Eosinophilia refers to an absolute peripheral blood eosinophil count of over 500/µL. With such a finding, a step-by-step diagnostic evaluation is required.

History

First, a careful history is taken, to elicit travel to places where helminthiasis is rife; exposure to drugs; ingestion of possibly helminth-infected food; family history of myeloproliferative disorders, allergies, and drug hypersensitivity.

Symptoms which should be inquired about include:

• Those of helminth infestation
• Symptoms of organ involvement such as lung infiltration, inflammatory bowel disease, or vasculitis of various organs
• Symptoms of allergies of the skin or respiratory system in particular
• Systemic symptoms such as fever, joint pain and swelling, or lymphadenopathy

Drug exposure should be classified based on the association between the drug and the chances of eosinophilia. For instance, anticonvulsants, allopurinol (a drug used in gout), and semisynthetic penicillins have a higher chance of causing eosinophilia.

Specific findings may also point to the culprit drug, such as:

• Anticonvulsants, if hepatitis or DRESS is present
• NSAIDs or semisynthetic penicillins in pneumonitis
• Immune vasculitis with phenytoin or allopurinol
• Nephritis with cephalosporins

Physical Examination

The patient should have a detailed examination of the cardiovascular, renal, respiratory, gastrointestinal, and neurologic systems. Such findings as a skin rash, asthma or lung congestion, or generalized lymphadenopathy are suspicious of underlying conditions such as pulmonary syndromes with eosinophilia, myeloproliferative disorders, and vasculitis or atopic disease. This examination will also help to pick up complications due to hypereosinophilia, the most important of which include pulmonary, neurologic, and cardiac dysfunction.

Diagnostic Testing

In addition to the basic blood counts which revealed the eosinophilia, other tests may include:

• Peripheral blood smear for abnormal eosinophils or blast cells suggestive of myeloproliferative disorders
• Stool ova and parasite tests on three consecutive specimens
• Parasite tests such as thin and thick blood smears
• Specific serological tests for parasites
• Anti-neutrophil cytoplasmic antibodies  which are raised in some types of immune disease
• Organ-specific tests such as renal or hepatic function tests as clinically indicated
• Chest radiograph
• Electrocardiogram, cardiac troponin
• IgE levels and other immunoglobulins
• Urinalysis
• Serologic tests for vasculitic and other connective tissue disorders
• Specialized tests such as bone marrow aspirate examination and cytogenetic tests for myeloproliferative disorders
• Echocardiography if other tests are normal, or if cardiac symptoms are present
• Pulmonary function tests
• Tissue biopsies for parasites such as Trichinella, filariasis, and visceral larva migrans
• Imaging tests based on symptoms or signs of organ involvement, such as CT scan of the chest or abdomen

Treatment

Treatment of eosinophilia is dependent on the cause.

In the case of secondary eosinophilia, the underlying cause requires to be adequately treated. Drugs which are thought to be responsible for the eosinophilia should be withdrawn.

Following this, if the eosinophil count remains high, low-dose corticosteroids may be given for a short duration to suppress the allergic response.

If the patient history suggests a possibility of exposure to the helminth Strongyloides stercoralis, even years before, great care should be taken to rule out this infection by serologic (ELISA) testing, stool ova testing being inefficient at picking it up. This precaution is warranted because the administration of corticosteroids in this condition may cause widespread and uncontrolled infection with the parasite. This phenomenon is not seen with other helminths.

When no cause is found, the patient may receive corticosteroids and should be followed over time to detect any organ-related complications due to continued eosinophil recruitment and chemokine release.

In case there are signs of any complications, such as cardiac damage, high-dose, short-term corticosteroid therapy should be initiated. However, if there is even a remote possibility of Strongyloides infection, ivermectin should be started at the same time to avoid the risk of disseminated infection following steroid administration.

Patients who do not respond to corticosteroids should be treated only by specialists, with drugs such as vincristine or imatinibmesylate.

If no cause is found after full testing, the patient needs to be evaluated for possible hypereosinophilia syndromes (HES). This testing may include:

• RT-PCR and FISH for presence of the F/P fusion
• T cell receptor (TCR) phenotyping
• PCR for TCR gene rearrangement
• karyotyping

For most HES, an initial course of corticosteroids is advised, with hydroxyurea or interferon-α being added as corticosteroid-sparing agents if long-term steroid therapy is required. Imatinibmesylate is first-line therapy for some types of HES.

References

• http://www.mayoclinic.org/symptoms/eosinophilia/basics/definition/SYM-20050752?p=1
• http://www.merckmanuals.com/professional/hematology-and-oncology/eosinophilic-disorders/eosinophilia#v973293
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2902584/