High Eosinophils: Causes (Allergy, Parasites and More)

Eosinophils are produced in the bone marrow, and their number is determined mainly by interleukin-5 (IL-5), a cytokine produced by type 2 helper T cells (Th2) and so-called type 2 innate lymphoid cells. The same pathway, known as type 2 inflammation, underlies allergy, atopic asthma, some cases of atopic dermatitis and defence against worms (helminths). When activated, eosinophils release granules containing proteins toxic to parasites but also damaging to the body's own tissues, which is why very high eosinophil counts can cause organ damage.

Two practical consequences follow from this mechanism. First, the blood eosinophil count reflects the intensity of type 2 inflammation only approximately: what happens in the tissues (lungs, oesophagus, skin, bowel) can matter more and does not always translate into the blood result. Second, the same number can mean entirely different things in different people, so interpretation always depends on symptoms, medicines and travel history.

It is also worth knowing that the eosinophil count changes over the day and under the influence of hormones. It is usually lower in the morning, when cortisol is highest, and higher at night, and glucocorticoid drugs, including inhaled ones at higher doses, can lower it. Conversely, adrenal insufficiency (Addison's disease), in which cortisol is lacking, can cause a rise in eosinophils. Small fluctuations between successive results are therefore normal and should not cause worry.

The reference range for eosinophils on a differential is usually about 1–5% of white cells, which at a typical white cell count corresponds roughly to 0.0–0.5 G/L (the exact values are given by the laboratory on the report). To assess eosinophilia you need to look at the absolute number, which is often given beside the percentage. If it is missing, you can calculate it: multiply the white cell count (G/L) by the eosinophil percentage and divide by 100. For example, 8 G/L white cells and 10% eosinophils give 0.8 G/L, that is, mild eosinophilia.

The thresholds of 0.5 G/L for eosinophilia and 1.5 G/L for hypereosinophilia are used in the haematological literature and in international consensus classifications of eosinophil disorders. Hypereosinophilic syndrome (HES) is diagnosed when hypereosinophilia is joined by organ damage caused by eosinophils, with no other explanatory cause. These limits are not sharp, and decisions are always made in clinical context.

In most people with slightly raised eosinophils the cause is atopy, that is, a tendency of the immune system to overreact to environmental allergens. This applies to allergic rhinitis (hay fever), atopic asthma, atopic dermatitis, food allergies and urticaria. Typically eosinophils rise in the pollen season, after contact with an allergen or during a flare of the disease, and fall after anti-inflammatory treatment.

In asthma the blood eosinophil count is used as a biomarker of so-called eosinophilic asthma, a phenotype that responds well to inhaled corticosteroids and, in severe forms, to biological treatment targeting IL-5. A high eosinophil count in a person with asthma is linked to a higher risk of exacerbations, but the result alone does not replace spirometry or clinical assessment. We describe the disease in more detail in our entry on asthma. It is worth adding that eosinophilia in a person with no allergy symptoms at all calls for a broader look and should not be explained away in advance as 'allergy'.

The practical conclusion: if eosinophils are mildly raised (below 1.5 G/L) and you have diagnosed atopy or compatible symptoms (wheeze, runny nose, itchy skin, rash), treating the underlying disease and a follow-up blood count after a few weeks or months is often enough. Failure to normalise despite treatment, or rising values, is a signal to look further.

Drugs are one of the most common causes of acquired eosinophilia over a lifetime. It has been described after many classes of drugs, among others antibiotics (especially beta-lactams, but also sulfonamides), non-steroidal anti-inflammatory drugs, anticonvulsants, allopurinol and some painkillers. It is usually mild and asymptomatic and resolves once the drug is stopped, but the decision to stop is always made by the treating doctor.

A special form is DRESS syndrome (drug reaction with eosinophilia and systemic symptoms), a rare but potentially dangerous hypersensitivity reaction that usually appears several weeks after starting a new drug. It is characterised by fever, an extensive rash, enlarged lymph nodes, eosinophilia and involvement of internal organs (liver, kidneys, lungs, heart). If fever with a rash appears after starting a new drug, it is a situation that needs urgent consultation, not waiting for a follow-up blood count.

A broad analysis of cases of severe eosinophilia from two New England hospitals shows the scale of this phenomenon in hospital conditions: drug reactions accounted for about one fifth of cases, of which about 90% occurred in the hospital setting. The details of that study are presented below.

Yes. Infections with multicellular parasites (worms) in which larvae migrate through tissues are a classic cause of eosinophilia. This applies among others to intestinal nematodes, strongyloidiasis (Strongyloides), trichinellosis, toxocariasis, schistosomiasis and filariasis. Infections with protozoa usually do not cause eosinophilia, and tapeworms and pinworms (common in children) only exceptionally. In countries with a low rate of infection, such as Poland, parasites are a relatively rare cause in the general population, but their importance grows with a travel or migration history.

