High Neutrophils in Cats: Causes, Normal Range & What It Means

Classic Stress Leukogram Pattern in Cats

1. Cat bite abscesses — The most common cause of neutrophilia in outdoor and indoor/outdoor cats. Cat bites inject bacteria deep under the skin; the wound seals over and traps the infection, forming an abscess. Neutrophil counts of 20,000–50,000/µL are typical. The abscess may be hidden under fur — your vet will palpate for a warm, painful, fluctuant mass. Treatment is drainage and antibiotics.
2. FIP (feline infectious peritonitis) — A mutation of feline coronavirus causes FIP, which triggers a severe inflammatory response. Neutrophilia is characteristic — often 20,000–60,000/µL with a left shift. The wet (effusive) form produces fluid in the abdomen or chest; the dry (non-effusive) form causes granulomas in organs. High neutrophils in a young cat with fever, weight loss, and fluid effusion should raise immediate suspicion for FIP. The FIP treatment landscape has changed significantly — GS-441524 antiviral therapy is now available. See our guide to FIP in cats →
3. Pyothorax — Bacterial infection of the chest cavity, often from bite wounds or migrating foreign material. One of the most serious causes of extreme neutrophilia in cats — counts of 40,000–80,000/µL with a significant left shift and toxic neutrophils are common. Cats are severely ill: labored breathing, reluctance to move, fever, and anorexia. Requires chest tube drainage and prolonged antibiotics, often with hospitalization.
4. Stress leukogram — Elevated cortisol from any source (illness, fear, pain, steroid medications) produces the characteristic pattern of high neutrophils, low lymphocytes, absent eosinophils — with no bands and no toxic changes. Does not indicate infection. Does not require antibiotics.
5. Corticosteroid medications — Prednisolone, dexamethasone, and injectable long-acting steroids reliably raise neutrophils within 24–48 hours. Counts of 15,000–25,000/µL are common. A predictable drug effect — not infection.
6. Dental disease — Severe periodontitis and tooth root abscesses are common in middle-aged and older cats and can cause persistent moderate neutrophilia (15,000–30,000/µL). Often underestimated as a cause of chronic illness. Dental radiographs and a dental cleaning under anesthesia may be needed to fully assess.
7. IBD and pancreatitis — Inflammatory bowel disease and pancreatitis can drive mild to moderate neutrophilia through cytokine release. These are common concurrent conditions in cats — the "triaditis" of pancreatitis, IBD, and cholangiohepatitis often travels together.
8. FeLV-associated leukemia — FeLV (feline leukemia virus) can cause malignant transformation of neutrophil precursors, producing very high counts (>80,000/µL) with abnormal cell morphology. Distinguished from infectious causes by the morphological abnormalities and positive FeLV test. Requires bone marrow evaluation.

1. Feline panleukopenia (feline parvovirus) — The most common cause of life-threatening neutropenia in unvaccinated cats, especially kittens. Panleukopenia destroys rapidly dividing cells — gut lining cells and bone marrow progenitors — causing profound neutropenia (often <500/µL) alongside low RBC and platelet counts. Without adequate neutrophils, gut bacteria invade the bloodstream, causing septicemia. Panleukopenia is highly contagious and often fatal without intensive supportive care. Vaccination is the single most important preventive measure in cats.
2. FeLV (feline leukemia virus) — FeLV infects and suppresses bone marrow, reducing production of neutrophils (and other blood cells). Cats with FeLV-associated myelosuppression develop persistent neutropenia alongside anemia and thrombocytopenia. FeLV also predisposes cats to secondary infections that further strain the neutrophil supply. Any cat with unexplained neutropenia should be tested for FeLV.
3. Chemotherapy — Cytotoxic drugs suppress bone marrow, reducing neutrophil production. Nadir (lowest point) typically occurs 7–10 days after treatment. Cats on chemotherapy need CBC monitoring at each cycle and may need antibiotics or dose adjustments if neutropenia is severe.
4. Overwhelming sepsis — When a bacterial infection is so severe that neutrophils are consumed faster than the bone marrow can replace them, total neutrophil count can fall despite an active infection. This degenerative left shift (more bands than mature neutrophils) with low or falling total neutrophils is a grave prognostic indicator requiring emergency intensive care.
5. Immune-mediated neutropenia — The immune system destroys its own neutrophils via antibody-mediated mechanisms. Can be primary (idiopathic) or secondary to drugs or other immune disease. Treated with immunosuppressive therapy (prednisolone, cyclosporine).
6. Drug reactions — Certain drugs beyond chemotherapy can cause neutropenia in cats, including methimazole (used for hyperthyroidism — can cause immune-mediated neutropenia in a small percentage of cats), chloramphenicol, and some antifungals. Any cat starting a new medication that develops neutropenia should have the drug evaluated as a possible cause.