Polycythemia, also called erythrocytosis, is a disorder in which the dog’s blood volume or concentration of the red blood cells (RBCs) increases. When this happens, the thickness or viscosity of the blood is increased and causes a multitude of problems.
There are three forms of polycythemia: absolute, relative, and transient. Each has its own cause and treatment, so it is very important that the form is properly diagnosed.
Absolute erythrocytosis refers to an increase in the number of RBCs in circulation due to an excess being produced in the bone marrow. There are two subtypes of absolute: primary and secondary.
Relative erythrocytosis, also called spurious polycythemia, refers to an apparent increase, but not an actual increase, of RBCs in circulation. This happens due to a reduction in volume of blood plasma. The overall RBC count remains the same but the decrease in plasma makes the blood thicker.
Transient erythrocytosis is a type of relative polycythemia, wherein the spleen puts large numbers of RBCs into the dog’s circulation due to stress, anger, surprise, excitement, or vigorous exercise. This increases the ratio of RBCs to plasma, and is a temporary condition.
No one knows what causes primary absolute polycythemia, and it is not very common in dogs. It occurs when the bone marrow tissue responsible for making RBCs increases, which in turn increases the number of red blood cells while plasma volume remains the same.
Secondary absolute polycythemia is caused when the kidneys produce and release an excess of erythropoietin, a hormone that stimulates bone marrow to produce RBCs. This has several causes, including Cushing’s disease, lung disease, adaptation to high altitudes, cancer, renal cysts or other renal diseases, pancreatitis, heart disease, and hyperthyroidism. Any time a dog develops hypoxia, it usually ends up with secondary absolute polycythemia as well.
Relative polycythemia occurs when plasma volume is reduced: blood loss, dehydration, decreased water intake, kidney disease, shock, diarrhea, vomiting, or hyperventilation. Blood loss may occur as the result of wounds, clotting disorders, parasites, or trauma.
Transient polycythemia, where the spleen releases adrenaline, which increases RBC volume, is caused when the dog requires adrenaline to prepare for fight or flight.
Relative polycythemia can be prevented by making sure the dog gets plenty of fluids, and by ensuring that all blood or fluid loss is treated promptly. Primary absolute polycythemia cannot be prevented, because no one knows what causes it. Secondary absolute polycythemia has many causes, some of which are treatable, which can help prevent or reverse the disorder. Transient polycythemia usually only lasts for a short while and doesn’t typically cause harm to the dog.
There are several general signs of the presence of too many red blood cells or unusual red blood cell volume: abnormal sensory skills, behaviour changes, cyanotic mucous membranes, nose bleeds, increased thirst and water intake, increased volume and frequency of urination, enlarged spleen, injected mucous membranes, enlarged liver, paroxysmal sneezing, and abnormal motor skills.
If left untreated, the disorder (especially absolute erythrocytosis) can cause weakness, difficulty breathing, confusion, lethargy, difficulty walking, exercise intolerance, seizures, uncontrolled shaking, lack of interest in play, lack of coordination, and vision impairment.
Dogs at Risk
All dogs are equally at risk – there is no age, gender, or breed predisposition to this disorder.
Testing and Diagnosis
When a dog shows any of the aforementioned symptoms, the vet will do a complete history and physical exam. This is followed by a baseline complete blood count, serum chemistry panel, total plasma protein measurement, and urinalysis, which can indicate polycythemia and help distinguish whether it is the relative or absolute form.
Other tests may be done to determine blood gas oxygen values, pulse oximetry oxygen saturation values; x-rays to show lung or heart disorders or any other disease that may contribute. Ultrasounds and electrocardiographs can identify any liver, adrenal gland, heart, or kidney problems.
Treatment and Prognosis
It is essential that the type and underlying cause of polycythemia be determined before any course of treatment is undertaken. Most cases are treatable, and it is best if treatment is prompt. Treatment goals include relieving the symptoms and restoring the dog’s plasma-red blood cell ratio.
Primary absolute polycythemia is treated by reducing the thickness of the blood. This is done by decreasing the number of RBCs and increasing the plasma volume via a procedure called phlebotomy. A portion of the blood is removed while isotonic fluids are used to restore the volume. It might take several courses of “bleeding” the animal to correct the disorder, and it may need to be repeated occasionally to maintain the proper ratio. Sometimes, drugs that suppress RBC production may be required.
Secondary absolute polycythemia requires the diagnosis and treatment of the underlying cause. Once that is resolved or controlled, the disorder may be eliminated. In some severe cases, phlebotomy may help return the dog to proper blood ratios.
The typical treatment for relative polycythemia is rehydration using subcutaneous or intravenous fluid therapy. Once fluid levels are restored, the underlying cause may be treated.
In most cases, once the underlying cause is treated, red blood cell count is normalized and plasma levels are restored, prognosis is quite good.
