Agents Of Atypical Pneumonia

M. pneumonia is seldom reported as a causative agent in CAP or LRTI in the very old (1-13%) (Janssens et al., 1996; Marrie et al., 1997; Riquelme et al., 1997; Lim et al., 2001; Kaplan et al., 2002), and has not been identified in elderly patients admitted to the ICU for severe CAP (El-Solh et al., 2001).

C. pneumoniae infection in the elderly generally is considered a mild disease and was reported in only 1% of patients admitted to ICU for CAP (El-Solh et al., 2001). However, C. pneumoniae has been reported in up to 18% of NHAP and 28% of CAP. Occasional C. pneumoniae outbreaks in nursing homes have been associated with a high attack rate (44-68%) and high mortality (~35%) of confirmed cases (Troy et al., 1997). C. pneumoniae has no specific clinical presentation but the combination of pharyngitis or hoarseness (laryngitis) and nonproductive cough should suggest C. pneumoniae infection. Duration of illness > 8 days before admission also increases the probability of C. pneumoniae infection (Socan et al., 2004). A case-control study comparing older patients admitted for C. pneumoniae CAP vs. non-C. pneumoniae CAP, showed that residing in a nursing home vs. living at home was associated with a marked increase in the probability of C. pneumoniae infection. C. pneumoniae infection can be identified by direct fluorescent antibody staining, nasopharyngeal swabs (PCR or culture), or retrospectively by serology.

Prevalence of L. pneumophilia in CAP shows important geographic variations, being in the range of 1.8 to 24% in hospital-based studies (Janssens et al., 1996). In Switzerland, 261 cases of definite L. pneumophilia infection were reported between 1999 and 2001 (incidence: 1.7 per 105 inhabitants). Median age of patients infected was 61 years; that of patients dying from the infection was 67 years; 34% of subjects infected were aged over 70 (OFSP, 2003). Legionella sp. infection was community-acquired in 60%, travel-related in 27%, and hospital-or nursing home-acquired in 10% and 3% of cases, respectively (OFSP, 2003). Colonization of potable water in long-term care institutions and geriatric hospitals is a potential hazard for Legionella sp. infection.

Because of a low to moderate sensitivity of diagnostic tests, the incidence of Legionella infection may be underestimated in clinical studies. Indeed, sensitivity of serology ranges from 40 to 60%; that of direct fluorescent antibody staining of sputum is 30 to 70% (specificity of 94-99%), and sputum culture has a sensitivity of approximately 80% (Stout et al., 1997). The most useful test, namely dosage of urinary Legionella antigen is highly specific (100%), yet has a sensitivity of 79 to 83%, increasing to 94% if only L. pneumophilia serogroup 1 is considered (Stout et al., 1997; Helbig et al., 2001).

Infection by Legionella is frequently heralded by an abrupt onset of malaise, weakness, headaches, and myalgia (Bentley 1984). Most patients cough; hemoptysis occurs in one-third of patients. Mental status changes are reported in 25 to 75% of older patients. Other associated features are bradycardia, liver dysfunction, diarrhea, and hyponatriemia, but none of these features are specific and all may occur with severe pneumonia of other etiologies.

The probability of L. pneumophilia infection increases in severe CAP or NHAP and must definitely be considered in this setting (El-Solh et al., 2001). Interestingly, in a study of older patients with pneumonia admitted to the ICU, L. pneumophilia infection was strongly associated with immunosuppression: 60% of patients had been under prolonged corticosteroid therapy. In up to 65% of patients, radiographic findings initially worsen after treatment has been started, and even after 10 weeks of therapy, only 50% of chest radiographs are normal (Macfarlane et al., 1984).

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