Staphylococcus aureus Bacteremia and Endocarditis

Staphylococcus aureus is fast becoming a leading cause of bacteremia, both as a hospital-acquired infection as well as a community-acquired infection. It is currently the second most common blood culture isolate. In the recent era, the number of cases of S. aureus bacteremia due to methicillin-resistant S. aureus (MRSA) has been steadily increasing [43]. Although not all cases of S. aureus bacteremia imply the diagnosis of IE, the incidence of proven S. aureus IE is also increasing and represents the most common bacteriologic cause of native valve IE amongst patients with intravenous drug use and second leading cause of IE overall.

In the setting of the S. aureus bacteremia, it is critical to consider the diagnosis of IE because of the therapeutic and prognostic implications. The reported prevalence of IE varies in different studies, depending on the population studied and the likelihood of IE. The incidence of IE among prospectively identified adults in the general population with S aureus bacteremia is ~ 12-25%, depending on the selection criteria

[44.45]. In a multicenter trial, Chang et al. found that in presence of S. aureus bacteremia, the prevalence of endocarditis was 21% in community-acquired infections, 35% in intravenous drug users, 5% in hospital acquired infections, and 12% in the hemodialysis population [43]. Sixty-nine percent of cases were secondary to methicillin-susceptible S. aureus (MSSA) and 31% were secondary to MRSA. Methicillin-susceptible S. aureus was more common in community-acquired infections, in patients with intravenous drug use, and in patients with previous endocarditis. MRSA was more likely to be hospital-acquired and to be found in patients on hemodialysis. MRSA was also more likely in patients with persistent bacteremia and prolonged fevers.

Certain features, such as those listed in Table 6.4, have been noted to be associated with higher risk for the presence of IE and presence of these features should prompt echocardiography.

All patients with S. aureus bacteremia should have repeat blood cultures performed three days after initiating antibiotics [44]. If persistent bac-teremia is noted, then endocarditis should be highly suspected; and echocardiography, if needed TEE, should be strongly considered. Also, investigation for metastatic foci of infection should also be performed. Usually echocardiography is not recommended in patients with nosocomial S. aureus bacteremia and low-risk features for endocarditis (absence of factors identified in Table 6.4).

Recently performed studies evaluating the role of transesophageal echocardiography in S. aureus bacteremia have raised some concerns

[45.46]. These studies found that neither assessment of clinical features nor transthoracic echocardiography was able to predict the risk of IE. In another study, Roder et al. identified a significant number of patients with IE at autopsy when it was not suspected clinically in patients with S. aureus bacteremia [47]. Also, the low risk of endocarditis in a patient with nosocomial infection as well as with an intravascular catheter has also been questioned in recent studies. Despite low clinical likelihood, Thangaroopan found TEE evidence of vegetations in 2 of 87 in his series, with both patients displaying the triad of S. aureus bacteremia, immunosoppression, and persistent fever [33]. The cost-effectiveness of TEE to determine the duration of antibiotic therapy in patients with clinically uncomplicated intravascular catheter associated S. aureus bacteremia has also been established [48]. These studies suggest a low threshold for consideration for TEE for IE with S. aureus bacteremia.

In spite of the above studies, the role of empiric initial TEE in risk stratification in every patient with persistent S. aureus bacteremia may be limited by factors including cost, limited resources because of the requirement of specialized physicians and equipment, time, and occasionally the presence of other indications for prolonged antibiotic therapy. Also, patients with endocarditis identified only by TEE compared to TTE generally have a better prognosis and outcome [49], likely because of the smaller size of the vegetation and fewer complications. Also the question has been raised whether endocarditis identified by TEE only may represent early diagnosis, which may require only a short course of antibiotics, as well as the possibility of a false-positive result when small masses are identified on valves, leading to overtreatment [45].

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