Erythrocytosis in Dogs

Erythrocytosis also referred to as polycythemia, is a disease that causes an excess of red blood cells to be produced. There are four types of erythrocytosis: relative, primary, secondary, and endocrinopathy-associated. Symptoms may include excessive thirst, weakness, sluggishness, changes in behavior, up to and including heart failure. Erythrocytosis is treated by reducing the amount of red blood cells in the body through phlebotomy and hydroxyurea, as well as managing tumors that cause an excess of serum erythropoietin (in secondary erythrocytosis). Follow-up exams will be conducted to measure the effectiveness of treatment.

• Red mucous membranes
• Bleeding
• Excessive production of dilute urine
• Excessive thirst
• Some loss of bodily movement control
• Weakness
• Seizures
• Blindness
• Changes in behavior
• Sluggishness
• Seizures
• Heart failure

Types

• Relative erythrocytosis - Relative erythrocytosis is a condition where red blood cell numbers increase without an increase in the total mass of red blood cells. This is typically caused by severe dehydration, normally the result of vomiting and diarrhea. Additionally, relative erythrocystosis may develop when fear or excitement causes splenic contraction, releasing red blood cells into circulation.
• Primary erythrocytosis (polycythemia vera) - In primary erythrocytosis, red blood cell production increases dramatically. Additionally, serum erythropoietin (a hormone in the blood) activity is decreased.
• Secondary erythrocytosis - Alternatively, this form of erythrocytosis is the result of an excess of serum erythropoietin activity. This form is more common in severe pulmonary diseases or the presence of heart problems where right-to-left shunting causes the blood to bypass the lungs.
• Endocrinopathy-associated erythrocytosis - This form of erythrocytosis results from hormones other than serum erythropoietin, such as cortisol, androgen, thyroxine, and growth hormones.

• Relative erythrocytosis is primarily caused by severe dehydration. This dehydration can be caused by lack of water intake, vomiting, and diarrhea. Separately, relative erythrocytosis can be caused by increased fear, excitement, or other states of arousal occur, releasing red blood cells into circulation.
• Primary erythrocytosis, or polycythemia vera, can be linked to congenital heart disease, kidney tumors, and some types of bone marrow cancer. The specific cause of this form of erythrocytosis is unknown.
• Secondary erythrocytosis is caused by decreases in the amount of oxygen reaching the tissues in the body, called hypoxia.
• Endocrinopathy-associated erythrocytosis is caused by hormones other than serum erythropoietin mimicking the effects of secondary erythrocytosis.

Typically, relative erythrocytosis is easily diagnosed by the severe dehydration that causes this condition. For cases where relative erythrocytosis is caused by some form of arousal, a blood test may show normal counts if the blood samples are collected when the dog is not aroused. Because there is an overlap between serum erythropoietin activity levels in primary and secondary erythrocystosis, diagnosis relies on ruling out causes that would suggest secondary erythrocytosis, such as tissue oxygenation levels. Below are some diagnostic tests you can expect your veterinarian to complete.

• Physical exam
• Blood tests
• Assessing of tissue oxygen levels
• Measuring of serum erythropoietin levels
• X-rays of hearts and lungs
• Electrocardiography
• Echocardiography
• Neurologic examinations
• Ultrasound of the stomach
• IV urography, an examination used to evaluate kidneys, ureters, and bladder
• CT scan
• MRI

Treatment of erythrocystosis is focused on reducing the number of red blood cells present in the bloodstream, thinning the blood and allowing it to flow properly and oxygenate the body. Possible treatments include:

• Treatment of relative erythrocytosis due to dehydration typically consists of rehydration through IV fluids and treatment of any underlying causes.
• Primary erythrocytosis (polycythemia vera) treatment typically takes one of two forms: removing some of the blood or medicine to slow the production of red blood cells. Phlebotomy, a procedure to withdraw blood, is typically the first step in the treatment of primary erythrocytosis. This is paired with fluid replacement. Hydroxyurea is a medication that is used to slow the production of red blood cells, and may be used in conjunction with phlebotomy.
• In secondary erythrocytosis, tumors that are creating excess than serum erythropoietin should be addressed with surgery, chemotherapy, or radiation therapy. Phlebotomy and hydroxyurea may be used to reduce red blood cell count.

If hydroxyurea is used, a complete blood count will be evaluated weekly for the first month, then monthly for three months, and then every three months (schedule may vary due to veterinarian specifications) to see if the treatment is effective. Depending on the body’s reaction to treatment, the pet may be weaned off in a few years, though some pets will need to stay on the medication for life. Depending on other causes, your veterinarian may schedule other follow-ups and testing to be done.