In recent clinical studies of older patients with either CAP or NHAP, the most frequently identified microorganisms were S. pneumoniae, H. Influenzae, Enterobacteriacae, and S. aureus.
S. pneumoniae is by far the predominant pathogen isolated in hospital-based studies of elderly patients with either CAP (up to 58%) or NHAP (up to 30%) (Riquelme et al., 1996; Lieberman et al., 1997; Marrie et al., 1997; Riquelme et al., 1997; Lim et al., 2001; Kaplan et al., 2002; Zalacain et al., 2003). In older subjects treated in the ICU, S. pneumoniae reportedly causes 14% of CAP and 9% of NHAP (El-Solh et al., 2001). The presence of S. pneumoniae is associated with coexisting lung disease, hepatic disorders, or alcohol abuse.
H. influenzae is reported in up to 14% of older patients with CAP or NHAP, and was identified in 7% of elderly subjects with severe pneumonia leading to admission to an ICU (El-Solh et al., 2001). H. influenzae is frequently linked to exacerbations of COPD and bronchiectasis and thus should be considered as a potential pathogen in these patients.
Staphylococcus aureus was documented in up to 7% of patients with CAP and 4% of patients with NHAP. One study shows an even higher occurrence of S. aureus-related pneumonia. Of 104 elderly patients with severe CAP or NHAP admitted to an ICU, 17% had (mostly methicillin-sensitive) S. aureus as causative agent (El-Solh et al., 2001). In this study, S. aureus was identified in 29%
of the patients with severe NHAP (78% methicillin-sensitive) vs. 7% of those with CAP (all methicillin-sensitive). Because of the increasing rate of MRSA colonization in the nursing home population, and the relatively high probability for MRSA carriers of developing symptomatic infection, MRSA pneumonia is likely to become a more frequently encountered entity. S. aureus is associated with lung abscess, empyema, and secondary bacterial pneumonia after viral respiratory infection.
Enteric Gram-negative bacteria (GNB) Both colonization by, and infection with, GNB is a function of the number and severity of comorbidities (Valenti et al., 1978). The likelihood of GNB pneumonia increases in nursing home patients and in patients with decreased functional status. In a community setting, GNB infection occurs primarily in debilitated and chronically ill patients. The presence of Pseudomonas suggests bronchiectasis (Fein et al., 1994). If appropriate, the presence of bronchiectasis should be investigated by high-resolution computed tomography; indeed, the presence of bronchiectasis warrants prolonged treatment for LTRI (14 to 21 days).
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