Infective Endocarditis

Infective endocarditis (IE) is a serious, potentially life-threatening condition characterized by microbial infection and inflammation of the endocardial surface of the heart, most commonly affecting the heart valves. In veterinary medicine, IE is predominantly seen in dogs and occasionally in cats. The infection typically involves the mitral and aortic valves, leading to vegetative lesions that can cause valvular dysfunction, heart failure, and systemic embolic complications. The disease is classified as acute or subacute based on the rapidity of onset and the virulence of the causative organism. Acute IE is often caused by highly virulent bacteria such as Staphylococcus aureus and can lead to rapid valvular destruction and fulminant heart failure. Subacute IE is more insidious, often caused by less virulent organisms like Streptococcus spp., and may present with chronic signs such as fever, lethargy, and weight loss. The condition is a diagnostic challenge due to its nonspecific clinical signs and requires a high index of suspicion, especially in dogs with pre-existing cardiac disease or those with a history of bacteremia. Untreated IE carries a grave prognosis, with mortality rates exceeding 50% in some studies. Early diagnosis and aggressive antimicrobial therapy are critical for improving outcomes.

The most common causative agents of infective endocarditis in dogs are Gram-positive bacteria, particularly Staphylococcus spp. (especially Staphylococcus aureus and Staphylococcus pseudintermedius), Streptococcus spp. (including Streptococcus canis and Streptococcus bovis), and Enterococcus spp. Gram-negative bacteria such as Escherichia coli, Pasteurella multocida, and Bartonella spp. are less common but can also cause IE. In cats, Bartonella henselae and Bartonella clarridgeiae are notable causes, often associated with flea infestation. Fungal endocarditis is rare but can be caused by Aspergillus spp., Candida spp., and Histoplasma capsulatum, particularly in immunocompromised animals. The pathogenesis involves transient bacteremia, which may arise from dental procedures, skin infections, urinary tract infections, or indwelling catheters. Bacteria adhere to damaged or abnormal endothelium, often at sites of high-velocity blood flow, such as the valve leaflets. Virulence factors include adhesins, such as fibronectin-binding proteins, which facilitate attachment to endothelial cells and platelets. Once adhered, bacteria proliferate and form vegetations composed of fibrin, platelets, and microorganisms. The immune response, including the release of pro-inflammatory cytokines, contributes to tissue damage and the formation of systemic emboli.

Infective endocarditis is an uncommon but serious disease in companion animals, with an estimated prevalence of 0.06% to 0.16% in dogs presented to referral hospitals. It is more frequently diagnosed in medium to large breed dogs, with a median age of 6 to 8 years. Male dogs may be slightly overrepresented. Certain breeds, such as German Shepherds, Golden Retrievers, and Boxers, appear to have a higher risk, possibly due to genetic predispositions to valvular abnormalities or immune dysfunction. Cats are less commonly affected, but when they are, it is often associated with Bartonella infection. No significant geographic or seasonal patterns have been consistently identified, although flea-borne Bartonella transmission may increase risk in warmer climates. Underlying cardiac conditions, such as chronic valvular disease or congenital heart defects, predispose to IE by creating turbulent blood flow and endothelial injury. Immunosuppressive therapy, chronic corticosteroid use, and concurrent systemic infections increase susceptibility. The incidence of IE may be rising due to increased use of invasive procedures and antimicrobial resistance.

The pathophysiology of infective endocarditis involves a complex interplay between the microorganism, the host's immune system, and the cardiac endothelium. The initial step is endothelial injury, which can result from turbulent blood flow, catheter-induced trauma, or immune-mediated damage. This injury exposes subendothelial collagen and tissue factor, promoting platelet aggregation and fibrin deposition, forming a sterile thrombus. During transient bacteremia, circulating bacteria can adhere to this thrombus via surface adhesins. Once adherent, bacteria stimulate further platelet activation and fibrin deposition, creating a mature vegetation. The vegetation provides a protected environment for bacterial proliferation, shielding them from host immune defenses and antibiotics. As the infection progresses, the valve leaflets may become thickened, ulcerated, or perforated, leading to valvular insufficiency or stenosis. The resulting hemodynamic alterations can cause volume overload, chamber enlargement, and ultimately congestive heart failure. Systemic embolization of septic thrombi can occur, leading to infarction and abscess formation in organs such as the kidneys, spleen, brain, and lungs. Immune-mediated mechanisms, including the formation of circulating immune complexes, can cause glomerulonephritis, vasculitis, and polyarthritis. The systemic inflammatory response syndrome (SIRS) may develop, leading to fever, leukocytosis, and multi-organ dysfunction.

