What is Rickettsial infection?

Rickettsial Infections: Overview, Diagnosis, and Management

Rickettsial infections are serious tick-transmitted bacterial diseases that can cause severe illness and death if not promptly diagnosed and treated with appropriate antibiotics. Doxycycline is the treatment of choice for all suspected rickettsial infections in both adults and children of any age, and early empiric therapy is critical to prevent severe morbidity and mortality. 1

Types of Rickettsial Infections

Tickborne rickettsial diseases in the United States include:

• Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, the most severe rickettsial illness in the US
• Other Spotted Fever Group (SFG) rickettsioses: Caused by Rickettsia parkeri and Rickettsia species 364D
• Ehrlichiosis: Including human monocytic ehrlichiosis (Ehrlichia chaffeensis), Ehrlichia ewingii infection, and Ehrlichia muris-like agent infection
• Anaplasmosis: Caused by Anaplasma phagocytophilum, also called human granulocytic anaplasmosis 1

Clinical Presentation

Rickettsial infections typically present with:

• Fever (often high)
• Headache
• Myalgia (muscle pain)
• Malaise
• Rash (varies by specific infection)
  • RMSF: Maculopapular or petechial rash appearing 2-5 days after fever onset
  • R. parkeri and 364D: Often present with an eschar (necrotic lesion) at the tick bite site
• Gastrointestinal symptoms (nausea, vomiting, diarrhea) - more common in ehrlichiosis 1

Key Clinical Distinctions

• RMSF: Most severe form with potential for rapid deterioration; rash typically appears on wrists, ankles, palms and soles, then spreads centrally
• R. parkeri infection: Milder than RMSF with characteristic eschar at bite site
• Ehrlichiosis: Can cause severe disease in immunocompromised patients; rash less common than in RMSF
• Anaplasmosis: Often presents without rash; can cause severe complications including respiratory failure 1

Diagnosis

Diagnosis is primarily clinical, as laboratory confirmation is rarely available during the acute phase when treatment decisions must be made:

• Clinical suspicion based on exposure history, symptoms, and epidemiologic factors is critical
• Laboratory findings may include:
  • Thrombocytopenia
  • Leukopenia (early) or leukocytosis (later)
  • Elevated liver enzymes
  • Hyponatremia 1

Confirmatory Testing (performed retrospectively)

• Serology (paired acute and convalescent samples)
• PCR testing of whole blood (most useful early in illness)
• Immunohistochemical staining of tissue specimens
• Blood smear examination may reveal morulae in ehrlichiosis/anaplasmosis 1

Treatment

Early empiric therapy is the most important factor in preventing severe disease and death from rickettsial infections. Treatment should never be delayed while awaiting laboratory confirmation.

First-line Treatment

• Doxycycline is the drug of choice for all rickettsial diseases in patients of all ages, including children under 8 years 1, 2, 3
  • Adults: 100 mg twice daily
  • Children <45 kg: 2.2 mg/kg twice daily
  • Duration: Minimum 5-7 days and at least 3 days after fever resolution

Special Considerations

• Pregnancy: Despite theoretical concerns, doxycycline is still recommended for life-threatening rickettsial infections in pregnant women as the benefits outweigh the risks 1
• Severe doxycycline allergy:
  • For RMSF: Chloramphenicol may be used, though it carries higher risk of treatment failure
  • For anaplasmosis: Rifampin may be considered for mild cases
  • For ehrlichiosis: No proven alternatives to doxycycline exist 1, 2

Severe Disease Management

Patients with severe rickettsial disease may require:

• ICU-level care
• Careful fluid and electrolyte management
• Vasopressor support for hypotension
• Management of complications including:
  • Acute respiratory distress syndrome
  • Renal failure
  • Meningoencephalitis
  • Myocarditis 1

Prevention

• Avoid tick-infested areas when possible
• Use tick repellents containing DEET
• Wear protective clothing
• Perform thorough tick checks after outdoor activities
• Remove attached ticks promptly
• Prophylactic antibiotics after tick bites are NOT recommended for prevention of rickettsial diseases 1, 2

Important Clinical Pitfalls

• Delaying treatment while awaiting laboratory confirmation significantly increases mortality risk
• Failing to consider rickettsial disease in patients with acute febrile illness and appropriate exposure history
• Relying on the presence of rash for diagnosis (may be absent early or entirely)
• Assuming children cannot receive doxycycline (it is the treatment of choice regardless of age)
• Using alternative antibiotics that have lower efficacy against rickettsial pathogens 1, 2

Early recognition and prompt initiation of appropriate antibiotic therapy are the most critical factors in reducing morbidity and mortality from rickettsial infections.