Wall-July PV-Review - VetLearn.com

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Compendium August 2002 Infective Endocarditis 617 IE is rare in dogs. 2,4–6,12,14,27,28 More than 50% of infections in dogs are the result of coagulase-positive staphylococci and streptococci. 2,6,10–12,29,30 Bartonella, Brucella, Coxiella, and Chlamydia have been identified as causative agents in culture-negative IE cases in humans. 31–34 Factors predisposing humans to fungal infections (e.g., prosthetic valve replacement, IV drug abuse, chronic immunosuppressive therapy, chronic broad-spectrum antibiotic usage 8,27 ) are less common in dogs. PATHOPHYSIOLOGY Aside from the constitutional signs of infection, which are mediated by cytokines, the clinical manifestations of IE are the result of valvular destruction, embolic phenomena, and immune-complex disease. 3,6–8 Congestive heart failure (CHF) is the usual outcome in dogs with aortic valve IE. 2,4,6,9–12 Embolization leads to organ infarction and metastatic infection. 6–9,24 Embolization is a frequent cause of clinical signs and clinical deterioration. Even in the absence of clinical evidence, dogs almost always have systemic emboli at necropsy, and the kidneys are almost always infarcted. 9–11 Emboli are usually present in many organs and tissues. 2,4,6,10,12 Persistent bacteremia stimulates both cell-mediated and humeral immune systems. 7–10 Circulating immune complexes containing IgG, IgA, IgM, and complement may be deposited subendothelially along glomerular basement membranes, in joint capsules, and in blood vessels, leading to glomerulonephritis, arthritis, and vasculitis. 3–5,7,8,10,35–40 Clinical signs of IE frequently mimic those of immune-mediated disorders, and positive immune-test results (e.g., Coombs test, antinuclear antibody test) can occur because of antigen–antibody–complement interaction. 6,35 Rheumatoid factor, antinuclear Clinical Findings That May Be Associated with Infective Endocarditis Heart murmur • Recent onset • Diastolic component Fever Inflammatory leukogram Systemic embolic phenomena • Saddle thrombus • Limb edema • Lameness Arthropathy • Joint swelling • Pain • Lameness www.VetLearn.com antibodies, and circulating immune complexes contribute to the development of polyarthritis, renal disease, and myocarditis. 6–9,35–40 CLINICAL FINDINGS Clinical signs and diagnostic abnormalities associated with IE are often nonspecific and often include extracardiac manifestations. The interval between the initiating bacteremic event and diagnosis of IE is usually uncertain, and a predisposing factor is not always evident. 6 Occasionally, there is a history of an illness that, in retrospect, may have initiated bacteremia 1 to 3 months before the diagnosis of IE. 6 Affected dogs often have signs of systemic infection, including fever and leukocytosis. Septic or sterile embolization/infarction, metastatic infections, and organ failure are responsible for many of the clinical signs. 6–8,12 Clinical signs associated with IE are diverse and misleading, and IE has frequently been called “the great imitator.” 10 Owners variably report lethargy, depression, weakness, anorexia, weight loss, lameness, neurologic signs, and signs consistent with CHF. 2–4,6,12,19 Fever, either current or a history thereof, is not always present and may be easily overlooked. 3,6,12 Fever is often absent at the time of diagnosis because the bacteremia is low grade or there was previous therapy with antibiotics, steroids, or NSAIDs. 6,12 Heart Murmurs Cardiac sequelae of IE include valvular insufficiency because infection destroys the valvular tissue and interferes with function. 7–10 The severity of destruction varies with chronicity and the virulence of the organism. 7–10 IE is the most common cause of a murmur of aortic regurgitation (the aortic valve is the most commonly infected 2,6 ), which can be detected over the left heart base or on the right thoracic wall midway between the tricuspid valve area and the manubrium. Systolic murmurs result from mitral regurgitation or aortic valve vegetation–generated systolic flow turbulence. A murmur of recent onset, especially with a diastolic component, accompanied with fever is very suggestive of IE. 2,6 The femoral pulses associated with mitral valve endocarditis are typically normal but may be weak if left-sided CHF is present. 2–4,6 The femoral pulses associated with aortic valve endocarditis may be normal or hyperdynamic and bounding. 2 The systolic pulse pressure with aortic regurgitation is often increased because of volume overload and resultant increased stroke volume. Diastolic pressure may be low because of arterial blood runoff back into the left ventricle during diastole; thus not only may the diastolic pressure be low, but the systolic pressure wave duration may be shortened.
