Chronic Idiopathic Myelofibrosis in Dogs

A persistent disease of bone marrow growth, chronic idiopathic myelofibrosis involves the replacement of bone marrow by fibrous scar tissue and changes in cellular composition of the blood. Chronic IMF dogs display an abnormally increased quantity of immature blood cells in the circulatory blood and within the bone marrow. This increase is due to proteins that send signals to the cells to incite proliferation. The bone marrow, spleen and liver are all affected as sites of the formation and development of these cells.

Signals released by large abnormal bone marrow cells cause immature fiber producing cells to deposit thick reticulin and small collagen fibers. There may eventually be a decrease in the cellular components of the blood due to decreased active bone marrow. The decrease in cellular components can cause serious health disorders, as the circulatory blood does not carry enough cells to carry out vital body functions.

Causes of Chronic Idiopathic Myelofibrosis

Chronic idiopathic myelofibrosis, or chronic IMF, can be a primary disorder that is self-originated and without a known cause or it can be secondary to other conditions. Toxin exposure, irradiation, chemotherapy and viruses are a few of the many questioned causes for the onset of primary chronic IMF. In its secondary form it is a condition related to numerous disorders including congenital or immune-mediated anemias, irradiation, marrow neoplasia or invasive tumors originating outside the marrow, drug-induced marrow damage or bacterial diseases caused by the organism Erlichia.


A dog with chronic IMF may gradually begin to exhibit symptoms such as:

  • Fatigue
  • Low exercise tolerance
  • Decreased appetite or anorexia
  • Slight weight loss to severe emaciation
  • Vomiting
  • Diarrhea
  • Fever

These symptoms can be observed by an owner but are also indicative of many other serious conditions. A regular physical examination by a veterinarian may detect symptoms such as pale mucous membranes or enlargement of the spleen or liver during abdominal palpation. Laboratory blood work may show anemia and a decrease or increase of platelets in circulating blood. A dog exhibiting severe anemia that is unresponsive to treatment is an indication that a bone marrow biopsy should be administered.


Definitive diagnosis of chronic IMF requires a bone marrow biopsy in which the core marrow sample is stained to detect myelofibrosis. The sample may exhibit varying degrees of cell mass, formation of fibrous tissue by fiber-producing cells and collagen deposits in the marrow cavity. Secondary myelofibrosis from related conditions can be differentiated in the observance of severe bone marrow damage and necrosis.

Courses of Treatment

Treatments for chronic IMF vary with its occurrence as a primary condition or secondary condition to underlying causes. Therapy involving agents that suppress immune response may be administered in conjunction with supportive blood transfusions. The corticosteroid Prednisolone can be administered to resolve anemia, with once daily oral doses that are gradually tapered after approximately three to four weeks. Anabolic steroids may be administered weekly for three weeks and then once every three weeks, but must be given in intramuscular injections. If anemia does not respond to initial treatments, Azathioprine may need to be administered orally at a rate of once every other day.