What is eosinophilia?

What is Eosinophilia?

Eosinophilia is defined as a peripheral blood eosinophil count greater than 0.5 × 10⁹/L (500 cells/μL), representing an abnormal elevation of eosinophilic white blood cells in the bloodstream. 1

Severity Classification

Eosinophilia is stratified by absolute eosinophil count into clinically meaningful categories:

• Mild eosinophilia: 500-1,500 cells/μL 2
• Moderate eosinophilia: 1,500-5,000 cells/μL 2
• Severe eosinophilia: >5,000 cells/μL 2
• Hypereosinophilia (HE): Persistent elevation >1.5 × 10⁹/L on two examinations at least one month apart and/or tissue hypereosinophilia 3

The degree of elevation provides diagnostic clues—values >20,000 cells/μL are highly suggestive of myeloproliferative disorders rather than allergic or parasitic causes. 2

Common Causes

Infectious Causes

Helminth (worm) infections are the most common identifiable cause of eosinophilia in returning travelers and migrants, with diagnosis rates ranging from 19-80%. 1, 3 Key parasitic infections include:

• Strongyloides stercoralis: Can persist lifelong asymptomatically but presents as fatal hyperinfection syndrome (mortality approaching 70%) in immunocompromised patients, even >50 years after initial exposure 3, 4
• Schistosomiasis: Associated with Katayama syndrome (fever, urticarial rash, eosinophilia) 4-8 weeks after freshwater exposure in endemic areas 4
• Hookworm, filariasis, and other tissue-migrating helminths: Eosinophilia typically appears 4-12 weeks post-exposure during tissue migration phases 4

Non-Infectious Causes

In countries with low endemic helminth infection rates, the most common causes are: 1

• Allergic disorders: Asthma, eczema, and hay fever account for approximately 80% of eosinophilia cases in non-tropical populations 4
• Medications: NSAIDs, beta-lactam antibiotics, and nitrofurantoin are frequent culprits 3, 5
• Malignancies: Myeloproliferative disorders, lymphomas, and solid tumors 2
• Autoimmune conditions: Eosinophilic granulomatosis with polyangiitis (EGPA) and other vasculitides 6

Critical distinction: Hypereosinophilia (≥1.5 × 10⁹/L) is never explained by allergy alone and always requires comprehensive workup to exclude secondary causes. 4

Organ-Specific Eosinophilic Disorders

Eosinophilic Esophagitis (EoE): Defined as ≥15 eosinophils per high-power field in esophageal biopsies, presenting with dysphagia/food impaction in adults or feeding problems/vomiting in children 3. This represents a chronic immune/antigen-mediated disease distinct from gastroesophageal reflux disease. 1

Clinical Significance and End-Organ Damage

Even mild persistent eosinophilia can cause significant end-organ damage if left untreated, particularly affecting the heart, lungs, central nervous system, and skin. 1, 3, 5

Cardiac Complications

Eosinophilic myocarditis is the leading cause of morbidity and mortality in hypereosinophilic syndrome, presenting in 20% of cases. 4 Long-standing hypereosinophilia causes damage through release of granule products, lipid mediators, and cytokines from eosinophils. 2

When to Escalate Care

• Urgent medical assessment required: Any patient with evidence of end-organ damage 1
• Hematology referral indicated: Eosinophilia ≥1.5 × 10⁹/L persisting >3 months without identified cause after infectious causes have been excluded or treated 1, 4

Important Clinical Pitfalls

Testing for eosinophilia alone is not an adequate screening strategy for helminth infection, as many people with helminth infections do not have eosinophilia. 1, 5 This means you cannot rule out parasitic infection based solely on a normal eosinophil count.

Idiopathic hypereosinophilic syndrome (or idiopathic hypereosinophilic syndrome with end-organ damage) is a diagnosis of exclusion made only after persistent eosinophilia ≥1.5 × 10⁹/L for more than 3 months with no identifiable cause. 1, 7, 8