618 Small Animal/Exotics Compendium August 2002 Lameness Lameness due to IE is an occasional finding rarely caused by septic emboli but usually the result of immune-mediated arthritis. 3,4,6,9,35,36,39 Synovial tissue may contain IgG, IgM, and complement, indicating an immune basis for the arthropathy. 35,36 Organ Failure Dogs with IE may present with signs consistent with CHF (usually left-sided), including coughing, dyspnea, pulmonary crackles (pulmonary edema), shortness of breath, weakness, and/or collapse. 3,6 CHF is most frequently associated with aortic valve IE. When the left heart appears radiographically normal in size, but pulmonary edema is present, IE should be considered because dramatic heart enlargement may not yet have occurred in the case of acute IE. The index of suspicion of IE rises in dogs that have no history of a heart murmur or are not prone to cardiomyopathy or myxomatous valvular degeneration. 6 Renal failure is another complication that may detract from recognizing IE. Renal failure is usually due to chronic infarction or glomerulonephritis (Figure 2). 3,6,13,37,38 Polysystemic Signs The constitutional signs of IE can mimic those of any polysystemic disease. 3,6 Many diagnostic differentials must be considered, including immune-mediated diseases, rickettsial infections, bacteremia alone, sequelae of neoplasia, causes of CHF, and causes of renal failure. 2,6,12,13,33,35,36 Localized signs, such as lameness, edema, and pain, often arise from complications of Figure 2—Gross pathology kidney specimen from a dog with aortic valve IE. The kidney has been sectioned in the sagittal plane. Multiple triangular-shaped, pale infarctions of variable age are visible within the renal parenchyma. www.VetLearn.com infection. 6,35,36,39 In order to avoid overlooking IE, the possibility should be investigated when one or more of the following signs are present: • An unexplained fever • A heart murmur of recent onset • A source of infection • Embolic phenomena Systemic Embolization IE always results in embolization. The kidneys consistently exhibit old and new infarcts that contribute to proteinuria and renal failure. The spleen, liver, and iliac or mesenteric arteries are sometimes involved. Splenic septic abscesses may contribute to persistent fever and leukocytosis. CLINICAL PATHOLOGY Hematologic findings of systemic inflammation are often absent in IE. This may be due to antibiotic or steroid administration or chronic, low-grade bacteremia. 6,12 Although an inflammatory leukogram is not always present, monocytosis is present in 90% of cases. 6,12 In some instances, mature neutrophilia and monocytosis indicate low-grade, chronic inflammation. Thrombocytopenia is another common finding and can be attributed to systemic vasculitis and immune or coagulatory consumption. 6,12 A variety of serum biochemical abnormalities may be observed. A septic triad may be present, characterized by low or decreasing albumin, increased serum alkaline phosphatase activity, and absolute or relative hypoglycemia. 6,13 Hypoalbuminemia may be explained by proteinuria, anorexia, or decreased liver function caused by toxins. 6,13 Increased serum alkaline phosphatase activity is usually due to toxin interference with bile metabolism or prior steroid use. 6,13,14 Hypoglycemia can be explained by bacterial toxin effects on intermediary metabolism. 13,14 Azotemia may be prerenal or caused by infarction and glomerulopathy. 3,9,10,37,38 The urine often contains protein, blood, casts, and/or bacteria. 6,13 Urine culture is recommended not only because bacteria are filtered from the blood into the urine but also because the urinary tract may be a source of infection. 6,13 SPECIAL DIAGNOSTICS The diagnosis of IE requires integration of clinical and laboratory findings. Arrhythmias (usually atrial or ventricular premature contractions) are present in at least 50% to 75% of documented cases; seldom life threatening; and best evaluated by long-term, continuous electrocardiographic monitoring (either cage-side
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