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Eosinophilia is defined as a peripheral blood eosinophil count > 500/μL. Causes and associated disorders are myriad but often represent an allergic reaction or a parasitic infection. Diagnosis involves selective testing directed at clinically suspected causes. Treatment is directed at the cause.

Eosinophilia has features of an immune response: an agent such as Trichinella spiralis invokes a primary response with relatively low levels of eosinophils, whereas repeated exposures result in an augmented or secondary eosinophilic response. Several compounds released by mast cells and basophils induce IgE-mediated eosinophil production. Such substances include eosinophil chemotactic factor of anaphylaxis, leukotriene B4, complement complex (C5-C6-C7), and histamine (over a narrow range of concentration).

Mild eosinophilia itself does not cause symptoms, but levels ≥1500/μL may cause organ damage. Although any organ may be involved, the heart, lungs, spleen, skin, and nervous system are typically affected. Organ damage typically occurs because of tissue inflammation and reaction to the cytokines and chemokines released by the eosinophils as well as to immune cells which are recruited to the tissues. Occasionally, patients with very severe eosinophilia (eg, eosinophil counts of >100,000/μL), usually with eosinophilic leukemia, develop complications of hyperleukocytosis (see Hypereosinophilic Syndrome ).


Eosinophilia may be primary (ie, clonal proliferation of eosinophils associated with hematologic disorders such as leukemias and myeloproliferative disorders), secondary to (or associated with) numerous nonhematologic disorders ( Important Disorders and Treatments Associated With Eosinophilia), or idiopathic (if other causes cannot be identified).

The most common cause in the US is

  • Allergic or atopic disorders (typically respiratory or dermatologic)

Other common causes include

  • Infections (typically parasitic)

  • Certain tumors (hematologic or solid, benign or malignant)

Almost any parasitic invasion of tissues can elicit eosinophilia, but protozoa and noninvasive metazoa usually do not.

Of hematologic tumors, Hodgkin lymphoma may elicit marked eosinophilia, whereas eosinophilia is less common in non-Hodgkin lymphoma, chronic myelocytic leukemia, and acute lymphoblastic leukemia.

The pulmonary infiltrates with eosinophilia syndrome comprises a spectrum of clinical manifestations characterized by peripheral eosinophilia and eosinophilic pulmonary infiltrates (see Overview of Eosinophilic Pulmonary Diseases ) but is usually of unknown cause.

Patients with eosinophilic drug reactions may be asymptomatic or have various syndromes, including interstitial nephritis, serum sickness, cholestatic jaundice, hypersensitivity vasculitis, and immunoblastic lymphadenopathy. Several hundred patients were reported to have developed an eosinophilia-myalgia syndrome after taking l-tryptophan for sedation or psychotropic support. This syndrome was probably caused by a contaminant rather than byl-tryptophan. The symptoms (severe muscle pain, tenosynovitis, muscle edema, rash) lasted weeks to months, and several deaths occurred. Drug reaction with eosinophilia and systemic symptoms (DRESS) is a rare syndrome characterized by fever, rash, eosinophilia, atypical lymphocytosis, lymphadenopathy, and signs and symptoms related to end-organ involvement (typically, heart, lungs, spleen, skin, nervous system).

Important Disorders and Treatments Associated With Eosinophilia

Cause or Associated Disorder


Allergic or atopic disorders


Allergic bronchopulmonary aspergillosis

Allergic rhinitis

Atopic dermatitis

Drug reactions (eg, to antibiotics or NSAIDs)


Episodic angioedema with eosinophilia

Milk-protein allergy

Occupational lung disease


Connective tissue, vasculitic, or granulomatous disorders (especially those involving the lungs)

Dressler syndrome

Eosinophilic fasciitis

Idiopathic eosinophilic synovitis

Inflammatory bowel disease

Polyarteritis nodosa

Progressive systemic sclerosis (scleroderma)



Sjögren syndrome


Endocrine disorders

Adrenal hypofunction

Immune disorders (often with eczema)

Congenital immunodeficiency syndrome (eg, IgA deficiency, hyper-IgE syndrome, Wiskott-Aldrich syndrome)

Graft-vs-host disease

Myeloproliferative disorders

Acute or chronic eosinophilic leukemia

Acute lymphoblastic leukemia (certain types)

