What are the signs of tick-borne disease on a Complete Blood Count (CBC)?

Signs of Tick-Borne Disease on Complete Blood Count (CBC)

Leukopenia, thrombocytopenia, and mild anemia are the most characteristic CBC findings in tick-borne diseases, particularly in ehrlichiosis, anaplasmosis, and Rocky Mountain spotted fever. 1

Key CBC Abnormalities in Tick-Borne Diseases

White Blood Cell (WBC) Abnormalities

• Leukopenia (low white blood cell count):

  • Present in up to 53% of patients with Human Monocytic Ehrlichiosis (HME)
  • Common in Human Granulocytic Anaplasmosis (HGA)
  • Less common in Rocky Mountain Spotted Fever (RMSF)
  • An inverse relationship exists between WBC count and probability of HGA 1

• Differential changes:

  • Increased bands (immature neutrophils) in RMSF
  • Monocytosis may be seen in ehrlichiosis
  • Lymphopenia can occur in several tick-borne diseases

Platelet Abnormalities

• Thrombocytopenia (low platelet count):
  • Present in up to 94% of patients with HGA
  • Common in HME
  • Frequently observed in RMSF
  • Often one of the earliest laboratory indicators of tick-borne disease 1
  • Can be severe (<50,000/μL) in some cases

Red Blood Cell Abnormalities

• Mild anemia may be present but is less common and less specific
• Hemoglobin typically remains within or slightly below normal range

Diagnostic Value of CBC Findings

• CBC abnormalities are particularly valuable when combined with:

  • History of tick exposure
  • Appropriate seasonal context (spring/summer)
  • Compatible clinical symptoms (fever, headache, myalgia, malaise) 1, 2

• The presence of leukopenia and thrombocytopenia in a patient with fever during tick season should prompt consideration of tick-borne diseases even in the absence of reported tick bite or rash 1

Peripheral Blood Smear Examination

• Morulae (intracellular bacterial clusters):
  • May be visible in peripheral blood smears in HGA or Ehrlichia ewingii infection
  • Appear as dark, round inclusions within white blood cells
  • Only observed in 1-20% of HME cases, making this finding specific but not sensitive 1
  • Most commonly seen in neutrophils (anaplasmosis) or monocytes (ehrlichiosis)

Other Laboratory Findings That May Accompany CBC Changes

• Liver function tests:

  • Mild to moderate elevations in hepatic transaminases (AST, ALT)
  • Common in most tick-borne diseases 1, 3

• Electrolyte abnormalities:

  • Mild hyponatremia may be seen, particularly in RMSF 1

Clinical Application

1. When to suspect tick-borne disease based on CBC:

   • Unexplained leukopenia and thrombocytopenia during tick season
   • Normal hemoglobin or mild anemia
   • Absence of other causes of these abnormalities

2. Diagnostic algorithm:

   • If CBC shows leukopenia and thrombocytopenia in a patient with fever:
     • Obtain blood cultures to rule out other infections
     • Consider specific testing for tick-borne diseases (PCR, serology)
     • Examine peripheral blood smear for morulae
     • Consider empiric treatment with doxycycline if clinical suspicion is high 1, 3

Pitfalls to Avoid

• Waiting for rash to appear: Many tick-borne diseases may not present with rash initially or at all. RMSF rash typically appears 2-4 days after fever onset 1, 2

• Delaying treatment: Do not wait for laboratory confirmation before initiating treatment if clinical suspicion is high, as delayed treatment can lead to severe complications or death 1, 2

• Overlooking tick-borne diseases in differential diagnosis: Consider tick-borne diseases in any patient with unexplained fever and CBC abnormalities during spring and summer months 3

• Relying solely on CBC findings: While helpful, CBC abnormalities are not specific to tick-borne diseases and must be interpreted in clinical context 1

Remember that CBC abnormalities may be subtle early in the disease course, and repeat testing may be necessary if clinical suspicion remains high despite initially normal results.