Several factors predispose animals to infective endocarditis. Pre-existing cardiac disease, such as chronic valvular degeneration (endocardiosis) or congenital defects (e.g., subaortic stenosis, patent ductus arteriosus), creates abnormal blood flow patterns that damage the endothelium and increase the risk of bacterial adherence. Immunosuppression, whether due to disease (e.g., diabetes mellitus, hyperadrenocorticism) or iatrogenic (e.g., corticosteroid or chemotherapy), impairs the host's ability to clear bacteremia. Indwelling intravenous catheters, urinary catheters, and surgical implants serve as portals for bacterial entry. Periodontal disease and dental procedures are well-documented sources of transient bacteremia. Skin infections, urinary tract infections, pneumonia, and gastrointestinal disease can also lead to bacteremia. In cats, flea infestation and exposure to Bartonella are significant risk factors. Age is a factor, as older animals are more likely to have degenerative valvular changes. Breed predispositions may reflect genetic differences in immune function or valve structure. Finally, the use of broad-spectrum antibiotics can disrupt normal flora and allow resistant organisms to flourish.

Clinical signs of infective endocarditis are often nonspecific and can be acute or chronic. In the acute form, animals may present with sudden onset of fever, lethargy, anorexia, and signs of septic shock, including tachycardia, tachypnea, and collapse. In subacute cases, signs may be more insidious, with intermittent fever, weight loss, and shifting lameness due to immune-mediated polyarthritis. Cardiac signs may include a new or changing heart murmur, most commonly a systolic murmur of mitral or aortic insufficiency, or a diastolic murmur if aortic regurgitation is present. Arrhythmias, such as atrial fibrillation or ventricular premature complexes, may occur. Signs of congestive heart failure, including cough, dyspnea, exercise intolerance, and ascites, can develop as valvular dysfunction progresses. Systemic embolic events can cause acute lameness (due to limb ischemia), renal infarction (hematuria, flank pain), cerebral infarction (seizures, altered mentation), or pulmonary embolism (acute dyspnea). Physical examination may reveal pale mucous membranes, prolonged capillary refill time, and weak or bounding pulses. Pets may exhibit signs of polyarthritis, such as joint swelling and pain. In cats, signs are often vague, including lethargy, anorexia, and fever, and may be associated with Bartonella infection.

Differential diagnoses for infective endocarditis include other causes of fever, heart murmur, and systemic illness. Key differentials include: 1) Chronic valvular disease (endocardiosis) – common in older small breed dogs, but typically lacks fever and systemic signs; echocardiography shows thickened valves without vegetations. 2) Myocarditis – inflammation of the myocardium, often viral or immune-mediated, may cause arrhythmias and heart failure, but no valvular vegetations. 3) Dilated cardiomyopathy – primary myocardial disease, characterized by chamber dilation and systolic dysfunction, without valvular lesions. 4) Heartworm disease – caused by Dirofilaria immitis, can cause pulmonary hypertension and right-sided heart failure, with eosinophilia and positive antigen test. 5) Sepsis or bacteremia from other sources – may cause fever and systemic signs, but echocardiography is normal. 6) Immune-mediated polyarthritis – presents with fever, lameness, and joint swelling, but lacks cardiac involvement. 7) Neoplasia, such as heart base tumors or hemangiosarcoma – may cause murmurs and arrhythmias, but imaging shows mass lesions. 8) Systemic fungal infections (e.g., histoplasmosis) – can cause fever and weight loss, with characteristic cytology or serology. 9) Hyperthyroidism in cats – can cause tachycardia and murmurs, but typically in older cats with elevated T4. 10) Anemia or polycythemia – can cause murmurs due to altered blood viscosity, but no vegetations. Definitive diagnosis relies on echocardiographic detection of vegetations and positive blood cultures.

