Eosinophils are a type of white blood cell produced in the bone marrow. A healthy cat has between 100 and 1,200 eosinophils per microliter of blood. The count may go up or down in number, depending on what's happening with your cat's health. An elevated eosinophil count in a cat indicates infection, parasites, allergies or a variety of other conditions.
Your Cat's White Blood Cells
Leukocytes, more commonly referred to as white blood cells, work to defend the body. They are the primary defenders of your cat's immune system. When a cat's white blood cell count is high, they're working to defend against what her body perceives as an intruder; when they're low, she's at risk for infection. Eosinophils are one type of white blood cell; the others are neutrophils, basophils, lymphocytes and monocytes. Each performs a specific role in the health of your cat. Eosinophils can consume foreign particles that shouldn't be in the blood or tissue.
A cat with a parasitic infection will have an increased eosinophil count, as they're called into action to take on the parasite. When your cat has fleas or an internal parasite, such as roundworm, her body reacts, sending signals her eosinophils respond to by releasing a chemical substance to kill the parasite.
If your cat has an allergy, her eosinophil level will rise as they respond to the perceived threat, whether it's food, dust or some other environmental irritant. Sometimes her eosinophils will overreact to the trigger, resulting in eosinophilic granuloma complex. This condition has three separate syndromes that result from an elevated eosinophil count, occurring separately or together in any combination. Eosinophilic plaque results in raised, round or oval lesions, typically on the abdomen or thighs, that often become ulcerated. Indolent ulcers are usually ulcerated lesions found on the upper lip. An eosinophilic granuloma is a mass or nodular lesion occurring on the face, in the mouth or back of the thighs.
Mast Cell Tumors and More
Mast cell tumors may be behind an elevated eosinophil reading. These tumors may be non-spreading -- benign -- or malignant and life-threatening. The cause of this type of cancer isn't known, and it tends to be a benign form in most cats.
Other potential causes of an elevated eosinophil count include inflammatory bowel disease, Addison's disease, heartworm infection and allergic pneumonitis. An elevated eosinophil count on its own is not sufficient information to make a diagnosis for what's ailing a cat. The vet will consider other test results, symptoms and medical history to determine the ultimate diagnosis.