Burden of Staphylococcus aureus endocarditis: how real is the threat?

Abstract

Staphylococcus aureus represents an interesting example of how strong the relationship between healthcare professionals and government policies can become after wide media involvement [1]. A Google News search for S. aureus for the last 3 years retrieved more than 4000 articles in non-scientific journals focused on the threat of the ‘superbug’ among hospitalized patients and the community. In 2009, an interesting paper by Boyce showed that the UK press exhibited a high interest in methicillin-resistant S. aureus (MRSA) as compared with that of the USA [2]. Healthcare workers, experts and professional bodies have strongly criticized the nature of media reporting, but have had little influence on or involvement in the press. However, the strong engagement of civil society and politicians led to attention and funds being devoted to this issue, which was only partly covered before the media interest. Therefore, it was not unexpected when the sharp increase in the rate of hospitalizations for bacterial infective endocarditis in the USA, mainly driven by S. aureus, recently reported at the Interscience Conference on Antimicrobials and Chemotherapy in Chicago, again attracted attention of the public and media interest (Federspiel et al., 51st ICACC, 2011, L1-387). The authors analysed, through a retrospective cohort study, the trends and characteristics of 83 700 hospitalizations for bacterial endocarditis during a 10-year period in the USA. The trends showed an increase in the overall rate of hospitalizations for bacterial endocarditis over time, from 11.4 to 16.6 discharges/100 000 population-years between 1999 and 2008. The annual rate increased by 46%, although most of the increase occurred before 2006. Among admissions where an organism was identified, the majority (71%) of the growth in incidence was attributable to S. aureus. Overall, S. aureus was the most common organism identified (46%), followed by streptococci/enterococci (33%) and coagulase-negative staphyloccoci (8%). After adjustment for demographics and comorbidities, S. aureus-related endocarditis was associated with significantly higher in-hospital mortality (risk difference: +5.9%) and inpatient charges (difference: +$19 280) and longer hospitalizations (difference: +2.26 days) than streptococcal/enterococcal endocarditis (Federspiel et al., 51st ICACC, 2011, L1-387). However, before generalizing results and deriving solutions for European hospitalizations, some threats to the validity of the study should be underlined. First, the researchers were able to determine the causative pathogen for 56% of the hospitalizations only, and, although the number of unidentified cases was roughly parallel with the number of total cases, this might have introduced a reporting bias. Second, and more important from an infection control point of view, distinction between cases caused by MRSA and methicillin-susceptible S. aureus (MSSA) and definition of the site of acquisition (community vs. hospital vs. healthcare facilities) were not possible. The lack of this information prevents public health officers, epidemiologists and infection control practitioners from analysing the origin of clones and suggesting future steps for prevention. However, some hypotheses might be outlined. First, if the increase has arisen from community-acquired cases, then possible changes in the outpatient population need to be ascertained. The rise might be attributed to the increase in the number of cases caused by intravenous drug usage or, as I suspect, to the change in the number of immunocompromised patients, mainly in relation to the wide usage of biological agents, as anti-tumour necrosis factor therapies [3]. Another reason for the increase can be derived from the observation reported by Furuno and coworkers on the US