The diagnostic approach to infective endocarditis should be systematic and thorough. Step 1: Obtain a detailed history and perform a complete physical examination, paying close attention to cardiac auscultation, pulse quality, and joint palpation. Step 2: If IE is suspected, perform baseline laboratory tests including complete blood count (CBC), serum biochemistry profile, and urinalysis. Look for leukocytosis, neutrophilia, anemia, thrombocytopenia, azotemia, and proteinuria. Step 3: Collect blood cultures – ideally three sets from different venipuncture sites, at least one hour apart, before starting antibiotics. Use aseptic technique and aerobic and anaerobic media. Step 4: Perform thoracic radiography to assess for cardiomegaly, pulmonary edema, or evidence of embolic pneumonia. Step 5: Echocardiography is the cornerstone of diagnosis. A transthoracic echocardiogram should be performed to identify vegetative lesions on valves, assess valvular regurgitation, and evaluate cardiac function. Transesophageal echocardiography may be needed in some cases for better visualization. Step 6: If Bartonella is suspected, consider serology or PCR on blood or tissue. Step 7: Additional imaging such as abdominal ultrasound or CT may be indicated to detect embolic complications (e.g., renal or splenic infarcts). Step 8: Apply the modified Duke criteria for diagnosis in humans, adapted for veterinary use, which include major criteria (positive blood culture, echocardiographic evidence of vegetation) and minor criteria (fever, predisposing heart condition, embolic phenomena, immunologic phenomena). A definitive diagnosis requires two major criteria, one major and three minor, or five minor criteria. Step 9: In cases where echocardiography is inconclusive but clinical suspicion remains high, consider repeat echocardiography in 7-10 days or advanced imaging. Step 10: Initiate empirical antimicrobial therapy after blood cultures are obtained, and adjust based on culture and sensitivity results.

Laboratory findings in infective endocarditis are variable but often include: Hematology: Leukocytosis with a left shift (neutrophilia) is common, but leukopenia may occur in severe sepsis. Anemia of chronic disease (normocytic, normochromic) is frequent. Thrombocytopenia may be present due to consumption or immune-mediated destruction. Serum Biochemistry: Hyperglobulinemia (especially beta and gamma fractions) due to chronic antigenic stimulation. Hypoalbuminemia may occur due to protein-losing nephropathy or hepatic involvement. Elevated liver enzymes (ALP, ALT) can result from hepatic congestion or sepsis. Azotemia (elevated BUN and creatinine) may indicate renal emboli or glomerulonephritis. Electrolyte imbalances, such as hyponatremia or hyperkalemia, may occur with renal dysfunction. Urinalysis: Proteinuria, hematuria, and casts may be present due to glomerulonephritis or renal infarction. Urine protein-to-creatinine ratio may be elevated. Blood Gas Analysis: Metabolic acidosis may be present in septic patients. Specific Biomarkers: Cardiac troponin I (cTnI) is often elevated, indicating myocardial damage. N-terminal pro-B-type natriuretic peptide (NT-proBNP) may be elevated in heart failure. C-reactive protein (CRP) is a sensitive marker of inflammation and is typically increased. Serology/PCR: Blood cultures are the gold standard for etiologic diagnosis. In suspected Bartonella, serology (IFA) or PCR on blood or tissue is recommended. Additional tests: Coagulation profile may show prolonged PT/PTT due to disseminated intravascular coagulation (DIC). Blood smears may reveal toxic neutrophils or organisms.

