Eosinophilia in cats

Eosinophils are a type of white blood cell produced in the bone marrow. They are easy to identify on a blood smear because the granules inside the cell are a beautiful red color when stained with eosin dye. Other types of white blood cells include monocytes, basophils, lymphocytes and neutrophils. Each white blood cell has a specific job to perform. Eosinophils are responsible for fighting parasites and other infections in cats as well as working with mast cells in allergic reactions.  Think asthma and allergies.

Eosinophilia is a term used to describe too many eosinophils. Most cats will have less than 1000/microliter of these cells circulating in their blood at any given time. Here is a list of common causes of eosinophila in cats. I have ranked them from what is most common in my practice to least common:

1) Parasites – Internal and external parasites will often cause a mild to moderate eosinophila in cats. Fleas are usually the culprit in most of the U.S. although I don’t see a lot of fleas in Phoenix, Arizona. The lack of humidity in the desert dries out the eggs making it tough to reproduce. Other external parasites include ticks, lice and mange. There are many types of internal parasites which can cause an increase in eosinophils including tapeworms that are transmitted by the fleas.  In addition, round worms, whip worms and hook worms can also be causative. If the cat has basophila in addition to  eosinophila, I also look for heartworms.

2) Allergies/Asthma – food or environmental. Cats can have allergic reactions just like humans. Some cats are allergic to their food while others are allergic to something in the environment. Cats with food allergies often have diarrhea, vomiting and gas. Sometimes, they even chew or pull out fur on their abdomens, probably from the pain. Environmental allergins can also trigger allergic reactions in cats. Feline asthma is a serious condition that can be life-threatening. I had one patient who suffered an asthma attack every Saturday night because he sat on the bathroom counter when his owner got ready to go out.

3) Eosinophilic granuloma complex – Eosinophilic granuloma complex is a painful disease that affects the lips, ears and face. For more information  and a picture of a cat with this disease go to https://drnelsonsveterinaryblog.com/tag/eosinophilic-granuloma-complex/.

4) Inflammatory bowel syndrome – Inflammatory bowel syndrome is a complex disease to diagnose and treat. Clinical signs include vomiting, diarrhea and weight loss. Cats with IBD usually require a limited antigen diet combined with medications that decrease inflammation in the intestines. Here is a link to my post on feeding cats with inflammatory bowel syndrome: https://drnelsonsveterinaryblog.com/2010/03/diet-recommendations-for-cats-with-inflammatory-bowel-disease/

5) Systemic mycoses – When fungal organisms infect, the disease is referred to in general terms as a “systemic mycoses”. There are several different types of mycotic disease that are found in specific environments.  The Midwestern United States has blastomycosis, the humid areas of the south have histoplasmosis and Arizona has a fungal organism called Coccidioides immitis  which causes “Valley Fever”. Although I diagnosis a lot of valley fever in dogs, it is rare for cats to get this.

6) Paraneoplastic syndrome – Eosinophilia may be secondary to cancer. Tumors sometimes secrete hormones or hormone-like products that cause abnormal reactions. In cats, transitional cell carcinoma of the urinary bladder, mast cell tumor and lymphosarcoma may cause eosinophilia.

7) Hyereosinophilic syndrome – HES is a rare condition in cats characterized by high numbers of circulating eosinophils. The cause is not known. The eosinophils infiltrate other organs including the spleen, liver and lungs causing damage. Even with aggressive immune-suppressive therapy, the prognosis is guarded.

8) Chronic Eosinophilic Leukemia – CEL is rare form of cancer that may be related to HES. Besides having high numbers of eosinophils in their blood, these cats have many ‘blast’ or immature cells in their bone marrow causing eosinophil counts of >50,000. In contrast, cats with HES usually have <50,000. Unfortunately, survival time is short with some cats within days of their diagnosis.


Rothrock, Kari. “Chronic Eosinophilic Leukemia” Associate Database, VIN updated 5/30/14.

Rothrock, Kari. “Hypereosinophilic Syndrome”, Associate Database, VIN Updated 5/30/14.

Tilley, L.P. and Smith, F.W.K., Blackwell’s Five-Minute Veterinary Consult: Canine and Feline, WILEY-BLACKWELL 2011.