Infective Endocarditis

11 Infective Endocarditis



Clay A. Calvert, Michelle Wall



1. What is infective endocarditis?

Infective endocarditis (IE) is the infection of the endocardial surface of the heart by microbes. In more than 90% of human patients IE is the result of bacterial colonization of the heart valves and adjacent endocardium with associated tissue destruction.


2. Which valves are most often infected?

The valves of the high-pressure left ventricle, the aortic and mitral, are most often involved. Valve involvement is directly related to the force of blood on each valve.


• The aortic valve is the single most commonly affected valve, and in some cases both the aortic and mitral valves are infected. Approximately 90% of infections involve the aortic valve.

• The pulmonic and tricuspid valves are seldom infected.

3. Why are the heart valves almost exclusively infected rather than other endocardial surfaces?

The valves are naturally exposed to trauma during each cardiac cycle as they open and close, and this results in microscopic damage to the endothelium.


4. What is the incidence of IE in dogs and cats?

Infective endocarditis is rarely diagnosed in cats. Infective endocarditis is diagnosed in 5 to 10 dogs annually at some teaching hospitals. Bacteremia alone is more common than IE. The prevalence of IE is much lower than that of dilated cardiomyopathy and myxomatous mitral valve degeneration.


5. Are certain types of dogs more likely to acquire IE than others?

In general, bacteremia is more common in tropical and semitropical regions and in medium to large dogs. Some reports cite a 2:1 male preponderance, possibly because of chronic bacterial prostatitis. German Shepherds, Boxers, Golden Retrievers, Labrador Retrievers, and Rottweilers comprise a substantial proportion of affected dogs.


6. Have any predisposing factors been identified?

Procedures involving the gingiva, oropharynx, gastrointestinal tract, or infected tissues can cause bacteremia. Chronic infections, including pyodermas, abscesses, foreign body infections, bacterial gingivitis and stomatitis, perianal infections, prior surgery (especially extensive or protracted procedures), bone infections, and bacterial prostatitis, have been associated with IE.


7. Do congenital heart defects sometimes lead to IE?

Infective endocarditis is seldom associated with congenital heart disease in dogs, with the exception of subaortic stenosis (SAS). With SAS, turbulent, high-velocity blood flow in the left heart produces microtrauma on the valve leaflets and facilitates microbe colonization. Boxers, Rottweilers, and Golden Retrievers have relatively high incidences of both IE and SAS. No more than 10% of dogs with SAS have IE, and no more than 10% of dogs with bacterial endocarditis have SAS.


8. Does immunosuppression predispose to IE?

The use of corticosteroids has been associated with IE. Debilitation from cancer chemotherapy or any abnormality that impairs host defenses may predispose to IE.


9. Other than corticosteroid use, can veterinary procedures or therapy cause IE?

Bacteremia can develop as a result of indwelling intravenous and urinary catheters or any procedure involving the gingiva, oropharynx, or gastrointestinal tract. Furthermore, manipulation of infected tissue, causing increased interstitial pressure, can force bacteria into veins and lymphatic channels.


• The body’s defense mechanisms (macrophages in the liver and spleen and spleen and capillary neutrophils) are generally able to eradicate transient bacteremia.

• Debilitated and immunosuppressed animals may have persistent bacteremia, and some of these will have IE.

10. Should prophylactic antibiotic therapy be administered to dogs undergoing invasive procedures, dentistry, or surgery?

There is little evidence to support a need for such an approach in normal dogs. Maintenance of oral hygiene is important. Dogs with congenital subaortic stenosis should receive antibiotics during dental treatment.


• Ampicillin, penicillin, and amoxicillin are not good choices because coagulase-positive staphylococci, the most frequent pathogens in cases of IE, are usually resistant to these agents.

• A first-generation cephalosporin, ticarcillin, or ticarcillin-clavulanate are better choices. One dose should be given during the procedure; therapy should not be initiated several days before the procedure.

11. What are the most common bacteria involved in bacterial endocarditis?

Coagulase-positive staphylococci and beta-hemolytic streptococci are most often isolated from blood or heart valves in cases of bacterial endocarditis. However, many different bacteria have been isolated, including E. coli and Pseudomonas spp. Recently Bartonella spp. have been implicated as a common cause of IE in dogs.


12. Is endocarditis ever caused by fungi?

Because of less frequent and less aggressive immunosuppressive treatments and the absence of prosthetic heart valve implantation in dogs as compared to human patients, the risk of fungal endocarditis in dogs is zero or nearly zero.


13. Why use the term IE if only bacteria are involved?

Occasionally IE is identified by echocardiography and blood cultures are negative. Results of polymerase chain reaction (PCR) analysis of blood cultures for Bartonella spp. have been positive in some of these dogs. Bartonella, Brucella, Coxiella, and Chlamydia may be causative agents in culture-negative cases.


14. Does preexisting heart disease predispose to IE?

Most dogs confirmed to have IE do not have known prior heart disease.


• Other than subaortic stenosis, neither congenital heart defects nor myxomatous degeneration of the mitral valve are major risk factors. In fact, breeds with the highest incidence of myxomatous mitral valve degeneration have a very low incidence of IE.

• Some bacteria, including some pathogenic staphylococci and streptococci, produce proteases that damage the endothelium and promote formation of platelet-fibrin meshes and deposition of red blood cells, predisposing to bacterial colonization of heart valves.

15. How can bacteremia be differentiated from IE?

Positive blood culture results, preferably more than one, are required for the diagnosis of bacteremia. Also, clinical signs and laboratory data consistent with bacteremia are important. Infective endocarditis is distinguished from bacteremia alone by the presence of endocardial bacterial colonization, which can be seen on echocardiography.


• A recent-onset cardiac murmur, especially if there is a diastolic component, indicates IE.

• Echocardiography greatly facilitates the diagnosis of IE.
• An oscillating cardiac mass is typical of IE.

• Aortic valve IE is easily identified.

16. Isn’t it true that blood cultures are often unrewarding?

Blood cultures do lack sensitivity and specificity, but they should always be performed when an infective organism is suspected. The most common causes for negative blood culture results are the absence of bacteremia and inadequate sampling.


• Excluding the possibility of contamination, if one performs blood cultures on dogs without bacteremia, the results will be negative.

• When multiple blood samples (preferably three samples over a 24-hour period) of 7 to 10 ml each are obtained from dogs with a strong likelihood of bacteremia, positive results are often obtained. It is possible to collect these samples simultaneously from different venipuncture sites with results similar to those obtained by collecting the samples at different times over 24 hours.

17. What factors contribute to negative blood culture results in dogs that actually have bacteremia?

The primary factors contributing to negative results are the following


• Technique and diligence of the microbiology laboratory

• Low concentration of bacteria in the blood

• Prior antibiotic therapy

• Chronic IE with encapsulated vegetative lesions

• Inadequate sample volume (<7 ml) or too few samples obtained

• Intermittent shedding of organisms

• Failure to culture for, or the laboratory’s inability to harvest, anaerobic organisms

• Infection by fastidious, slow-growing organisms

• Endogenous bactericidal factors (phagocytes and complement)

• Infection by Bartonella

18. How do we know that a bacterium isolated by blood culture is not a contaminant?

Strict attention to disinfection of the venipuncture site is critical. Sterile technique should be used when collecting blood samples.


• Isolating the same organism from multiple samples indicates that it is the pathogen.
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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Infective Endocarditis

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