Imaging plays a crucial role in the diagnosis and management of infective endocarditis. Thoracic Radiography: May show cardiomegaly, especially left atrial and left ventricular enlargement, due to valvular regurgitation. Pulmonary edema or interstitial infiltrates may indicate congestive heart failure. In cases of septic emboli, nodular or alveolar patterns may be seen in the lungs. Echocardiography: Transthoracic echocardiography (TTE) is the primary imaging modality. Findings include: 1) Vegetations – echogenic masses attached to valve leaflets, typically on the atrial side of the mitral valve or the ventricular side of the aortic valve. They may be sessile or pedunculated and vary in size. 2) Valvular thickening and destruction – the valve leaflets may appear thickened, irregular, or flail. 3) Valvular regurgitation – color Doppler demonstrates regurgitant jets. 4) Chamber enlargement – left atrial and ventricular dilation due to volume overload. 5) Myocardial dysfunction – reduced ejection fraction may be seen in severe cases. Transesophageal echocardiography (TEE) provides higher resolution and is more sensitive for detecting small vegetations, especially on the aortic valve. It is particularly useful when TTE is inconclusive. Computed Tomography (CT): CT angiography can be used to evaluate for embolic complications, such as pulmonary embolism, renal infarction, or cerebral abscesses. It may also help assess the extent of valvular damage. Magnetic Resonance Imaging (MRI): MRI is useful for detecting cerebral embolic events or abscesses. It is rarely used in veterinary medicine due to cost and availability. Fluoroscopy: May be used to guide catheter-based interventions, but is not routinely employed. In summary, echocardiography is essential for diagnosis, while other imaging modalities are adjunctive for detecting complications.

Cytology and histopathology are important for confirming the diagnosis and identifying the causative organism. Fine Needle Aspiration (FNA): FNA of peripheral lymph nodes, spleen, or liver may reveal reactive hyperplasia or, in cases of fungal infection, organisms such as Histoplasma or Aspergillus. Joint fluid analysis in cases of polyarthritis may show neutrophilic inflammation. Blood smears may occasionally demonstrate bacteria, but this is rare. Histopathology: On post-mortem examination, the heart valves show vegetative lesions composed of fibrin, platelets, neutrophils, and bacterial colonies. The underlying valve tissue may show necrosis, ulceration, and neovascularization. In chronic cases, fibrosis and dystrophic calcification may be present. Special stains, such as Gram stain, can help identify Gram-positive or Gram-negative bacteria. Silver stains (e.g., Warthin-Starry) are used to detect Bartonella organisms. In cases of embolic complications, infarcts may be seen in the kidneys, spleen, or brain. Histopathology of the kidneys may reveal glomerulonephritis characterized by mesangial proliferation, immune complex deposition, and crescent formation. Immunohistochemistry can be used to identify specific pathogens. In summary, histopathology is definitive but is often performed post-mortem, while cytology may provide supportive evidence during life.

Treatment of infective endocarditis requires aggressive antimicrobial therapy, supportive care, and management of complications. The primary goals are to eradicate the infection, prevent embolic events, and manage heart failure. Antimicrobial Therapy: Empirical therapy should be initiated immediately after blood cultures are collected, using broad-spectrum bactericidal antibiotics. A common protocol is a combination of ampicillin (22 mg/kg IV q8h) and enrofloxacin (10 mg/kg IV or PO q24h) or amikacin (15-20 mg/kg IV q24h) to cover Gram-positive and Gram-negative organisms. Once culture and sensitivity results are available, therapy should be adjusted to the most appropriate antibiotic. For susceptible Staphylococcus or Streptococcus, a beta-lactam such as ampicillin or amoxicillin-clavulanate (20 mg/kg PO q8h) may be used. For methicillin-resistant Staphylococcus, vancomycin (15 mg/kg IV q8h) or linezolid (10 mg/kg PO q12h) may be considered, though these are not commonly used in veterinary medicine. For Bartonella, doxycycline (5 mg/kg PO q12h) or azithromycin (10 mg/kg PO q24h) is recommended. Antimicrobial therapy should be continued for at least 4-6 weeks, often longer. Supportive Care: Hospitalization is usually required initially. Intravenous fluids may be needed for hydration, but caution is advised in animals with heart failure. Oxygen therapy is indicated for dyspnea. Antiemetics and appetite stimulants may be used if nausea or anorexia is present. Management of Heart Failure: If congestive heart failure develops, standard therapy includes furosemide (2-4 mg/kg IV or PO q8-12h), pimobendan (0.3 mg/kg PO q12h), and an ACE inhibitor such as enalapril (0.5 mg/kg PO q12h). Antiarrhythmic drugs may be needed for significant arrhythmias. Embolic Complications: Anticoagulant therapy is not routinely recommended in veterinary patients due to bleeding risks. Surgical Intervention: In severe cases with refractory heart failure or persistent infection, surgical valve replacement or debridement may be considered, but this is rarely performed in veterinary medicine. Nutritional Support: A high-quality, easily digestible diet is recommended. Omega-3 fatty acids may have anti-inflammatory benefits. Physical Rehabilitation: Gentle exercise is encouraged, but strict rest is advised during the acute phase.

