What is eosinophilia?

Eosinophilia: Definition, Causes, and Clinical Significance

Eosinophilia is defined as an elevated peripheral blood eosinophil count greater than 0.5 × 10⁹/L (or 500 cells/μL). This condition represents an important clinical finding that warrants appropriate investigation due to its association with various underlying conditions that can impact morbidity and mortality 1.

Classification and Causes

Eosinophilia can be categorized into several types based on etiology:

1. Infectious Causes

• Helminth infections: The most common identifiable cause of eosinophilia in returning travelers and migrants, with diagnosis rates varying from 19% to 80% 1
• Notable helminth infections include:
  • Strongyloides stercoralis (can persist lifelong with risk of fatal hyperinfection syndrome in immunocompromised patients)
  • Schistosomiasis (associated with squamous cell bladder carcinoma)
  • Other tissue-invasive helminths

2. Non-Infectious Causes

• Allergic/atopic conditions: Most common non-infectious causes in countries with low rates of endemic helminth infections 1
  • Asthma
  • Eczema
  • Hay fever
• Drug reactions: Particularly NSAIDs, beta-lactam antibiotics, nitrofurantoin 1
• Eosinophilic gastrointestinal disorders: Including eosinophilic esophagitis 1
• Autoimmune and connective tissue diseases
• Malignancies: Particularly hematologic neoplasms 1

3. Primary Eosinophilias

• Hypereosinophilia (HE): Defined as persistent elevated eosinophil count >1.5 × 10⁹/L 1
• Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusion genes 1
• Chronic eosinophilic leukemia, not otherwise specified (CEL, NOS)
• Idiopathic hypereosinophilic syndrome (HES): When HE with associated organ damage is detected with no apparent underlying disease 1

Clinical Significance

The clinical importance of eosinophilia lies in its potential to cause end-organ damage:

• Persistent eosinophilia of any degree, especially at high levels (>1.5 × 10⁹/L), can cause significant end-organ damage 1
• Target organs most commonly affected include:
  • Heart
  • Lungs
  • Central nervous system
• Specific complications from underlying causes:
  • Strongyloidiasis: Hyperinfection syndrome with high mortality in immunocompromised patients
  • Schistosomiasis: Spinal cord compression or bladder carcinoma
  • Hypereosinophilic syndrome: Multi-organ damage

Diagnostic Approach

When eosinophilia is detected, a systematic approach is warranted:

1. Confirm true eosinophilia (>0.5 × 10⁹/L)
2. Assess for end-organ damage - urgent medical assessment needed if present
3. Evaluate travel and migration history - particularly important for helminth infections
4. Consider common non-infectious causes in developed countries (allergy, drugs)
5. Investigate for hematologic malignancies if eosinophilia is persistent and high-grade (≥1.5 × 10⁹/L for more than 3 months)

Management Considerations

• Urgent assessment for patients with evidence of end-organ damage
• Treat underlying cause when identified
• Hematology referral for persistent high-grade eosinophilia (≥1.5 × 10⁹/L for >3 months) after infectious causes have been excluded or treated 1
• Targeted therapies for specific conditions (e.g., tyrosine kinase inhibitors for myeloid/lymphoid neoplasms with specific gene rearrangements)

Important Pitfalls to Avoid

1. Not recognizing potentially serious causes - some helminth infections can persist for decades and cause serious complications later
2. Focusing only on eosinophil count - many people with helminth infections do not have eosinophilia, so testing for eosinophilia alone is not an adequate screening strategy 1
3. Overlooking end-organ damage - any patient with evidence of end-organ damage needs urgent medical assessment
4. Missing hematologic malignancies - persistent unexplained eosinophilia warrants hematologic evaluation

Eosinophilia represents an important clinical finding that should prompt appropriate investigation to identify potentially serious underlying conditions that may impact patient morbidity and mortality.