Circulatory System

Blood supplies cells with water, electrolytes, nutrients, and hormones and removes waste products. The cellular elements supply oxygen (RBCs), protect against foreign organisms and antigens (WBCs), and initiate coagulation (platelets). Because of the diversity of the hematopoietic system, its diseases are best discussed from a functional perspective. Function may be classified as either normal responses to abnormal situations (eg, leukocytosis and left shift in response to inflammation) or primary abnormalities of the hematopoietic system (eg, pancytopenia from marrow failure). Furthermore, abnormalities may be quantitative (ie, too many or too few cells) or qualitative (ie, abnormalities in function). (Also see The Biology of the Immune System.)

Anemia is an absolute decrease in RBC numbers, hemoglobin concentration, or PCV. Signs include pale mucous membranes, increased heart rate, and hypotension. Diagnosis can be made by CBC, but may be refined by additional tests. Treatment can be symptomatic, but often requires addressing the underlying cause.

Blood groups are determined by genetically controlled, polymorphic, antigenic components of the RBC membrane. The allelic products of a particular genetic locus are classified as a blood group system. Some of these systems are highly complex, with many alleles defined at a locus; others consist of a single defined antigen. Blood group systems, in general, are independent of each other, and their inheritance conforms to Mendelian dominance. For polymorphic blood group systems, an animal inherits one allele from each parent and thus expresses no more than two blood group antigens of a system. An exception is in cattle, in which multiple alleles are inherited.

Anaplasmosis, formerly known as gall sickness, traditionally refers to a disease of ruminants caused by obligate intraerythrocytic bacteria of the order Rickettsiales, family Anaplasmataceae, genus Anaplasma. Cattle, sheep, goats, buffalo, and some wild ruminants can be infected with the erythrocytic Anaplasma. Anaplasmosis occurs in tropical and subtropical regions worldwide (~40°N to 32°S), including South and Central America, the USA, southern Europe, Africa, Asia, and Australia.

Canine lymphoma is a cancer of malignant lymphocytes. Derived from the hematopoietic system, canine lymphoma can involve any anatomic site in which lymphocytes reside and/or traffic. Dogs with lymphoma most commonly present with enlarged, nonpainful, generalized lymphadenopathy, and when tumor burden is significant, clinical signs including lethargy, weight loss, and constitutional decline are frequently displayed. A diagnosis of lymphoma is readily achieved through either fine-needle aspirate cytology or lymph node biopsy with histopathologic evaluation. Infrequently, when a definitive diagnosis remains elusive, additional molecular techniques including flow cytometry and PCR for antigen receptor rearrangements (PARR) can provide ancillary information to aid in definitive diagnosis. Effective treatment of canine lymphoma most often requires the institution of systemic chemotherapy, with the majority of dogs receiving systemic cytotoxic strategies achieving objective responses with correlative improvements in quality of life and extension in survival times.

Effective hemostasis depends on an adequate number of functional platelets, an adequate concentration and activity of plasma coagulation and fibrinolytic proteins, and a normally responsive blood vasculature. The diagnosis, treatment, and monitoring of hypo- and hypercoagulable animals is difficult with regard to both progression of disease and monitoring blood component and/or anticoagulation therapy. Citrated plasma samples are often used in veterinary medicine to determine fibrinogen concentration, activated partial thromboplastin time (APTT), prothrombin time (PT), and d-dimer or fibrinogen degradation product (FDP) concentration.

Leukocytes, or WBCs, in the blood of healthy mammals include segmented neutrophils, band neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Abnormal leukocytes include neutrophils less mature than bands (eg, metamyelocytes, myelocytes, progranulocytes), blast cells of any hematopoietic lineage, mast cells, and other neoplastic cells. WBCs vary in their site of production, their duration of circulation, and the stimuli that affect their release into and migration out of the vascular system. Differences in leukocyte physiology account for species differences in normal blood cell concentrations and their responses in disease.