British guidelines for returning travellers and migrants note that eosinophilia is a common finding in this group, often pointing to worm infection that may be asymptomatic or cause non-specific symptoms, and that some of these infections can lead to severe disease. The recommendations provide for choosing tests according to the region of travel (serological tests, stool examination for parasites and, in certain situations, empirical antiparasitic treatment) and have recently been updated, among others with regard to managing asymptomatic eosinophilia.

In practice: if in recent months or years you have been to tropical or subtropical regions, eaten undercooked meat or fish, or your water came from an uncertain source, or you have lived outside Europe, tell your doctor. A single stool test is often not enough, because many parasites are shed irregularly and some (like Strongyloides) are better detected serologically. Deworming 'blindly' without a diagnosis is not the right strategy, because drugs are chosen for the specific parasite.

What follows from this? Above all, that the distribution of causes depends on the eosinophil level and on the population. In people with severe eosinophilia (above 5 G/L) treated in hospital, blood diseases and cancers come first, not allergy or parasites. In mild eosinophilia, which is usually found in outpatient clinics, atopic diseases dominate. An important caveat: the study concerns hospital patients from a particular region of the USA with severe eosinophilia, so it does not translate directly into how often something dangerous lies behind a result of 0.6–0.9 G/L in a Polish clinic. It cannot be used to conclude that mild eosinophilia means a blood disease.

Eosinophilic oesophagitis (EoE) is a chronic immune disease in which eosinophils infiltrate the lining of the oesophagus. It presents with difficulty swallowing, a feeling of food getting stuck, heartburn that does not respond to treatment or chest pain, more often in young men with atopy. Diagnosis requires endoscopy with biopsy; according to the 2018 AGREE international consensus it rests on symptoms of oesophageal dysfunction and the presence of eosinophilic infiltration in the biopsies, after other causes have been excluded. Blood eosinophils may be raised but not always, so a normal blood count does not exclude it.

Among skin diseases, apart from atopic dermatitis, eosinophilia is also caused by urticaria, bullous pemphigoid and some other dermatoses. As for the bowel, eosinophilic gastroenteritis is rare but possible. Autoimmune diseases include eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg-Strauss syndrome), which affects people with asthma, often with accompanying nasal polyposis, and presents with neuropathy, skin changes and lung or heart involvement. Such diagnoses are made in a specialist clinic (rheumatology, allergology, gastroenterology).

Rarely, a high eosinophil count is a sign of a disease of the blood itself. There are marrow neoplasms in which the eosinophils themselves are the neoplastic clone, for example myeloid and lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (PDGFRA, PDGFRB, FGFR1), as well as hypereosinophilic syndrome (HES) and lymphoid neoplasms such as lymphomas, which can cause secondary eosinophilia. The landmark 2012 consensus, and then its 2023 update, unified the classification: primary (clonal), reactive (where eosinophils are a result of an underlying disease, including cancer) and idiopathic forms are distinguished, and the diagnosis of HES requires organ damage caused by eosinophils.

Signals that raise suspicion of such a cause are: very high and rising eosinophils (especially above 1.5 G/L persisting for months), not explained by allergy, drugs or parasites, together with fever, weight loss, night sweats, enlarged nodes or spleen, changes in the other blood count parameters (anaemia, platelet abnormalities, abnormal white cells) or signs of organ damage (breathlessness, palpitations, neurological problems, skin changes). In such a situation the patient should be referred to a haematologist, who will order molecular tests and possibly a bone marrow biopsy. A 2026 review of the practical approach to hypereosinophilia stresses precisely early recognition of organ damage and the red flags indicating the need for haematological assessment.

This sequence matches the logic described in reviews and consensus documents: first the common and reversible causes (drugs, allergy, parasites) are sought, then organ-based and autoimmune diseases, and last the haematological causes. It is important not to stop the work-up at the first fitting answer if eosinophils are very high or rising, because several causes can coexist.

On the other hand, a single mildly raised result (0.5–0.8 G/L) in a person with allergy or asthma and no additional symptoms rarely means anything dangerous. It is worth repeating and discussing with a doctor, but there is no reason for panic.

It is also worth remembering that eosinophils are not a 'bad' cell to be fought. They are a normal part of the immune response, and lowering them is not a goal in itself. In severe asthma a drug targeting eosinophils is used for clear indications, not to improve a blood count result.

High eosinophils are one of those results whose interpretation depends on context more than on the number itself. In most cases allergy or an atopic disease stands behind the rise, and the work-up comes down to a good history and simple tests. Common sense nevertheless requires remembering drugs, travel history and rare but important causes such as DRESS syndrome, eosinophilic oesophagitis or hypereosinophilic syndrome. When the numbers are very high, rising or accompanied by worrying symptoms, there is no point delaying a haematologist and an assessment of the organs. In our series on unusual blood count results we also cover high and low platelets, low white cells and high MCV.