The prognosis for infective endocarditis is guarded to poor. Mortality rates range from 50% to 80% in dogs, despite appropriate therapy. Factors associated with a worse prognosis include: 1) Infection with Staphylococcus aureus or Gram-negative organisms. 2) Presence of congestive heart failure at diagnosis. 3) Aortic valve involvement, which is more likely to cause severe regurgitation and heart failure. 4) Large vegetations (>1 cm) on echocardiography. 5) Embolic complications, such as renal failure or cerebral infarction. 6) Delayed diagnosis and initiation of treatment. 7) Underlying immunosuppression. Short-term survival is possible if the infection is controlled and heart failure is managed, but long-term survival is rare. Some animals may survive for months to years with chronic antimicrobial therapy and cardiac medications, but recurrence is possible. Serial blood cultures and echocardiography are recommended to monitor response to therapy. Negative blood cultures after 48-72 hours of appropriate antibiotics are a favorable sign. However, even with successful treatment, valvular damage may progress, leading to chronic heart failure. Owners should be counseled about the guarded prognosis and the potential for significant financial and emotional investment.

Follow-up care for animals with infective endocarditis is intensive and long-term. During the initial hospitalization, blood cultures should be repeated every 48-72 hours until negative. Clinical parameters such as temperature, heart rate, respiratory rate, and body weight should be monitored daily. Echocardiography should be repeated at 1 week, 1 month, and then every 3-6 months to assess vegetation size and valvular function. Serum cardiac troponin I and NT-proBNP can be measured serially to monitor myocardial damage and heart failure status. After discharge, recheck appointments should be scheduled at 2 weeks, 1 month, 2 months, and then every 3-6 months. At each visit, a thorough physical examination, CBC, serum biochemistry, and urinalysis should be performed. Blood cultures should be repeated if fever or other signs of infection recur. Antimicrobial therapy should be continued for at least 4-6 weeks, and the animal should be monitored for adverse drug effects, such as gastrointestinal upset or renal toxicity. If the animal is on long-term antibiotics, periodic monitoring of renal and hepatic function is advised. Heart failure medications may need dose adjustments based on clinical status and renal function. Owners should be educated to monitor for signs of recurrence, such as lethargy, fever, or lameness, and to seek immediate veterinary care if these occur. Dental prophylaxis and other invasive procedures should be avoided or performed with antibiotic prophylaxis to prevent bacteremia. Long-term management may include a cardiac diet, exercise restriction, and regular monitoring of blood pressure and renal function.

Pearls: 1) Maintain a high index of suspicion for infective endocarditis in any dog with a new or changing heart murmur, especially if accompanied by fever or systemic signs. 2) Blood cultures should be obtained before starting antibiotics, and multiple sets increase the yield. 3) Echocardiography is the most valuable diagnostic tool; look for vegetations on the aortic and mitral valves. 4) Bartonella should be considered in cats with unexplained fever and cardiac signs, especially if they have flea exposure. 5) Early and aggressive antimicrobial therapy is crucial; use bactericidal antibiotics and continue for at least 4-6 weeks. 6) Monitor for embolic complications, such as acute lameness or renal failure. 7) Serial echocardiography is essential to assess response to therapy. Pitfalls: 1) Do not delay antibiotic therapy while waiting for culture results; empirical therapy should be started immediately after cultures are taken. 2) Avoid using bacteriostatic antibiotics (e.g., tetracyclines) as monotherapy for severe infections. 3) Do not rely solely on clinical signs; many animals do not have a fever. 4) Be cautious with fluid therapy in animals with heart failure; overhydration can worsen pulmonary edema. 5) Do not discontinue antibiotics prematurely; relapse is common. 6) Do not ignore the possibility of fungal endocarditis, especially in immunocompromised animals. 7) Remember that negative blood cultures do not rule out infective endocarditis, especially if antibiotics were given prior to sampling. 8) Do not forget to evaluate for underlying predisposing conditions, such as dental disease or immunosuppression.

