Overproduction of White Blood Cells in the Bone Marrow in Cats


PetMD Editorial

Updated Aug. 24, 2022

Hypereosinophilic Syndrome in Cats


Hypereosinophilic syndrome, characterized by persistent eosinophilia -- that is, sustained overproduction of eosinophils (white blood cells of the immune system) in the bone marrow, is a disorder of unknown cause. However, its suspected cause is a link to a severe reaction to an unidentified antigen, or impairment of the immune response and control of eosinophil production. This is a multi-system syndrome, with invasion of the tissues by eosinophils and subsequent organ damage and dysfunction.


Organ damage can result from the effects of eosinophil granule products and eosinophil-derived cytokines, a category of regulatory proteins that are released by cells in the immune system into the tissues. Common sites of infiltration include the gastrointestinal tract (especially the intestine and liver), spleen, bone marrow, lungs, and lymph nodes (especially those in the abdominal area). It frequently has a fatal outcome.


Less common sites of infiltration include the skin, kidney, heart, thyroid, adrenal glands and pancreas. Middle-aged domestic short hair cats appear to be predisposed, and female cats are diagnosed more frequently than males.




  • Lethargy
  • Fever
  • Loss of appetite (anorexia)
  • Intermittent vomiting and diarrhea
  • Weight loss
  • Emaciation
  • Enlargement of the liver and spleen
  • Thickened (diffuse or segmental) intestine that is non-painful
  • Abdominal masses
  • Itching and seizures (less frequently)
  • Mesenteric and possibly peripheral lymphadenopathy (swollen lymph nodes in the abdominal region or other areas of the body)
  • Mass lesions caused by eosinophilic granulomatous (inflamed masses of tissue) involving the lymph nodes and/or organs



The cause of hypereosinophilic syndrome is unknown. However, it is believed to be cause by a severe reaction to an underlying, as yet unidentifiable antigenic stimulus that may be composed of two different strains of a virus. An early form of this disorder may be apparent in some cats; eosinophilic enteritis, an inflammatory disease of the small intestine that is directly related to infiltration of eosinophils in the connective tissue of the intestinal lining.




The veterinary examination will consist of standard laboratory work, including a complete blood profile, chemical blood profile, a complete blood count, and a urinalysis. You will need to give a thorough history of your cat's health and onset of symptoms. Additional diagnostics will include a bone marrow aspiration and/or core biopsy of the cells, and a biopsy of the affected organ or mass. It is typical for the blood test results to show increased amounts of multiple types of white blood cells, most notably leukocytosis (leukocyte), basophilia (basophil), and eosinophilia (eosinophil). The results of the blood tests may also show anemic conditions, and the biochemical profile may show abnormalities in the case of organ dysfunction.


Diagnostic imaging can be helpful in determining the extent of organ damage as well. Radiographic contrast, which uses an injection of a radiocontrasting agent into the area to be viewed, may be used to improve visibility of the internal organs. These X-rays may show thickened intestines and abnormalities in the lining of the intestines. Other findings may be reactive hyperplasia (abnormal enlargement) of the lymph nodes due to eosinophil infiltration, and fibrosis (excess fibrous connective tissue) and thrombosis (coagulation in the arteries) surrounding the heart.




Long-term maintenance therapy will be employed to control or reduce the eosinophilia and organ damage. High serum immunoglobulin concentrations (the fraction of the blood serum that contains antibodies) can signify a good response to treatment with prednisone, a corticosteroid given to reduce inflammation, and therefore a better prognosis. Prednisone can be effective at suppressing eosinophil production. In some cases, chemotherapy may be appropriate for inhibiting DNA synthesis, in effect, reducing the reproduction of cells. Massive tissue infiltration can impede treatment and usually leads to a poor prognosis.


Living and Management


Your veterinarian will schedule follow-up examinations for your cat to monitor eosinophil counts (not always indicative of tissue infiltrates) and myelosuppression (by which bone marrow activity is decreased) if chemotherapeutic drugs are being used. Clinical signs will also be monitored along with any physical abnormalities (e.g. loss of appetite, lethargy, vomiting, and diarrhea).

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