Key Differences and Considerations in Lyme Disease and Ehrlichiosis

Causative Organisms and Cell Tropism

The Centers for Disease Control and Prevention (CDC) states that Lyme disease is caused by the spirochete Borrelia burgdorferi in North America 2
The CDC notes that Ehrlichia chaffeensis (monocytic ehrlichiosis) or Ehrlichia ewingii (granulocytic ehrlichiosis) are the causative agents of ehrlichiosis, which specifically infect monocytes or granulocytes 3, 4
The American Academy of Pediatrics recommends awareness that ehrlichial bacteria multiply in cytoplasmic membrane-bound vacuoles as microcolonies called morulae, which can occasionally be visualized on Wright or Giemsa-stained blood smears 1, 5

The CDC reports that Lyme disease is most common in Northeast, upper Midwest, and focal West Coast areas, transmitted by Ixodes scapularis (blacklegged tick) in the Northeast/Midwest and Ixodes pacificus in California 6, 2
The CDC states that ehrlichiosis (HME) occurs predominantly in southeastern and south-central United States, transmitted by Amblyomma americanum (lone star tick) 6, 5

The Infectious Diseases Society of America (IDSA) notes that erythema migrans (EM) rash occurs in 70-80% of Lyme disease cases, which is an expanding annular lesion that is the hallmark of early infection 2
The CDC reports that rash is present in only approximately one-third of ehrlichiosis patients and is more common in children than adults, with fever, headache, malaise, and myalgia being the predominant symptoms 3, 4
The IDSA states that neurologic manifestations occur in approximately 20% of ehrlichiosis patients 3, 4
The CDC notes that the case-fatality rate is approximately 3% for E. chaffeensis ehrlichiosis, significantly higher than Lyme disease 4

The IDSA recommends that diagnosis of Lyme disease is clinical for erythema migrans in endemic areas, with laboratory confirmation needed for other manifestations, using two-tiered serologic testing (ELISA followed by Western blot) 2
The CDC states that characteristic laboratory findings for ehrlichiosis include leukopenia, thrombocytopenia, and elevated hepatic transaminases in the first week of illness, with mild-to-moderate hyponatremia possibly present 3, 4
The IDSA notes that blood smear examination may reveal morulae in monocytes (E. chaffeensis) or granulocytes (E. ewingii), though sensitivity is limited 1, 5

The IDSA recommends doxycycline 100 mg twice daily for 14-21 days as the preferred treatment for Lyme disease, with amoxicillin as an alternative for early-stage disease 7, 1, 5
The CDC states that amoxicillin is not effective for ehrlichiosis, making doxycycline the treatment of choice 1, 5

The CDC notes that because Ixodes ticks transmit both Borrelia burgdorferi and Anaplasma phagocytophilum, simultaneous infections occur and have been documented, making it essential to consider coinfection in endemic areas 1, 6
The IDSA recommends that discerning mixed infection is vital because it affects antimicrobial choice, with doxycycline covering both infections and preferred in endemic areas where coinfection is possible 7