Pulmonary hypertension is an elevated blood pressure of the lung vasculature (the pulmonary vein, pulmonary artery, or pulmonary capillaries). It can lead to heart failure if left untreated.
By definition, any pulmonary arterial systolic pressure over 30 mm Hg, pulmonary arterial mean pressure over 20 mm Hg, or pulmonary arterial diastolic pressure over 15 mm Hg constitutes pulmonary hypertension.
In a healthy dog with normal pulmonary blood pressure, the pulmonary arteries have distensible walls and are elastic. In dogs with the disorder, the pulmonary arteries are thickened, hypertrophic. In some severe cases, the blood vessels may develop necrosis.
Dogs with pulmonary hypertension are divided into classes depending upon the severity of the signs ranging from functional class I (the dog shows no signs of the disease) to functional class IV (the dog is unable to perform physical activity without severe signs of pulmonary hypertension, and show symptoms even at rest). Dogs with the most severe form have right-sided heart failure as well.
Pulmonary hypertension is always secondary to an underlying cause. Anything that affects pulmonary arterial vasodilation or vasoconstriction, smooth muscle cell proliferation, blood flow, or platelet activation will increase pulmonary arterial pressure. Left atrium high blood pressure can lead to pulmonary hypertension. Underlying causes may be cancer, thrombosis, pneumonia, vascular blockage, bronchitis, adult respiratory distress syndrome, overactive adrenal glands, infection, obesity, congenital shunts, heartworm disease, inflammation of the pancreas, hypoxia, protein-losing nephropathy, high altitude disease, and paralysis.
The main cause of elevated pulmonary pressure is heart valve disease and the second main cause is heartworm infection.
Because of the wide variety of primary causes, there is no definitive way to prevent pulmonary hypertension.
There are several general signs a dog owner may notice in a dog suffering from elevated pulmonary pressure: trouble breathing, fatigue, coughing or vomiting blood, exercise intolerance, coughing, fainting, weight loss, and enlarged abdomen. It is important that any dog showing any of these symptoms be examined immediately, because untreated pulmonary hypertension leads to permanent damage and eventual death.
Dogs at Risk
Any dog may develop pulmonary hypertension as the result of an underlying disorder. Some underlying diseases may affect some breeds more than others. One of the main causes of pulmonary hypertension is degenerative mitral valve disease, which occurs most often in small breed dogs that are middle-aged and older. Cavalier King Charles Spaniels are prone to mitral valve disease, as are the Norwich Terrier, Norfolk Terrier, English Toy Spaniel, Chihuahua, and Saluki.
Dogs prone to heartworm infection are also at higher risk for pulmonary hypertension. This includes all working dogs, middle-aged male outdoor dogs, and all those dogs that live in endemic areas.
Testing and Diagnosis
A thorough physical exam and history, blood chemistry profile, heartworm antigen test, urinalysis, complete blood count, and arterial blood gas test should be performed. Any fluid in the lungs or abdomen should be drawn off for testing. The vet should listen for a split s2 sound, heart murmurs, and abnormal lung sounds.
Thoracic x-rays can show heart disease and/or heart and lung abnormalities. An electrocardiogram can test for pulmonary hypertension. An echocardiogram, currently considered the gold standard in diagnosis for this disorder, can indicate pulmonary blood clots and heart abnormalities that didn’t show on x-ray. ECG can also help to measure the pressure in the dog’s blood vessels when the heart contracts.
Right heart catheterization is another option to diagnose this disorder, but it is invasive and generally not done very often. It requires general anesthesia and is high-risk. It is also expensive.
When pulmonary hypertension is suspected, and then diagnosed, the first goal is to determine the underlying cause. Second, the degree of severity of the hypertension must be determined. The third goal of diagnosis is to assess for hemodynamic impairment, and the fourth goal is to decide upon the best treatment available.
Treatment and Prognosis
Treatment goals include decreasing clinical signs, decreasing hospitalization duration, increasing survival time, and improving quality of life.
Dogs with severe breathing problems will be hospitalized and placed on oxygen. Treatment for the underlying disease or disorder varies, and can bring about a reversal of the elevated pulmonary blood pressure.
Supportive therapy may include brochodilating medicines, chemotherapy, diuretics, corticosteroids, oxygen supplementation, heartworm therapy, and anti-inflammatory agents, depending on the primary disease. Dogs with functional class III through IV must be monitored to assess improvement or decline in function.
Prognosis depends entirely upon the underlying cause and the functional class at which the dog is diagnosed. In many cases, the underlying cause cannot be cured, only treated. This means that the goal is to make the patient more comfortable. Most dogs only live up to 91 days past diagnosis of functional class III and IV, if they receive immediate treatment. Some dogs may live up to two years past diagnosis, but this is rare.