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What are Malignant Histiocytosis?

A disease affecting middle aged dogs, malignant histiocytosis produces multiple tumors in the skin on the limbs or trunk. Inside the body, tumors frequently affect many organs systems simultaneously, including the spleen, liver, lymph nodes, lungs, bone marrow, brain and central nervous system, and joints. It is important for this condition to be diagnosed and treated immediately, as it can quickly result in life-threatening conditions.

Malignant histiocytosis, or disseminated histiocytic sarcoma, is a fairly rare and aggressive disease that produces multiple cancerous tumors in the skin and vital organs throughout the body. These tumors progress rapidly and are very often fatal, as they spread to various organs and destroy the surrounding tissues. Symptoms are varied and depend on what part of the body is affected.

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Symptoms of Malignant Histiocytosis in Dogs

Many different types of symptoms can be involved in this disease, and are wholly dependent on which organs are affected by tumor growth. If the lungs are affected, breathing can be difficult, while a mass in the brain can result in seizures and paralysis. Be sure to note all the symptoms you see so that your veterinarian will have an accurate view of which organs may be involved. Some signs can include:

  • Loss of appetite
  • Weight loss
  • Vomiting
  • Fever 
  • Lethargy
  • Depression
  • Coughing
  • Difficulty breathing
  • Diarrhea
  • Limping
  • Lameness
  • Incoordination
  • Neurological disturbances 
  • Paralysis
  • Seizures
  • Anemia 
  • Jaundice

Causes of Malignant Histiocytosis in Dogs

The definitive cause of malignant histiocytosis is still unknown. While some believe that these tumors are a soft tissue sarcoma, others have categorized malignant histiocytosis as a type of histiocytic sarcoma, which may be influenced by an abnormally large amount of histiocytes, a type of white blood cells that are integral to immune system functions. 

While it is unknown if there exists a genetic component that may predispose a dog to develop malignant histiocytosis, it does seem to develop in certain breeds, including: 

  • Bernese Mountain Dogs
  • Boxers
  • Briards
  • Bull Terriers 
  • Chinese Shar Peis
  • Cocker Spaniels
  • Collies
  • Dachshunds
  • Flat Coated Retrievers
  • German Shepherds
  • Golden Retrievers
  • Great Danes
  • Rottweilers
  • Shetland Sheepdogs

Diagnosis of Malignant Histiocytosis in Dogs

A veterinary visit is often scheduled once your dog is showing symptoms, or there are visible tumors on his skin. Your veterinarian will perform a physical exam and look for signs of other tumors. Be sure to relate all symptoms you have seen, as they can help point to organs that may be affected. Blood samples may be taken and analyzed, and may show signs of anemia or increased liver enzymes. A urinalysis may also be taken to look for abnormalities. 

X-rays, CT scans, ultrasounds, and MRIs may be employed to assess if the tumors have spread to internal organs and tissues.  They can also alert your veterinarian to complications that may be occurring, such as an enlarged liver or spleen, or the presence of fluid in lung cavities. 

Next, a biopsy or tissue sample of the tumors, or any excessive fluids that have collected in body cavities, may be taken and analyzed. Immunohistochemistry can help to differentiate between benign and malignant tumors, as well distinguish the type of cancer your dog may have. It can also help to determine where the tumors may have originated from, which can aid the diagnosis. A bone marrow evaluation may also be recommended, which is not usually painful, but does require sedation.

Treatment of Malignant Histiocytosis in Dogs

Treatment will attempt to remove and reduce tumor growths, and is best performed as early as possible to catch the tumors before they spread. Once tumors have spread into internal organs, the disease is usually fatal and any treatment will aim to make your dog more comfortable. 

Immediate surgery is required to remove any tumors found. If tumors are affecting joint tissues in limbs, an amputation may be recommended. 

Radiation and chemotherapy can be used with surgery to stop the spread of the cancer, and are often administered over several weeks. These therapies may increase survival times from months to years. Supportive therapies may include fluid and electrolyte treatments.

Recovery of Malignant Histiocytosis in Dogs

The prognosis for recovery from malignant histiocytosis is poor. If tumors have not spread into internal organs, treatments can give many more months or years of life to your dog. Tumors are rapidly progressive, and can quickly spread into internal organs, emphasizing the need to diagnose and treat this condition as soon as possible. If tumors have infiltrated the organs, the disease is usually fatal, either by complications or euthanasia. 

While any cancer is difficult to predict, you can help a quick diagnosis by monitoring any lumps, masses, or abnormal growths that may appear on your dog’s skin. Getting these growths checked by a veterinarian can result in quick and life-saving treatment before the condition progresses too far.