Based on Plumb's Veterinary Drug Handbook, the following antimicrobial protocols are recommended for infective endocarditis in dogs and cats. Empirical therapy (before culture results): Ampicillin 22 mg/kg IV q8h PLUS Enrofloxacin 10 mg/kg IV or PO q24h (dogs) or 5 mg/kg q24h (cats). Alternatively, Ampicillin 22 mg/kg IV q8h PLUS Amikacin 15-20 mg/kg IV q24h (dogs) with careful monitoring of renal function. For confirmed Gram-positive infections (Staphylococcus, Streptococcus): Ampicillin 22 mg/kg IV or PO q8h, or Amoxicillin-clavulanate 20 mg/kg PO q8h. For methicillin-resistant Staphylococcus: Vancomycin 15 mg/kg IV q8h (slow infusion over 60 min) or Linezolid 10 mg/kg PO q12h (dogs). For Gram-negative infections: Enrofloxacin 10 mg/kg PO or IV q24h (dogs) or 5 mg/kg q24h (cats), or Cefotaxime 25-50 mg/kg IV q8h. For Bartonella: Doxycycline 5 mg/kg PO q12h for 4-6 weeks, or Azithromycin 10 mg/kg PO q24h for 3-5 days, then every other day. For fungal endocarditis: Fluconazole 5-10 mg/kg PO q12h or Amphotericin B 0.5-0.8 mg/kg IV three times weekly, with monitoring for nephrotoxicity. Supportive medications: Furosemide 2-4 mg/kg IV or PO q8-12h for heart failure; Pimobendan 0.3 mg/kg PO q12h; Enalapril 0.5 mg/kg PO q12h; Digoxin 0.005-0.01 mg/kg PO q12h (dogs) for atrial fibrillation. Antiarrhythmics: Lidocaine 2 mg/kg IV bolus then 50-80 mcg/kg/min CRI for ventricular arrhythmias; Sotalol 1-2 mg/kg PO q12h. Analgesics: For pain, consider opioids such as buprenorphine 0.01-0.02 mg/kg IV q8-12h. Always adjust dosages for renal or hepatic impairment and monitor for drug interactions.

Infective endocarditis in dogs and cats has been the subject of several retrospective studies and case series. A landmark study by Calvert et al. (1985) described the clinical and pathological features of bacterial endocarditis in dogs, highlighting the prevalence of Staphylococcus aureus and Streptococcus spp. and the poor prognosis. More recent studies, such as that by Peddle et al. (2009), evaluated the utility of echocardiography in diagnosing IE and found that vegetations were most commonly located on the aortic and mitral valves. The modified Duke criteria have been adapted for veterinary use, as proposed by MacDonald et al. (2003), to improve diagnostic accuracy. Regarding treatment, a study by Sykes et al. (2006) reported successful outcomes with long-term antimicrobial therapy, but emphasized the need for early intervention. The ACVIM consensus statement on infective endocarditis (2015) provides evidence-based guidelines for diagnosis and management, recommending blood cultures and echocardiography as essential diagnostic tools, and prolonged antibiotic therapy (4-6 weeks) with bactericidal drugs. Bartonella-associated endocarditis in cats has been documented in several case reports, and doxycycline or azithromycin is recommended. Overall, the literature underscores the high mortality rate and the importance of a high index of suspicion and prompt, aggressive therapy.