically, amoxicillin treats Lyme disease but is completely ineffective for ehrlichiosis, making accurate differentiation essential to prevent mortality. 1

Causative Organisms and Cell Tropism

Lyme Disease

• Caused by the spirochete Borrelia burgdorferi in North America 2
• Does not infect blood cells and cannot be visualized on routine blood smears 2
• Spirochetes disseminate through tissues and organs rather than residing in leukocytes 2

Ehrlichiosis

• Caused by obligate intracellular bacteria: Ehrlichia chaffeensis (monocytic ehrlichiosis) or Ehrlichia ewingii (granulocytic ehrlichiosis) 1
• E. chaffeensis specifically infects monocytes, while E. ewingii infects granulocytes 1
• Bacteria multiply in cytoplasmic membrane-bound vacuoles as microcolonies called morulae, which can occasionally be visualized on Wright or Giemsa-stained blood smears 1
• Morulae are seen in monocytes in peripheral blood in some E. chaffeensis cases, providing a diagnostic clue not available in Lyme disease 1

Geographic Distribution and Vectors

Lyme Disease

• Most common in Northeast, upper Midwest, and focal West Coast areas 1, 2
• Transmitted by Ixodes scapularis (blacklegged tick) in the Northeast/Midwest and Ixodes pacificus in California 1
• Causes approximately 300,000 illnesses annually in the United States 2

Ehrlichiosis

• HME (monocytic ehrlichiosis) occurs predominantly in southeastern and south-central United States 1
• Transmitted by Amblyomma americanum (lone star tick) 1
• Geographic distribution differs significantly from Lyme disease, though some overlap exists 1

Clinical Presentation

Lyme Disease

• Erythema migrans (EM) rash occurs in 70-80% of cases—an expanding annular lesion that is the hallmark of early infection 2
• Progresses through three stages: early localized, early disseminated, and late disseminated 2
• Early disseminated disease manifests as multiple skin lesions, facial palsy, meningitis, or carditis 2
• Late disease presents as recurrent large-joint arthritis 2
• Fever may be present but is less prominent than in ehrlichiosis 3

Ehrlichiosis (E. chaffeensis)

• Rash is present in only approximately one-third of patients and is MORE common in children than adults—this is the opposite pattern of many other tick-borne diseases 1
• Fever, headache, malaise, and myalgia are the predominant symptoms 1
• Gastrointestinal symptoms are common 1
• Neurologic manifestations occur in approximately 20% of patients 1
• Patients with ehrlichiosis are typically older and more likely to have fever, chills, or dyspnea compared to Lyme disease patients 3
• Case-fatality rate is approximately 3% for E. chaffeensis ehrlichiosis—significantly higher mortality than Lyme disease 1

Laboratory Findings

Lyme Disease

• Normal complete blood count in most cases 4
• Liver enzymes may be mildly elevated 4
• Diagnosis is clinical for erythema migrans in endemic areas; laboratory confirmation needed for other manifestations 2
• Two-tiered serologic testing (ELISA followed by Western blot) is standard 2
• Serology has low sensitivity in early disease but increases to 70-100% in disseminated disease 2

Ehrlichiosis

• Characteristic laboratory findings include leukopenia, thrombocytopenia, and elevated hepatic transaminases in the first week of illness 1
• During recovery, relative and absolute lymphocytosis develops 1
• Mild-to-moderate hyponatremia may be present 1
• Blood smear examination may reveal morulae in monocytes (E. chaffeensis) or granulocytes (E. ewingii), though sensitivity is limited 1
• Confirmatory testing requires serology, PCR, or immunostaining 1

Critical Treatment Differences

Lyme Disease

• Doxycycline 100 mg twice daily for 14-21 days is preferred 5
• Amoxicillin can be used to treat early-stage Lyme disease 1
• Treatment failure rate is approximately 1% 5

Ehrlichiosis

• Amoxicillin is NOT effective for ehrlichiosis—this is a critical distinction 1
• Doxycycline is highly efficacious and is the treatment of choice 6
• Delayed therapy is associated with severe or fatal outcomes 6

Coinfection Considerations

Because Ixodes ticks transmit both Borrelia burgdorferi and Anaplasma phagocytophilum (causing human granulocytic anaplasmosis, similar to ehrlichiosis), simultaneous infections occur and have been documented 1. This is particularly important because:

• Coinfection rates can be significant—one study found unsuspected Ehrlichia infection in 13% of patients diagnosed with Lyme disease 3
• Discerning mixed infection is vital because it affects antimicrobial choice—amoxicillin alone would fail to treat the ehrlichial component 1
• Most patients with apparent coinfection do not have hematologic abnormalities, making clinical diagnosis challenging 3
• Doxycycline covers both infections and is therefore preferred in endemic areas where coinfection is possible 5

Common Pitfalls to Avoid

• Do not assume absence of rash rules out Lyme disease—30% of Lyme patients never develop erythema migrans 2
• Do not assume presence of rash rules out ehrlichiosis—one-third of ehrlichiosis patients have rash 1
• Do not rely solely on morulae detection for ehrlichiosis diagnosis—blood smears are insensitive and cases will be missed 1
• Do not use amoxicillin empirically for tick-borne illness in areas where ehrlichiosis or anaplasmosis occur—this could result in treatment failure and mortality 1
• Do not delay treatment while awaiting serologic confirmation in clinically suspected cases—both diseases require prompt antibiotic therapy 5
• Do not fail to consider coinfection in endemic areas—ticks can transmit multiple pathogens simultaneously 1, 3