ehrlichiosis

• ehrlichiosis is a tick-borne bacterial infection caused by Ehrlichia species, predominantly Ehrlichia chaffeensis and Ehrlichia ewingii
• epidemiology and vectors: almost entirely confined to North America, though imported cases occur in the UK and should be considered in symptomatic patients with a relevant travel history:
  • main vector is the Lone Star tick (Amblyomma americanum)
  • primary animal reservoirs are the white-tailed deer and the white-footed mouse (also a major reservoir for Lyme disease in the USA)
  • co-infections: in the USA, co-infections with Lyme disease and/or babesiosis are relatively frequent and must be accounted for during diagnosis and management (co-infections are rarely seen elsewhere)

Clinical features

• human monocytic ehrlichiosis (HME): caused by E. chaffeensis, which targets monocytes and causes significantly more severe disease than anaplasmosis or Lyme disease:
  • initial symptoms are non-specific, including fever, gastrointestinal symptoms, muscle aches, and headaches
  • severe progression: although most cases follow a benign course, rapid deterioration with sepsis, multi-organ failure, and death can occur
  • key clinical and laboratory clues: suspect HME in patients with relevant exposure presenting with sepsis, particularly alongside abnormal liver function tests, thrombocytopenia (falling platelet count), and leukopenia (falling white cell count)
• human ewingii ehrlichiosis (HEE): caused by E. ewingii, which targets neutrophils rather than monocytes; presents similarly to HME but is generally less severe

Diagnosis

• blood film: careful examination of a peripheral blood smear may demonstrate characteristic intracytoplasmic morulae within infected leukocytes
• molecular testing limitations: due to its rarity in the UK, no direct routine molecular testing is currently available in the UK (though research assays may be available for confirmation)
• empirical approach: if a patient presents with a compatible clinical syndrome and exposure history, treatment should be initiated immediately on clinical suspicion without waiting for laboratory confirmation

Management and prognosis

• seek expert advice
• antibiotic therapy: doxycycline is the treatment of choice and should be started promptly upon credible clinical suspicion of HME or HEE
• specialist advice: clinical guidance can be obtained from the UKHSA Rare and Imported Pathogens Laboratory (RIPL)
• prognosis: most infections resolve without complications, and chronic infection has not been reported; among symptomatic cases in the USA, the overall mortality rate is approximately 1%, with higher mortality in children under 5 years of age and increased disease severity in older or immunosuppressed patients

Reference:

UK Health Security Agency (UKHSA). Guidance: Anaplasmosis and Ehrlichiosis. Published 9 November 2026.