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Peter F. Weller

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date: 07 March 2021

Eosinophilia (eosinophil count >0.45 × 109/litre) is associated with some infections, some allergic diseases, and a variety of other conditions, sometimes neoplastic. Parasitic diseases—eosinophilia is a characteristic feature of infection by multicellular helminth parasites (e.g. Strongyloides stercoralis) with diagnosis typically based on geographical/dietary history, serological tests, and examination of stool or tissues for parasite forms. Other diseases—eosinophilia can be caused by the fungal disease coccidioidomycosis, and modest eosinophilia may accompany retroviral infections such as HIV and HTLV-1. Common allergic diseases—asthma, rhinitis, and atopic dermatitis are associated with modest eosinophilia. Drug reactions—these are a frequent cause of eosinophilia, at times in reactions characterized by rashes and pyrexia. More severe reactions may also manifest with (1) pulmonary eosinophilia and lung infiltrates; (2) interstitial nephritis; (3) hepatitis; (4) myocarditis; (5) drug-induced hypersensitivity vasculitis; (6) gastroenterocolitis; and (7) DRESS syndrome. Other conditions—these include (1) eosinophilic granulomatosis with polyangiitis; (2) hyper-IgE syndromes; (3) chronic myeloid leukaemia, acute myeloid leukaemia, and lymphoma; (4) a variety of pulmonary, skin, gastrointestinal, and endocrine diseases. Hypereosinophilic syndromes are defined by (1) eosinophilia (>1.5 × 109/litre) sustained over a month, (2) lack of an identifiable cause precipitating a secondary eosinophilia, and (3) symptoms and signs of organ involvement. About 30% of patients will have either a myeloproliferative condition (chronic eosinophilic leukaemia) or hypereosinophilia mediated by clonal expansion of specific T cells producing interleukin-5 (IL-5). Treatment—patients without organ damage do not require treatment. Aside from supportive care, chronic eosinophilic leukaemia may respond to tyrosine kinase inhibitors (e.g. imatinib), and nonmyeloproliferative hypereosinophilic syndrome may respond to high-dose corticosteroids, with hydroxyurea, interferon-α‎ or anti-IL-5 monoclonal antibody used in refractory cases.

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