By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
Eosinophils are the white cells that respond to parasites and allergic inflammation, so a raised count classically points to worms, fleas, or allergic disease. A low count is usually just the body's stress response and rarely means anything on its own.
Eosinophils are a distinctive granulocyte, packed with granules whose contents are designed to attack parasites — and which, when released inappropriately, drive allergic inflammation. They are usually a small minority of the white cell count, so proportionally large swings are common and the absolute count is what matters.
Eosinophils target parasites and drive allergic inflammation — so a raised count classically points to worms, fleas, or allergic disease.
Reading a high result
A raised eosinophil count (eosinophilia) has a short, useful differential:
Parasites — intestinal worms, lungworm, fleas and other external parasites. This is the first thing to exclude, and often the easiest to treat.
Allergic and hypersensitivity disease — flea-allergic dermatitis, atopy, allergic airway disease and asthma.
Certain inflammatory syndromes — eosinophilic gastroenteritis, and in cats the eosinophilic granuloma complex.
Addison's disease — the absence of a normal stress response can leave eosinophils inappropriately normal or high in a sick animal, a subtle but genuinely useful clue.
Rarely, some tumours (including mast cell tumours) drive eosinophilia.
So a persistent eosinophilia in a dog or cat is a prompt to think parasites first, allergy second, and to look further if neither fits.
Reading a low result
A low eosinophil count (eosinopenia) is very common and usually means little: cortisol — from stress, illness, or steroid medication — reliably drives eosinophils down as part of the classic stress leukogram. It is expected in an unwell or anxious patient and is not pursued on its own. Its main interpretive value is the reverse: when a genuinely sick animal has failed to produce that expected eosinopenia, it raises the question of Addison's disease.
In chronic disease
Followed over time, the eosinophil count tracks parasite burden and allergic disease activity, and can be used to gauge response to parasite treatment or allergy management. In cats with asthma or eosinophilic disease, a falling count alongside clinical improvement is reassuring; a persistently raised count suggests ongoing antigenic drive.
Cat vs dog
Eosinophils behave similarly in both species, with allergic and parasitic causes dominating in each. Cats have some distinctive eosinophilic conditions — feline asthma and the eosinophilic granuloma complex — that make a raised count particularly worth pursuing, and feline eosinophil counts can run relatively higher. In both, the stress-driven low count is common and unremarkable.
About the reference range
This page shows no range strip: Pawline draws a band only where it holds a verified reference interval, and eosinophil intervals are analyser- and species-specific. Read your result against the range printed on your own report, and interpret the absolute count rather than the percentage.
Two published examples show why the lab matters: Cornell University (Advia 2120) reports absolute eosinophils of roughly 0.1–2.1 ×10⁹/L (dog) and 0.1–1.8 ×10⁹/L (cat), while IDEXX Reference Laboratories (updated July 2025) gives 0.14–1.93 ×10⁹/L (dog) and 0.21–1.21 ×10⁹/L (cat) — the same units, two different analysers, visibly different intervals (×10⁹/L is numerically identical to the ×10³/µL some reports use). Your own report's range is the one to read.
What to do next
A raised eosinophil count is a practical prompt to review parasite control (including lungworm and fleas) and to consider allergic disease with your vet — both very treatable. A low count in an unwell pet is expected and needs no action; the notable exception is a sick animal without eosinopenia, which is one of the small clues that can point towards Addison's disease.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