Caseous lymphadenitis (CL) is a chronic, contagious bacterial disease that manifests clinically as abscesses of peripheral and/or internal lymph nodes and organs. The characteristic purulent material is very thick and nonodorous. Whereas the peripheral form presents as abscesses of single or multiple peripheral palpable lymph nodes, internal CL typically manifests as chronic weight loss and ill thrift. Culture of active lesions for Corynebacterium pseudotuberculosis is diagnostically definitive. When eliminating animals from the herd/flock is undesirable, treatment consists of consistent, sustained antimicrobial therapy to reduce the numbers of active draining lesions and isolation from other herd mates until lesions are dry and/or resolved.

The cardiovascular system comprises the heart, veins, arteries, and capillary beds. The atrioventricular (mitral and tricuspid) and semilunar (aortic and pulmonic) valves keep blood flowing in one direction through the heart, and valves in large veins keep blood flowing back toward the heart. The rate and force of contraction of the heart and the degree of constriction or dilatation of blood vessels are determined by the autonomic nervous system (sympathetic and parasympathetic) and hormones produced either by the heart and blood vessels (ie, paracrine or autocrine) or at a distance from the heart and blood vessels (ie, endocrine).

Congenital anomalies of the cardiovascular system are defects that are present at birth and can occur as a result of genetic, environmental, infectious, toxicologic, pharmaceutical, nutritional, or other factors or a combination of factors. For several defects, an inherited basis is suspected based on breed predilections and breeding studies. Congenital heart defects are significant not only for the effects they produce but also for their potential to be transmitted to offspring through breeding and thus affect an entire breeding population. In addition to congenital heart defects, many other cardiovascular disorders have been shown, or are suspected, to have a genetic basis. Diseases such as hypertrophic cardiomyopathy, dilated cardiomyopathy, and degenerative valvular disease of small breeds of dogs may have a significant heritable component.

Heart disease is defined as any functional, structural, or electrical abnormality of the heart. It encompasses a wide range of abnormalities, including congenital abnormalities (see Congenital and Inherited Anomalies of the Cardiovascular System) as well as anatomic and physiologic disorders of varying causes. Heart disease can be classified by various characteristics, including whether the disease was present at birth or not (eg, congenital or acquired), cause (eg, infectious, degenerative, genetic or heritable), duration (eg, chronic or acute), clinical status (eg, left heart failure, right heart failure, or biventricular failure), by anatomic malformation (eg, ventricular septal defect), or by electrical disturbance (eg, atrial fibrillation, ventricular premature complexes).

Heartworm disease (dirofilarosis), produced by Dirofilaria immitis, primarily affects the pulmonary arteries, producing inflammation, vascular dysfunction, and pulmonary hypertension. Secondarily, this results in right-side cardiac pressure overload and progressive pulmonary insufficiency. Tertiary problems include aberrant worm migration, caval syndrome, coagulopathies (DIC), and heart failure. Most infected dogs are asymptomatic, but the most common clinical signs are weight loss, exercise intolerance, cough, and labored breathing. The most important therapy is prevention, which is done with macrocyclic lactones. Clearing a heartworm infection typically requires an organic arsenic compound, melarsomine, administered intramuscularly in three injections. Chronic use of certain macrocyclic lactones with doxycycline has also been effective when melarsomine cannot be used or is unavailable.

Bovine high-mountain disease is a condition that occurs in cattle pastured at high altitude. Low oxygen saturation in the air leads to pulmonary hypertension, with subsequent edema in ventral tissues of the chest and abdomen. Treatment typically requires movement to lower elevation, and prevention through selective breeding for resistant stock is often preferred.

Thrombosis (clot formation within a blood vessel), embolism (process by which unattached material (emboli) such as a blood clot, fat or cholesterol deposit, gas, tissue, or foreign material travels within the bloodstream and occludes flow within a vessel), and aneurysm (dilation or outpouching of a blood vessel wall) are pathologic abnormalities that can occur within the vasculature. Clinical signs, diagnostic testing, and therapy vary based on the classification, location, severity, chronicity, and associated or concurrent comorbidities. Typically, mainstays of treatment include addressing the underlying disease process and antithrombotic drugs.