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Hypereosinophilic Syndrome

Activated eosinophils in the blood of a patient with hypereosinophilic syndrome
Activated eosinophils in the blood of a patient with hypereosinophilic syndrome.
Credit: NIAID

Hypereosinophilic syndrome (HES) describes a group of chronic disorders in which very high numbers of eosinophils are present in the blood and tissues. People diagnosed with HES have elevated blood eosinophil levels on at least two occasions and signs or symptoms of eosinophil infiltration of different tissues. HES can affect any organ in the body and often involves the skin, lungs, gastrointestinal tract, heart, and nervous system.

HES is a rare group of diseases affecting an estimated 0.36 to 6.3 people per 100,000 in the United States. Although HES characteristically develops between the ages of 20 and 50 years, young children and the elderly also can be affected.


There are many different types of HES. In 10 to 20 percent of cases, HES is caused by a problem in the eosinophil itself. Most of these cases are due to an abnormality in chromosome 4 that leads to the fusion of two genes. This fused gene, called FIP1L1-PDGFRA, produces an abnormal protein that may cause changes in bone marrow cells, leading to increased levels of eosinophils. In other cases, HES is caused by abnormalities in a different type of blood cell called a lymphocyte. These abnormal lymphocytes produce substances that lead to increased eosinophil production. A very small number of cases are hereditary. In more than half of cases, the cause of HES is unknown.


HES symptoms are extremely varied and depend on the organs involved and the severity of the disease. People with HES commonly experience fatigue, muscle and joint pains, rashes, itching, abdominal pain, shortness of breath, and cough. Chest pain and symptoms of nervous system involvement, such as weakness or numbness, can be signs of life-threatening complications.


Currently, there are no specific diagnostic tests for HES. Doctors perform blood tests to measure eosinophil counts. More than 1,500 eosinophils per microliter of blood may indicate HES. Healthy people typically have less than 500 eosinophils per microliter of blood.

To diagnose HES, doctors also conduct tests to measure tissue and organ damage caused by eosinophil invasion and to rule out other known causes of high eosinophil levels, such as parasitic infections and certain bone marrow diseases. These may include more blood tests, ultrasounds, X-rays, or bone marrow biopsies.


HES is a life-long condition, and there currently is no cure. Treatments for HES aim to prevent or control organ damage by decreasing eosinophil levels.


Standard treatment for most variants of HES includes corticosteroids, which often reduce eosinophil counts effectively. However, eosinophil levels may rise and symptoms of organ damage may reappear when the drug is stopped. Long-term use of corticosteroids carries the risk of harmful side effects, including bone loss, fractures, weight gain, depression, and diabetes.

Imatinib mesylate (Gleevec)

People with HES who have the FIP1L1-PDGFRA fusion gene are treated with imatinib mesylate (Gleevec). Imatinib blocks the action of the abnormal protein produced from the fusion gene. Studies have indicated that imatinib can reduce eosinophil levels and stop disease progression within one to two weeks of starting treatment. Some people with HES who do not have the fusion gene also respond to imatinib treatment.


Doctors may prescribe hydroxyurea in combination with corticosteroids or other HES therapies. Sometimes, hydroxyurea may be used alone to treat HES patients who do not respond to corticosteroids. Hydroxyurea blocks the growth of cells by interfering with DNA synthesis.


Interferon-alpha can be an effective and sometimes life-saving treatment for people with certain HES variants who do not respond to corticosteroids or other drugs. Interferon-alpha may sometimes be used in combination with other drugs. However, the drug has several common side effects, including flu-like symptoms, depression, and fatigue.

Development of new treatments

Researchers are working to develop new HES drugs with fewer side effects. In clinical studies, an experimental asthma drug called mepolizumab safely and effectively lowered participants’ eosinophil counts and reduced their need for high doses of corticosteroids. Mepolizumab is an antibody that blocks interleukin-5, a molecule that helps control the activation of eosinophils in the blood.

To learn more about mepolizumab as a potential HES treatment, see Investigational Asthma Drug is Effective in Treating Chronic Hypereosinophilic Syndrome.

NIAID researchers continue to evaluate potential new HES treatments. Learn more about current efforts in the Research section.

Last Updated April 24, 2014

Last Reviewed April 24, 2014