What tick-borne illness is Ehrlichia related to?

Ehrlichia is Closely Related to Anaplasmosis Among Tick-Borne Illnesses

Ehrlichia is most closely related to Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis, as both belong to the family Anaplasmataceae and share similar clinical presentations, pathophysiology, and treatment approaches. 1

Relationship Between Ehrlichia and Anaplasmosis

Ehrlichia and Anaplasma are closely related genera of obligate intracellular bacteria that cause tick-borne illnesses with similar clinical presentations:

• Both are transmitted through tick bites
• Both infect white blood cells (though different cell types)
• Both produce similar clinical syndromes characterized by fever, headache, myalgia, and laboratory abnormalities
• Both are treated with the same antibiotic regimen (doxycycline)

Key Similarities

• Pathophysiology: Both Ehrlichia and Anaplasma multiply in cytoplasmic membrane-bound vacuoles as microcolonies called morulae 1
• Clinical presentation: Both cause fever, headache, malaise, and myalgia 1, 2
• Laboratory findings: Both typically cause leukopenia, thrombocytopenia, and elevated hepatic transaminases 1
• Treatment: Both respond to doxycycline as first-line therapy 3

Different Ehrlichia Species and Their Characteristics

There are three main species of Ehrlichia that cause human disease in the United States:

1. Ehrlichia chaffeensis:

   • Causes human monocytic ehrlichiosis (HME)
   • Transmitted by Lone Star ticks (Amblyomma americanum)
   • Infects mononuclear phagocytes (monocytes)
   • Case-fatality rate approximately 3% 1, 2

2. Ehrlichia ewingii:

   • Similar clinical features to E. chaffeensis but with less gastrointestinal symptoms and rash
   • Also transmitted by Lone Star ticks
   • Infects granulocytes
   • No deaths reported 1

3. Ehrlichia muris-like agent (EML):

   • Recently described pathogen (2011)
   • Transmitted by blacklegged ticks (Ixodes scapularis)
   • Target cell type not yet known
   • No fatal cases reported 1

Anaplasma phagocytophilum

Anaplasma phagocytophilum causes human granulocytic anaplasmosis (HGA):

• Infects predominantly neutrophils (granulocytes)
• Transmitted by Ixodes species ticks
• Occurs mostly in upper midwest and northeast United States
• Case-fatality rate <1% 1, 2

Clinical Distinctions Between Ehrlichiosis and Anaplasmosis

While similar, there are some clinical differences:

• Rash: Present in approximately one-third of E. chaffeensis cases but <10% of anaplasmosis cases 1
• Gastrointestinal symptoms: More common in ehrlichiosis than anaplasmosis 1
• Neurologic involvement: Neurologic manifestations occur in approximately 20% of E. chaffeensis cases but are rare in anaplasmosis 1, 3
• Severity: Severe or life-threatening manifestations are less frequent with anaplasmosis than with E. chaffeensis ehrlichiosis 1

Diagnostic Considerations

• Morulae (microcolonies) can be observed in:

  • Monocytes in E. chaffeensis infection
  • Granulocytes in E. ewingii and A. phagocytophilum infections 1

• Laboratory confirmation typically requires:

  • PCR testing
  • Serologic testing
  • Culture (in specialized laboratories) 1, 4

Important Clinical Pearls

• Empiric treatment with doxycycline should never be delayed pending laboratory confirmation due to risk of severe complications and death 4
• Immunocompromised patients are at higher risk for severe disease and death 1, 5
• Co-infection with other tick-borne pathogens like Borrelia burgdorferi (Lyme disease) may occur and should be considered 3

In summary, Ehrlichia species are most closely related to Anaplasma phagocytophilum, with both causing similar tick-borne illnesses that differ primarily in the cell types they infect and some clinical features.