Hypereosinophilic syndrome (HES) is a rare condition in which the body produces far too many eosinophils, a type of white blood cell, and those excess cells infiltrate organs and damage tissue. It most often affects people between ages 20 and 50. The defining feature is a persistently elevated eosinophil count in the blood combined with signs that organs are being harmed, after other causes of high eosinophils (like parasitic infections, severe allergies, or drug reactions) have been ruled out.
What Eosinophils Do and Why Too Many Are Dangerous
Eosinophils are part of your immune system. In normal amounts, they help fight off parasites and play a role in allergic responses. But when eosinophil levels stay abnormally high, the cells begin accumulating in tissues where they don’t belong: the heart, lungs, skin, gut, and nervous system. Once embedded in tissue, eosinophils release toxic proteins and enzymes designed to kill invaders. Without an actual invader to target, those chemicals damage your own cells instead. This is what makes HES dangerous. It’s not the high blood count itself that causes problems; it’s the tissue destruction that follows.
The Three Main Subtypes
HES isn’t a single disease. It’s classified into subtypes based on what’s driving the overproduction of eosinophils, and the subtype determines both prognosis and treatment approach.
Myeloproliferative HES
In this variant, the problem starts in the bone marrow. A genetic mutation causes blood-forming stem cells to overproduce eosinophils. The most common culprit is a fusion gene called FIP1L1-PDGFRA, found in roughly 50% to 60% of HES cases in studied populations. This fusion gene creates an always-on growth signal that drives uncontrolled eosinophil production. People with this variant tend to have an enlarged spleen, low red blood cell or platelet counts, and are at particularly high risk for heart damage. The good news is that this subtype responds well to targeted therapy.
Lymphoproliferative HES
Here, the problem originates with abnormal T cells (a different type of immune cell) that pump out chemical signals telling the bone marrow to make more eosinophils. This variant tends to show up with more skin-related symptoms like hives, swelling, and eczema, along with elevated antibody levels. It’s identified through specialized testing that detects the abnormal T cell population.
Idiopathic HES
When no underlying genetic mutation or abnormal T cell population can be identified, the condition is classified as idiopathic, meaning the cause is unknown. This is sometimes called hypereosinophilia of uncertain significance. The organs affected and the severity vary widely from person to person.
How HES Affects the Body
HES can affect virtually any organ, but certain systems are hit more often and more seriously than others. The pattern of organ involvement often depends on which subtype you have.
Heart
Cardiac involvement is the most dangerous complication and a leading cause of death in HES. Heart damage from eosinophils progresses through three stages. The first stage is often silent: eosinophils infiltrate the heart muscle and release their toxic contents, causing microscopic areas of tissue death. You may feel completely fine during this phase, with a normal physical exam.
In the second stage, blood clots begin forming on the damaged inner lining of the heart. These clots can break loose and travel to the brain, causing stroke, or to the limbs, cutting off blood flow. In the third and final stage, scar tissue replaces the damaged heart muscle. This scarring stiffens the heart, leading to a form of heart failure where the heart can’t relax properly to fill with blood. It can also affect the heart valves and disrupt the heart’s electrical system, causing dangerous rhythm problems. Because the earliest stage produces no symptoms, heart monitoring is a critical part of HES management.
Skin
Skin problems are among the earliest and most visible signs of HES. They include eczema, hives, swelling (especially of the face and extremities), and itchy red bumps, patches, or raised plaques on the trunk and limbs. Skin involvement is especially common in the lymphoproliferative variant.
Lungs
Lung involvement mimics asthma: shortness of breath, coughing, and wheezing. Imaging may show hazy patches in the lung tissue, fluid around the lungs, or swollen lymph nodes in the chest. Blood clots in the lungs can also occur.
Digestive System
When eosinophils infiltrate the gut, they cause inflammation of the stomach, intestines, or both. This can show up as difficulty swallowing, abdominal pain, nausea, vomiting, diarrhea, and unexplained weight loss. In some cases, the liver and bile ducts are affected.
Nervous System
Neurologic symptoms appear in a significant number of HES patients. Peripheral nerve problems, including numbness, tingling, and weakness in the hands and feet, account for more than half of all neurologic symptoms. Less commonly, the central nervous system is affected, causing confusion, memory problems, or behavioral changes.
How HES Is Diagnosed
Diagnosing HES is largely a process of elimination. The first step is a blood test showing a persistently elevated eosinophil count, typically measured on more than one occasion. But high eosinophils alone aren’t enough. Many common conditions raise eosinophil levels: parasitic infections, allergic diseases, drug reactions, adrenal insufficiency, and certain cancers. All of these secondary causes need to be investigated and excluded before HES is considered.
Once secondary causes are off the table, the next step is determining whether the excess eosinophils are actually damaging organs. Blood work, imaging studies like echocardiograms and CT scans, and sometimes tissue biopsies help map out which organs are involved. Genetic testing, particularly for the FIP1L1-PDGFRA fusion gene, and specialized immune cell analysis help pinpoint the subtype. The 2022 World Health Organization classification now groups certain eosinophil-driven conditions under a broader category of “myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions,” reflecting the growing understanding that some HES cases are closely related to blood cancers.
Treatment Options
Treatment depends heavily on the subtype and which organs are affected.
For patients with the myeloproliferative variant carrying the PDGFRA fusion gene, a targeted therapy that blocks the overactive growth signal is remarkably effective. This oral medication (a tyrosine kinase inhibitor) can normalize eosinophil counts rapidly, often at relatively low doses. It works because it precisely shuts down the molecular switch that’s driving eosinophil overproduction. Patients with this mutation often see dramatic and sustained improvement.
For patients without this mutation, corticosteroids are typically the first-line treatment. They suppress the immune system broadly and lower eosinophil counts, but they come with significant long-term side effects, so the goal is always to use the lowest effective dose. When steroids aren’t enough or cause too many problems, other immune-suppressing medications may be added.
A newer option is mepolizumab, a biologic medication that targets a specific chemical signal (interleukin-5) responsible for eosinophil growth and survival. It’s FDA-approved for adults and children 12 and older with HES lasting six months or more, when no other treatable cause has been identified. It’s given as an injection under the skin once every four weeks. By cutting off the signal that tells the bone marrow to produce eosinophils, it can reduce counts without the broad immune suppression of steroids.
Living With HES and Long-Term Outlook
HES is a chronic condition that requires ongoing monitoring. Regular blood counts track eosinophil levels, while periodic echocardiograms watch for early heart damage, even in patients who feel well. Lung function tests and imaging are used as needed depending on symptoms.
The prognosis varies enormously by subtype. Patients with the PDGFRA-positive myeloproliferative variant who respond to targeted therapy can do very well long-term. For others, the outlook depends on which organs are involved and how well the disease responds to treatment. In a long-term study at Mayo Clinic spanning 19 years, the most common identified cause of death was cardiac dysfunction, responsible for about a third of deaths. Infections, blood clots, and vascular disease accounted for most of the remaining mortality. Early detection of heart involvement, before symptoms appear, is one of the most important factors in improving outcomes.
Because HES is rare and its presentation varies so widely, management typically involves specialists in hematology, and often coordination with cardiologists, pulmonologists, or dermatologists depending on which organs are affected. The disease can go through periods of relative stability punctuated by flares, making consistent follow-up essential even when you’re feeling well.