Print OverviewHypereosinophilic syndrome is organ damage or dysfunction caused by an excess of disease-fighting white blood cells called eosinophils. Hypereosinophilic syndrome (hy-per-ee-o-SIN-o-phil-ik) is rare. It can affect any tissues in the body. Common targets include the skin, lungs, digestive tract, heart and nervous system. If a condition is found to cause hypereosinophilic syndrome, then treating the underlying condition also may lower eosinophil levels. Other treatments include medicines that reduce the number of eosinophils in the bloodstream.Products & ServicesA Book: Mayo Clinic Family Health BookNewsletter: Mayo Clinic Health Letter — Digital EditionShow more products from Mayo Clinic SymptomsSymptoms of hypereosinophilic syndrome (HES) vary depending on what organs are affected. Early symptoms of HES may include: Fatigue. Cough. Shortness of breath. Muscle pain. Swelling in deep layers of the skin. Rash. Fever. When to see a doctorMany different conditions can cause the symptoms related to HES. See your healthcare professional as soon as possible if symptoms do not improve. Request an appointment CausesEosinophils are important in fighting parasites and bacteria. They also regulate other immune system cells and proteins that cause allergic reactions. HES occurs when high levels of these white blood cells last a long time. The cells themselves or their byproducts can move into organs, where they can cause damage or dysfunction. Terms to knowYou may hear the following terms regarding HES. Eosinophilia is a higher than typical number of eosinophils circulating in the bloodstream. Hypereosinophilia is a high number of eosinophils that lasts for an extended period, usually at least one month. Hypereosinophilia syndrome is organ damage or destruction caused by long-term high eosinophils. Types of hypereosinophiliaPersistent increases in these white blood cells may be caused by a number of factors. General categories of hypereosinophilia based on cause include the following: Primary hypereosinophilia is caused by cancers of the bone marrow or blood that directly increase the production of eosinophils. Secondary hypereosinophilia, also called reactive hypereosinophilia, is caused by conditions that typically activate white blood cells, such as: Parasitic or bacterial infections. Allergies. Immune system disorders. Long-lasting inflammatory diseases. Drug reactions. Familial hypereosinophilia is an inherited disorder. Idiopathic hypereosinophilia has no cause that can be found. Most cases are idiopathic. Risk factorsHES can affect anyone, but it occurs more often in men. By Mayo Clinic Staff Hypereosinophilic syndrome care at Mayo Clinic Request an appointment Diagnosis & treatment June 27, 2025 Print Living with hypereosinophilic syndrome? Connect with others like you for support and answers to your questions in the Blood Cancers & Disorders support group on Mayo Clinic Connect, a patient community. Blood Cancers & Disorders Discussions Diagnosed: MGUS 113 Replies Sat, Jul 19, 2025 chevron-right Anyone want to talk about Myelofibrosis? 169 Replies Sat, Jul 19, 2025 chevron-right What are most common side effects when starting HU (Hydroxyurea)? 174 Replies Sat, Jul 19, 2025 chevron-right See more discussions Show references Roufosse F, et al. Hypereosinophilic syndromes: Clinical manifestations, pathophysiology, and diagnosis. https://www.uptodate.com/contents/search. Accessed Nov. 18, 2024. Ferri FF. Hypereosinophilic syndrome. In: Ferri's Clinical Advisor 2025. Elsevier; 2025. https://www.clinicalkey.com. Accessed Nov. 18, 2024. Hoffman R, et al. Eosinophilia, eosinophilic neoplasms, and the hypereosinophilic syndromes. In: Hematology: Basic Principles and Practice. 8th ed. Elsevier; 2023. https://www.clinicalkey.com. Accessed Nov. 18, 2024. Eosinophil production and function. Merck Manual Professional Version. https://www.merckmanuals.com/professional/hematology-and-oncology/eosinophilic-disorders/eosinophil-production-and-function. Accessed Nov. 22, 2024. Marra AM, et al. Eosinophil-associated diseases: The allergist's and clinical immunologist's perspective. European Annals of Allergy and Clinical Immunology. 2024; doi:10.23822/EurAnnACI.1764-1489.339. Rosenberg CE, et al. Diagnosis and management of pediatric hypereosinophilic syndrome. Journal of Allergy and Clinical Immunology: In Practice. 2022; doi:10.1016/j.jaip.2022.02.007. Roufosse F, et al. Hypereosinophilic syndromes: Treatment. https://www.uptodate.com/contents/search. Accessed Nov. 18, 2024. Klion AD. Approach to the patient with suspected hypereosinophilic syndrome. Hematology, American Society of Hematology Education Program. 2022; doi:10.1182/hematology.2022000367. Related Products & Services A Book: Mayo Clinic Family Health Book Newsletter: Mayo Clinic Health Letter — Digital Edition Show more products and services from Mayo Clinic Hypereosinophilic syndromeSymptoms&causesDiagnosis&treatmentDoctors&departmentsCare atMayoClinic Advertisement Mayo Clinic does not endorse companies or products. Advertising revenue supports our not-for-profit mission. Advertising & Sponsorship Policy Opportunities Ad Choices Mayo Clinic Press Check out these best-sellers and special offers on books and newsletters from Mayo Clinic Press. NEW: Mayo Clinic Guide to Better Sleep Listen to Health Matters Podcast Mayo Clinic on Incontinence The Essential Diabetes Book FREE Mayo Clinic Diet Assessment Mayo Clinic Health Letter - FREE book CON-20306102 Diseases & Conditions Hypereosinophilic syndrome