High eosinophils, a condition called eosinophilia, happens when your blood contains more than 500 eosinophils per microliter. Normal levels fall between 30 and 350 cells per microliter. The causes range from common allergies and infections to rare blood cancers, and figuring out which one is driving your count up is the key to treatment.
Eosinophils are white blood cells that help your body fight parasites and play a role in inflammation. Problems arise when your body makes too many of them or can’t turn off their production. The excess cells can accumulate in tissues and cause damage to organs including the lungs, skin, heart, and digestive tract.
Severity Levels of Eosinophilia
Doctors classify eosinophilia into three tiers based on your blood count. Mild eosinophilia means 500 to 1,500 cells per microliter, which is common with seasonal allergies or mild infections. Moderate eosinophilia falls between 1,500 and 5,000 cells per microliter. Severe eosinophilia exceeds 5,000 cells per microliter and almost always requires further investigation because the risk of organ damage increases significantly at that level.
Allergies and Asthma
Allergic conditions are the most common cause of high eosinophils in developed countries. When you inhale an allergen like pollen, dust mites, or pet dander, your immune system ramps up production of a signaling molecule called IL-5. This molecule is essentially the on-switch for eosinophils. It drives their creation in bone marrow, pulls them into the bloodstream, activates them, and keeps them alive longer than they’d normally survive.
Asthma, hay fever, eczema, and food allergies can all push your eosinophil count into the mild or moderate range. In allergic asthma specifically, the bone marrow becomes more responsive to IL-5 after allergen exposure, producing eosinophil precursors at a faster rate. This is why people with poorly controlled asthma often have persistently elevated counts.
Parasitic Infections
Parasitic worms are the most potent trigger of eosinophilia worldwide and often produce the highest counts. Your immune system deploys eosinophils specifically to fight these organisms, so a spike in eosinophils is sometimes the first clue that someone has picked up a parasite, particularly after international travel.
The CDC documents dozens of parasitic infections that cause eosinophilia, many of which produce severe elevations during their acute phase. Hookworm, roundworm, and strongyloides infections all cause severe eosinophilia when you’re first infected, though counts often settle to mild or normal levels in chronic infection. Schistosomiasis follows the same pattern. Trichinella (from undercooked meat) and gnathostoma infections tend to produce moderate to severe elevations. Liver flukes can push counts to severe levels during acute illness but typically drop to mild elevations once the infection becomes chronic.
Single-celled parasites like malaria and giardia generally do not cause eosinophilia. It’s the tissue-invading worms, the ones that migrate through organs, that provoke the strongest response.
Eosinophilic Gastrointestinal Disorders
Eosinophils can accumulate in the lining of the digestive tract even without a parasite. The most well-known example is eosinophilic esophagitis (EoE), where eosinophils build up in the esophagus and cause difficulty swallowing, food getting stuck, and chest pain. It’s diagnosed by biopsy: finding 15 or more eosinophils per high-power field under the microscope, a threshold that has remained the standard since 2007 and was reaffirmed in the 2025 American College of Gastroenterology guidelines.
Similar conditions can affect the stomach and intestines, causing pain, nausea, and diarrhea. These eosinophilic GI disorders are often linked to food allergies but can also be idiopathic, meaning no clear trigger is found.
Medication Reactions
Certain drugs can trigger your body to overproduce eosinophils. The most serious form of this is a condition called DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms), which typically appears two to eight weeks after starting a new medication. It causes fever, rash, swollen lymph nodes, and can damage the liver, kidneys, or heart.
The most common culprits are anti-seizure medications, with carbamazepine being the single most frequent cause. Allopurinol (used for gout), sulfa-containing drugs like sulfamethoxazole and dapsone, and certain antibiotics including minocycline and vancomycin are also well-documented triggers. More recently, some vaccines and biologic drugs have been linked to DRESS as well. If you’ve started a new medication in the past few weeks and develop a rash with fever, that timeline matters for diagnosis.
Autoimmune and Vasculitis Conditions
Some autoimmune diseases drive eosinophil production as part of a misdirected immune response. One of the most distinctive is eosinophilic granulomatosis with polyangiitis (EGPA), formerly known as Churg-Strauss syndrome. This condition inflames blood vessels throughout the body and tends to unfold in three phases.
The first phase is asthma, which may appear years or even decades before the disease is fully recognized. Nasal polyps and chronic sinus problems often develop alongside it. The second phase involves rising eosinophil counts, typically above 10% of total white blood cells. The third and most dangerous phase is full-blown vasculitis, where inflamed blood vessels damage the skin, lungs, kidneys, and nerves. Nerve involvement is particularly common and devastating: it can cause severe tingling, numbness, shooting pain, and muscle wasting in the hands or feet. The long gap between the first asthma symptoms and later phases is why EGPA is often diagnosed late.
Blood Cancers and Bone Marrow Disorders
In some cases, high eosinophils are a sign that something has gone wrong in the bone marrow itself. Certain genetic mutations cause bone marrow cells to produce eosinophils uncontrollably. The World Health Organization recognizes a specific category of cancers called myeloid and lymphoid neoplasms with eosinophilia, which are driven by rearrangements in genes that control cell growth.
Chronic eosinophilic leukemia is one such condition, where eosinophils are part of a cancerous clone. Systemic mastocytosis, a disorder involving another type of immune cell, is accompanied by eosinophilia in roughly 20% to 30% of cases. These diagnoses are uncommon but important to identify because some respond dramatically well to targeted therapies that block the specific mutated protein driving the disease.
Hypereosinophilic Syndrome
When eosinophil counts exceed 1,500 cells per microliter for six months or more and there’s evidence of organ damage, the condition is classified as hypereosinophilic syndrome (HES). An alternative definition requires that the count exceed 1,500 on at least two separate blood draws taken at least two weeks apart. HES is not a single disease but a label applied when high eosinophils are causing harm and no underlying cause, such as a parasite or cancer, can be identified.
The organs most vulnerable to eosinophil-mediated damage are the heart, lungs, skin, and nervous system. Eosinophils release toxic proteins that were designed to kill parasites but can injure your own tissues when they accumulate in the wrong places. Heart involvement is the most serious complication and can lead to scarring that restricts how well the heart pumps.
Less Common Triggers
Several other conditions can raise eosinophil counts. Adrenal insufficiency, where the body doesn’t produce enough cortisol, removes a natural brake on eosinophil production. Some inflammatory bowel diseases, particularly Crohn’s disease, can be associated with mild eosinophilia. Certain fungal infections, radiation exposure, and cholesterol crystal embolism are rarer causes. In a significant number of people with mild eosinophilia, no cause is ever identified. These idiopathic cases are monitored over time but often don’t progress to anything concerning.