Fluoroquinolones in the Management of Community-acquired Pneumonia in Primary Care

Brian Wispelwey; Katherine R Schafer


Expert Rev Anti Infect Ther. 2010;8(11):1259-1271. 

In This Article

Common Pathogens in CAP

Bacterial pathogens are the primary cause of CAP, although viral causes have been reported in 1–34% of all CAP cases, with significant variations between studies.[23–25] Many studies indicate that when the etiology of CAP is pursued, the cause is 'unknown' in most cases. This phenomenon is likely due to the lack of specimen collection in an outpatient setting. When a cause is identified, S. pneumoniae is the most common pathogen. The importance of S. pneumoniae was confirmed by a Spanish study that used trans-thoracic needle biopsy in patients for whom a causative pathogen was not identified by conventional methods.[26] This study found that S. pneumoniae was implicated in 25% of all cases and approximately one-third of those where the cause was not initially ascertained.[26]

The prevalence of typical CAP pathogens in the primary care setting was determined by the results of the 1999–2000 Respiratory Surveillance Study. In this study, the most prevalent pathogen found in primary care patients treated for CAP was Haemophilus influenzae (38%), followed by S. pneumoniae (18%), and then Moraxella catarrhalis (15%).[27] More recent surveillance of CAP in ambulatory settings found a higher percentage of atypical pathogens, although etiologic diagnosis was not ascertained in all patients tested.[28,29] However, most studies have found S. pneumoniae to be the most prevalent pathogen in CAP, and the authors noted that this organism is sensitive to improper handling techniques and thus may be underestimated. In addition, 28% of the study population had prior antibiotic treatment, and this may have affected the distribution of pathogens.[27] At the same time, such variability in distribution could also be a function of patient demographics or location of treatment. S. pneumoniae remains the most common etiologic agent in many inpatient studies, as well as in geriatric populations.[29,30]

Table 1 presents the most common causative pathogens of CAP, listed in order of prevalence in three patient populations: outpatients, general hospital ward and ICU.[1,31]S. pneumoniae is the most common cause of CAP in all three patient populations, accounting for 20–60% of all episodes.[2,25] Other common pathogens include H. influenzae, M. catarrhalis and atypical organisms such as Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella species.[1,2] Among outpatients, M. pneumonaie is more common in those younger than 50 years of age without significant comorbid conditions, while S. pneumoniae is the most common pathogen in older patients and those with comorbidities.[1] In addition, particular pathogens are identified more commonly in patients with CAP who have certain risk factors, such as a history of alcoholism, chronic obstructive pulmonary disease, smoking, aspiration, HIV infection or CAP acquired during influenza season.[1]

Although S. pneumoniae is the most common etiologic pathogen for CAP overall, it is important to consider the possibility of polymicrobial infection. One prospective cohort study found 38.4–47.7% of CAP involved multiple pathogens.[32]

Resistance issues are important in CAP and increased resistance to penicillin and macrolides has been noted in S. pneumoniae. Multidrug-resistant S. pneumoniae (MDRSP) poses a particular challenge to clinicians. H. influenzae, M. catarrhalis and a number of enteric Gram-negative bacteria have also shown resistance to various antibiotics, which can be a problem in the management of CAP.[2,27] As stated above, one must recognize the difficulty in interpreting in vitro resistance and its relationship to clinical outcomes.

An emerging risk of infection with less common, usually hospital-associated pathogens, such as Pseudomonas and Acinetobacter species, and methicillin-resistant Staphylococcus aureus (MRSA), has also been identified in patients with CAP.[1] Risks for MRSA infection include hemodialysis, HIV, insulin-requiring diabetes mellitus and intravenous drug use.[1] Community-acquired MRSA should be considered in all cases of post-influenza pneumonia.[1]


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