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Overproduction of White Blood Cells in the Bone Marrow in Dogs

Hypereosinophilic Syndrome in Dogs

Hypereosinophilic syndrome is a condition characterized by an unknown origin, marked by persistent eosinophilia—an excessive production of eosinophils (a type of white blood cell) in the bone marrow. While its precise cause remains elusive, it is believed to be associated with a severe reaction to an unidentified antigen or a malfunction in the immune system’s regulation of eosinophil production. This syndrome affects multiple bodily systems, as eosinophils invade tissues, leading to organ damage and dysfunction. Sadly, it often culminates in a fatal outcome.

Organ damage arises from the actions of eosinophil granule products and eosinophil-derived cytokines, which are regulatory proteins released by immune system cells into tissues. Common sites of infiltration include the gastrointestinal tract—particularly the intestines and liver—along with the spleen, bone marrow, lungs, and abdominal lymph nodes.

In rarer cases, infiltration may occur in the skin, kidneys, heart, thyroid, adrenal glands, and pancreas. Although uncommon in dogs, certain breeds, such as Rottweilers, may have a predisposition to this condition.

Symptoms and Types

  • Fatigue
  • Elevated body temperature (Fever)
  • Reduced desire to eat (Anorexia)
  • Intermittent episodes of vomiting and diarrhea
  • Decline in body weight
  • Emaciation
  • Enlargement of the liver and spleen
  • Thickened segments of the intestine that are non-painful
  • Presence of abdominal masses
  • Occasional symptoms of itching and seizures
  • Swollen lymph nodes in the abdominal area (Mesenteric) and possibly in other parts of the body (Peripheral)
  • Formation of masses resulting from eosinophilic granulomatous inflammation involving lymph nodes and/or organs


The exact cause of hypereosinophilic syndrome remains unknown. However, it is suspected to stem from a severe reaction to an underlying antigenic stimulus that has yet to be identified. This stimulus might be comprised of two distinct strains of a virus.


The veterinary assessment will involve standard laboratory procedures, including a comprehensive blood profile, chemical blood analysis, complete blood count, and urinalysis. You will be required to provide a detailed history of your dog’s health and the onset of symptoms. Additional diagnostic measures may encompass bone marrow aspiration and/or core biopsy, as well as biopsy of the affected organ or mass. Blood test results typically reveal elevated levels of various types of white blood cells, notably leukocytosis, basophilia, and eosinophilia. Anemia may also be indicated, and abnormalities in the biochemical profile may signal organ dysfunction.

Diagnostic imaging techniques can aid in assessing the extent of organ damage. Radiographic contrast, involving the injection of a radiocontrasting agent, may enhance visualization of internal organs. X-rays may reveal thickened intestines and abnormalities in intestinal lining. Other observations may include reactive lymph node enlargement due to eosinophil infiltration, as well as fibrosis and arterial thrombosis around the heart.


A long-term maintenance regimen will be initiated to manage or diminish eosinophilia and mitigate organ damage. Elevated serum immunoglobulin levels, indicating a favorable response to prednisone—a corticosteroid administered to alleviate inflammation—may correlate with a better prognosis. Prednisone proves effective in suppressing eosinophil production. In select cases, chemotherapy may be warranted to inhibit DNA synthesis, thereby curtailing cell reproduction. Extensive tissue infiltration often hampers treatment and typically portends a bleak prognosis.

Recovery and Management

Your veterinarian will arrange follow-up appointments to assess your dog’s eosinophil count, which may not always reflect tissue infiltrates, and to monitor for myelosuppression if chemotherapy drugs are administered. Clinical symptoms and physical abnormalities, such as loss of appetite, lethargy, vomiting, and diarrhea, will be closely monitored during these examinations.

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