Eosinophils in cats and dogs
Eosinophils are white blood cells that take part in allergic reactions and in fighting parasites. On a blood count they are a small group, and the count most often rises with allergies, fleas, worms or heartworm. Stress hormones lower it, so a count near zero in a sick or anxious animal is usually part of the stress pattern rather than a separate finding.
What it shows
Eosinophils are made in the bone marrow and carry orange granules that are easy to see on a stained smear. Their main jobs are killing parasites and taking part in allergic inflammation, especially in the skin, airways and gut. In the blood they are only a small share of white cells, and most of them work in tissues.
Because eosinophils react both to allergy and parasites and to the animal’s hormonal state, their count is a pointer, not an answer: a high value suggests where to look, and a low one often only reflects stress at the moment of sampling.
Higher than reference — what it may be a sign of
- Allergy and hypersensitivity — a common cause, for example allergic skin disease or allergic inflammation of the airways or gut.
- Parasites — common: fleas, intestinal worms, migrating larvae. In dogs heartworm (Dirofilaria) is a frequent cause. Most blood parasites, such as Mycoplasma, do not cause eosinophilia.
- Tumours: most often mast cell tumours; less often T-cell lymphoma and some other cancers.
- Hypoadrenocorticism (Addison’s disease) in dogs — usually a mild rise. In a sick dog, the absence of the usual stress pattern can be a clue the vet may check — a sign, not a diagnosis.
- Hypereosinophilic syndrome — rare: a persistent, marked rise with eosinophils in organs, described in cats and in Rottweilers.
Lower than reference
A low eosinophil count is most often caused by corticosteroids — the body’s own cortisol in a stressed or sick animal, or steroid medicines — and is part of the stress pattern. It can also be seen in bacterial sepsis. Because the lower limit of many reference ranges is zero, a decrease is often impossible to document, and Merck notes that low eosinophils have no pathological relevance on their own.
Look at together with
- Basophils — they often rise together with eosinophils in allergy and parasitic disease.
- Neutrophils and lymphocytes — more neutrophils with fewer lymphocytes and eosinophils is the stress pattern.
- Total white cell count (WBC) — needed to turn the percentage into an absolute number.
- Parasite tests — faecal tests, flea checks and, in dogs, a heartworm test, as the vet decides.
Units and conversion
The absolute count (EOS, Eos#) is reported as ×10⁹/L (×10^9/л on Russian forms). The percentage (EOS%) is the share of eosinophils among all white cells. Absolute count = percentage × total WBC ÷ 100. A vet relies on the absolute count: with a very high total WBC, even a modest percentage can mean a real eosinophilia. In the app each row keeps the unit printed on the form.
Reference range
The reference range is on your lab’s form. It differs between species and analyzers, and its lower limit is often zero. A mild rise in an animal with itchy skin or fleas is read differently from a persistent marked rise without an obvious reason; the vet decides which further tests make sense. A series of tests says more than one result.
Cats and dogs
In cats eosinophilia is often linked to fleas, intestinal worms and allergic disease of the skin. Idiopathic hypereosinophilic syndrome — persistent eosinophilia with eosinophils in organs and no cause found — has been reported in cats, but it is rare.
In dogs heartworm is a common cause of eosinophilia, along with fleas, worms and allergic skin disease. In greyhounds, other sighthounds and golden retrievers eosinophils often lack visible granules and look grey and vacuolated — a normal breed variant. In a sick dog, the absence of the usual stress pattern (low lymphocytes and eosinophils) is one of the features that may lead a vet to think of Addison’s disease.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Individual WBC
- eClinPath (Cornell University) — Normal leukocytes
- Merck Veterinary Manual — Leukogram Abnormalities in Animals
- Merck Veterinary Manual — Physiology of Leukocytes in Animals
- Merck Veterinary Manual — White Blood Cell Disorders of Dogs (dog owners)
- Merck Veterinary Manual — White Blood Cell Disorders, Leukemia, and Lymphoma of Cats (cat owners)
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.