Chronic myelocytic leukemia

Hypereosinophilic syndrome

Nonparasitic infections



Cat-scratch fever

Chlamydial pneumonia of infancy

Coccidioidomycosis (acute)

Infectious lymphocytosis

Infectious mononucleosis

Mycobacterial disease

Scarlet fever

Parasitic infections (especially due to tissue-invasive metazoans)



Cysticercosis (caused by Taenia solium)




Hookworm infection


Pneumocystis jirovecii infection





Visceral larva migrans

Skin disorders

Dermatitis herpetiformis

Exfoliative dermatitis



Syndromes of pulmonary infiltration with eosinophilia

Allergic bronchopulmonary aspergillosis

Chronic eosinophilic pneumonia

Eosinophilic granulomatosis with polyangiitis (Churg-Strauss syndrome)

Simple pulmonary eosinophilia (Löffler syndrome)

Tropical pulmonary eosinophilia


Carcinomas and sarcomas of the lung, pancreas, colon, cervix, or ovary

Hodgkin lymphoma

Immunoblastic lymphadenopathy

Non-Hodgkin lymphomas



Familial eosinophilia

Peritoneal dialysis

Radiation therapy


The number of possible causes and associated disorders is very large. Common causes (eg, allergic, infectious, or neoplastic disorders) should be considered first, but even they are often difficult to identify, so a thorough history and physical examination are always required.


The questions most likely to be helpful pertain to the following:

  • Travel (suggesting possible parasite exposure)

  • Allergies

  • Drug use

  • Use of herbal products and dietary supplements, including l-tryptophan

  • Systemic symptoms (eg, fever, weight loss, myalgias, arthralgias, rashes, lymphadenopathy)

Systemic symptoms suggest that a minor allergic or drug cause is less likely, and a detailed evaluation for an infectious, neoplastic, connective tissue, or other systemic disorder should be done. Other important parts of the history include family history of blood dyscrasias (eg, plasma cell disorders) and a complete review of systems, including symptoms of allergies and pulmonary, cardiac, GI, and neurologic dysfunction.

Physical examination

General physical examination should focus on the heart, skin, and neurologic and pulmonary systems. Certain physical findings may suggest causes or associated disorders. Examples include rash (allergic, dermatologic, or vasculitic disorders), abnormal lung findings (asthma, lung infections, or syndromes of pulmonary infiltration with eosinophilia), and generalized lymphadenopathy or splenomegaly (myeloproliferative disorders or cancer).


Eosinophilia is typically recognized when CBC is done for other reasons. Additional testing often includes the following:

  • Stool ova and parasite testing

  • Other tests to detect organ damage or for specific causes based on clinical findings

In general, if a drug or allergic cause is not suspected based on clinical findings, 3 stool specimens should be examined for ova and parasites; however, negative findings do not rule out a parasitic cause (eg, trichinosis requires a muscle biopsy; visceral larva migrans and filarial infections require other tissue biopsies; duodenal aspirates may be needed to exclude specific parasites, eg, Strongyloides sp—see Strongyloidiasis ).

Other specific diagnostic tests are determined by the clinical findings (particularly travel history) and may include chest x-ray, urinalysis, liver and kidney function tests, and serologic tests for parasitic and connective tissue disorders. If patients have generalized lymphadenopathy, splenomegaly, or systemic symptoms, blood tests are done. An elevated serum vitamin B12 level or abnormalities on the peripheral blood smear suggest an underlying myeloproliferative disorder, and a bone marrow aspirate and biopsy with cytogenetic studies may be helpful. Also, if routine evaluation does not reveal a cause, tests are done to detect organ damage. Testing can include some of the tests previously mentioned as well as LDH and liver function tests (suggesting liver damage or possibly a myeloproliferative disorder), echocardiography, and pulmonary function tests. Once a specific cause has been determined, additional testing may be needed.


  • Sometimes corticosteroids

For corticosteroid treatment of hypereosinophilic syndrome, see Hypereosinophilic Syndrome : Immediate therapy.

Drugs known to be associated with eosinophilia are stopped. Other identified causes are treated.

If no cause is detected, the patient is followed for complications. A brief trial with low-dose corticosteroids may lower the eosinophil count if eosinophilia is secondary (eg, to allergy, connective tissue disorders, or parasitic infection) rather than primary. Such a trial is indicated if eosinophilia is persistent and progressive in the absence of a treatable cause.

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