What level of eosinophilia (elevated eosinophil count) is indicative of parasitism?

Eosinophilia Threshold for Parasitic Investigation

An absolute eosinophil count >0.5 × 10⁹/L (>500 cells/μL) should trigger consideration and investigation for parasitic infection, particularly in patients with travel history to endemic regions. 1, 2

Defining Eosinophilia Levels

The most recent UK guidelines (2025) establish a clear threshold:

• Mild eosinophilia: 0.5-1.5 × 10⁹/L - Most commonly caused by allergic disorders or medications in non-endemic areas, but helminth infections remain the leading identifiable cause (19-80% of cases) in returning travelers or migrants 1, 3

• Moderate to severe eosinophilia: ≥1.5 × 10⁹/L - Requires hematology referral if persisting >3 months after infectious causes excluded or treated 1, 3

• Marked eosinophilia: ≥5.0 × 10⁹/L - Carries significant risk of morbidity and mortality at any time point 3

Clinical Context That Elevates Parasitic Concern

The absolute eosinophil count alone is insufficient for determining parasitic etiology; travel and exposure history are critical. 1, 4

Key historical features that increase parasitic likelihood:

• Fresh water exposure in Africa or tropical regions - raises concern for schistosomiasis 3, 4
• Raw or undercooked meat consumption - suggests tissue-invasive helminths 3
• Travel to West Africa - necessitates specific filarial investigations 4
• Any travel to helminth-endemic areas - warrants full parasitic workup regardless of eosinophil level 1, 5

Critical Parasitic Infections Requiring Urgent Recognition

Strongyloides stercoralis deserves special attention as it can persist lifelong and cause fatal hyperinfection syndrome (high mortality) in immunocompromised patients. 1, 3, 4

Other high-risk parasitic infections:

• Schistosoma haematobium - associated with squamous cell bladder carcinoma 1, 3
• Chronic schistosomiasis - can cause spinal cord compression or portal hypertension with esophageal varices 3
• Tropical pulmonary eosinophilia (filarial hypersensitivity) - typically presents with eosinophil count >3 × 10⁹/L 1

Diagnostic Approach Based on Eosinophil Level

For any eosinophilia >0.5 × 10⁹/L with travel history to endemic areas:

• Stool microscopy for ova and parasites - three separate concentrated specimens on different days 1, 4, 5
• Strongyloides serology and culture - essential given hyperinfection risk 1, 4, 5
• Schistosomiasis serology - if fresh water exposure in endemic areas 1, 4, 5
• Terminal urine microscopy - for all patients with African travel history 5
• Filarial serology and day-blood for microfilaria - specifically for West African travelers 4, 5

For asymptomatic eosinophilia 0.5-1.5 × 10⁹/L in returning travelers:

• Empirical treatment with albendazole 400 mg single dose PLUS ivermectin 200 μg/kg single dose may be considered while awaiting test results (for patients >24 months of age) 4, 6

Important Caveats

Many helminth-infected patients do not have eosinophilia, so normal eosinophil counts do not exclude parasitic infection. 1, 4 Testing for eosinophilia alone is inadequate as a screening strategy for helminth infection. 1

Only tissue-invasive helminthic parasites cause eosinophilia, limiting its application as a general screening tool for all parasitic infections. 2 Intraluminal parasites (e.g., Giardia, Entamoeba) typically do not cause eosinophilia. 2

Eosinophilia may resolve spontaneously over time as parasites complete their life cycle, so timing of testing relative to exposure is crucial. 2 Repeated stool examinations or serology may be necessary if initial testing is negative but clinical suspicion remains high. 2

Multiple helminth species frequently coexist - 17% of patients with parasitic eosinophilia have more than one helminth species, and median eosinophilia increases with number of concurrent infections. 5 Complete parasitic workup should not stop after identifying one pathogen.

Critical warning for Loa loa: Do not use diethylcarbamazine if microfilariae are seen on blood film (particularly if >8000/mL), as it may cause fatal encephalopathy. 1, 3 Use corticosteroids with albendazole first to reduce microfilarial load before definitive treatment. 1, 3