At what level of eosinophilia (EOS) should parasitic infection be considered versus allergy?

From the Guidelines

Eosinophilia should be considered as a potential indicator of helminth infection (worms) when the peripheral blood eosinophil count is greater than 0.5 × 10⁹/L, as defined in the UK guidelines for the investigation and management of eosinophilia in returning travellers and migrants 1.

Key Considerations

• The threshold for eosinophilia is defined as a peripheral blood eosinophil count of > 0.5 × 10⁹/L, according to the UK guidelines 1.
• Helminths (worms) are the most common identifiable cause of eosinophilia in returning travelers or migrants, with diagnosis rates varying from 19%−80% 1.
• Persistent eosinophilia of any degree, especially at very high levels, can cause significant end-organ damage, particularly affecting the heart, lungs, and central nervous system 1.
• If patients have eosinophilia ≥ 1.5 × 10⁹/L for more than 3 months but no evidence of end-organ damage, consider referring them to a haematologist for further investigations once infectious causes have been excluded or treated 1.

Distinguishing Between Causes

• Elevated eosinophil counts can indicate either parasitic infections (including worms) or allergic reactions, but there's no standard threshold that definitively distinguishes between these causes.
• The term "allergy tea" is not a recognized medical term and does not relate to the diagnosis or management of eosinophilia.
• A healthcare provider should interpret specific lab values in the context of symptoms and medical history to determine the cause of elevated eosinophil counts.

From the Research

Eosinophilia and Parasite Infection

• Eosinophilia is defined as an absolute count of > 500 eosinophils per mm3 of peripheral blood 2
• Helminth infestation of the digestive tract or organ tissues is a common and curable cause of high eosinophil counts 3
• Tissue-invasive helminthic parasites can cause eosinophilia, but not all parasites will cause this condition 2

Diagnosis and Treatment

• Diagnosis of eosinophilia caused by parasites can be difficult, especially when the level of infestation is low or the helminth is in a larval stage 3
• Clinical improvement after treatment can be a criteria for positive diagnosis, and normal eosinophil counts followed later by normal serology confirms the diagnosis 3
• Albendazole has been found to be highly-effective in the management of patients with eosinophilia without obvious causes, with 80% of patients having a reduction in eosinophil count to < 1,000/mm3 after treatment 4

Eosinophilia Threshold

• There is no specific threshold mentioned in the studies for how high EOS has to be to suspect worms or allergy tea
• However, eosinophilia is generally defined as an absolute count of > 500 eosinophils per mm3 of peripheral blood 2, and patients with eosinophil counts > 1,000/mm3 were treated with albendazole in one study 4