ERYTHROCYTE DISORDERS

Erythrocyte disorders are frequently diagnosed in the dog and cat and may be associated with decreased production, increased destruction, or inappropriate loss of red blood cells (hemorrhage). Included in this category of disorders are anemias, hemorrhage, and neoplasia.

Anemia is one of the most common laboratory findings encountered in veterinary medicine and is usually secondary to a primary disorder elsewhere in the body. The major causes of anemia are varied and include hemorrhage, hemolysis, blood parasites, iron deficiencies, immune-mediated disease, and toxins.

A systematic diagnostic approach to anemia is necessary and should include a good history, physical examination, and a complete blood count, including blood films. Treatment should be aimed at correcting the primary disorder and supporting the patient. Therefore it is important to establish whether the anemia is regenerative or nonregenerative. This can be done by evaluating the reticulocyte count. Regenerative anemias are usually the result of hemorrhage or hemolysis, whereas nonregenerative cases may involve the bone marrow.

Anemia Caused by Hemorrhage

The most common cause of hemorrhage is trauma, although platelet abnormalities and abnormal clotting chemistries must be considered when determining the diagnosis. Acute hemorrhage that occurs as a result of trauma or laceration is usually an easily diagnosable problem. With acute blood loss internally the hematocrit does not reflect the severity of the problem, and as fluid shifts occur to compensate for blood loss, shock may result. Treatment should consist of controlling the hemorrhage and volume replacement.

Thrombocytopenia accounts for many cases of generalized bleeding in pet animals. In these cases, it may be more difficult for the veterinarian to diagnose blood loss. Signs of platelet deficiency include petechial hemorrhages on earflaps, mucous membranes, and on nonhaired areas such as the abdomen. Treatment involves steroid therapy, platelet-rich or whole-blood transfusions, and avoidance of trauma.

Iron-Deficiency Anemia

Dogs experiencing chronic external blood loss can experience development of iron-deficiency anemia. Severe flea infestation, gastrointestinal parasites, gastric ulceration, and bleeding neoplasms can cause significant blood loss over time. The iron and hemoglobin lost with this external bleeding result in the formation of altered red blood cells with decreased life spans. Treatment consists of correcting the cause of the blood loss and supplementing iron orally for 30 to 60 days.

Hemolysis

When immune components attach directly or indirectly to the red blood cell membrane, they alter its structure. The body, in an attempt to regain homeostasis, begins to remove these altered cells. Macrophages interact with the altered cells, resulting in extravascular hemolysis. This disease, when seen in the dog, appears to be related to the presence of an underlying inflammatory process. Affected animals acutely develop exercise intolerance, pale mucous membranes, tachycardia, and icterus if the condition is severe. In cats, the most common cause of hemolytic anemia is haemobartonellosis. Chronic infections with feline leukemia virus (FeLV) may also stimulate immunohemolytic disease in the cat.

Treatment is aimed at suppressing the immune system (steroid therapy) and supportive therapy. Transfusion should be considered if the hematocrit of the cat declines to life-threatening levels. Tetracycline should be used to treat cats with haemobartonellosis.

A special form of immune-mediated hemolytic disease is seen in neonates. This occurs in horses and, rarely, in cats and dogs. The dam passes antibodies against fetal red blood cells in her colostrum. The neonate’s red blood cells are attacked and lysed because they are coated with these antibodies. This problem can be avoided by blood-typing breeding animals and by fostering young born to incompatible dams.

Blood-Borne Parasites

Several commonly seen blood parasitic diseases produce anemia through hemolysis. Mycoplasma hemofelis is a common cause of anemia in cats. The parasite attaches to the erythrocyte membrane, causing increased destruction of the cells. Animals that have nonspecific signs of weight loss, anorexia, fever, hepatomegaly, and splenomegaly should have blood films examined for the presence of this microorganism. Some of these animals may be icteric on examination.

Babesia canis and Babesia gibsoni both produce hemolytic disease in the dog (Fig. 5-1). The brown dog tick Rhipicephalus sanguineus transmits these parasites. The presence of this intracellular parasite results in hemolysis of the infected red blood cells. Diagnosis is accomplished by finding the intracellular organism on blood films or by serology testing. Symptoms exhibited in the dog include hemoglobinuria, dehydration, fever, anorexia, and depression. Treatment involves tetracycline administration (for M. hemofelis) and supportive care.