Community-acquired pneumonia in ambulatory patients: relative importance of atypical pathogens

Differences in the etiology of community-acquired pneumonia according to site of care: A population-based study

BACKGROUND

A few population-based studies assessing the etiology of community-acquired pneumonia in both hospitalized and ambulatory patients, with special emphasis on the etiologic role of viral infections, have been reported. The purpose of this study was to assess microbiological differences according to initial site of care in patients with community-acquired pneumonia.

METHODS

We studied 496 patients > 14 years of age collected from the study samples of three population-based studies carried out in the same geographical area ("Maresme" region in the Mediterranean coast in Barcelona, Spain) with the same methodology over an 8-year period (1987-1995).

RESULTS

Fifty-six percent of patients were hospitalized and 44% were treated at home. Of the 474 patients with etiological evaluation, 195 patients had an identifiable etiology (overall diagnostic yield 41%). Streptococcus pneumoniae was the most common causative organism. Viral infection was diagnosed in 26.5% of hospitalized patients vs. 13.2% of ambulatory patients (P=0.03). Twenty-five percent of the 68 patients with documented etiology treated at home had Chlamydia pneumoniae infection compared with 14.3% of those treated in the hospital. Ten percent of hospitalized patients had pneumonia caused by two pathogens compared with 9.7% of ambulatory patients. The association of viruses and bacteria was the most frequent cause of dual infection (79% inpatients, 67% outpatients).

CONCLUSIONS

This study has provided information on etiology of community-acquired pneumonia in hospitalized patients and in patients treated at home. A considerable proportion of patients had viral pneumonia, frequently requiring hospital admission for inpatient care.

The atypical pneumonias: clinical diagnosis and importance

The most common atypical pneumonias are caused by three zoonotic pathogens, Chlamydia psittaci (psittacosis), Francisella tularensis (tularemia), and Coxiella burnetii (Q fever), and three nonzoonotic pathogens, Chlamydia pneumoniae, Mycoplasma pneumoniae, and Legionella. These atypical agents, unlike the typical pathogens, often cause extrapulmonary manifestations. Atypical CAPs are systemic infectious diseases with a pulmonary component and may be differentiated clinically from typical CAPs by the pattern of extrapulmonary organ involvement which is characteristic for each atypical CAP. Zoonotic pneumonias may be eliminated from diagnostic consideration with a negative contact history. The commonest clinical problem is to differentiate legionnaire's disease from typical CAP as well as from C. pneumoniae or M. pneumonia infection. Legionella is the most important atypical pathogen in terms of severity. It may be clinically differentiated from typical CAP and other atypical pathogens by the use of a weighted point system of syndromic diagnosis based on the characteristic pattern of extrapulmonary features. Because legionnaire's disease often presents as severe CAP, a presumptive diagnosis of Legionella should prompt specific testing and empirical anti-Legionella therapy such as the Winthrop-University Hospital Infectious Disease Division's weighted point score system. Most atypical pathogens are difficult or dangerous to isolate and a definitive laboratory diagnosis is usually based on indirect, i.e., direct flourescent antibody (DFA), indirect flourescent antibody (IFA). Atypical CAP is virtually always monomicrobial; increased IFA IgG tests indicate past exposure and not concurrent infection. Anti-Legionella antibiotics include macrolides, doxycycline, rifampin, quinolones, and telithromycin. The drugs with the highest level of anti-Legionella activity are quinolones and telithromycin. Therapy is usually continued for 2 weeks if potent anti-Legionella drugs are used. In adults, M. pneumoniae and C. pneumoniae may exacerbate or cause asthma. The importance of the atypical pneumonias is not related to their frequency (approximately 15% of CAPs), but to difficulties in their diagnosis, and their nonresponsiveness to beta-lactam therapy. Because of the potential role of C. pneumoniae in coronary artery disease and multiple sclerosis (MS), and the role of M. pneumoniae and C. pneumoniae in causing or exacerbating asthma, atypical CAPs also have public health importance.

Kinetic bactericidal activity of telithromycin, azithromycin and clarithromycin against respiratory pathogens

The present study assessed the comparative in vitro killing kinetics of telithromycin, azithromycin and clarithromycin. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) were determined against Streptococcus pneumoniae, beta-haemolytic streptococci, Haemophilus influenzae and Moraxella catarrhalis strains characterized by different susceptibilities to beta-lactams and macrolides. For each bacterial species, representative strains were chosen for time-kill studies. Telithromycin showed high activity against all the tested strains with MIC ranging from < or = 0.004 to 0.5 mg/L for streptococci, from 0.008 to 8 mg/L for H. influenzae, and from 0.008 to 0.5 mg/L for M. catarrhalis. In time-kill studies, telithromycin showed an overall superior bactericidal activity in respect to macrolides, particularly against resistant strains. In conclusion, telithromycin proved to possess bactericidal activity against a wide range of respiratory pathogens, including strains resistant to common macrolides.

Guidelines for Empiric Antimicrobial Prescribing in Community-Acquired Pneumonia

Empiric antimicrobial prescribing for community-acquired pneumonia remains a challenge, despite the availability of treatment guidelines. A number of key differences exist between North American and European guidelines, mainly in the outpatient setting. The North American approach is to use initial antimicrobial therapy, which provides coverage for Streptococcus pneumoniae plus atypical pathogens. Europeans tend to focus on providing pneumococcal coverage with less emphasis on covering for an atypical pathogen. Ambulatory patients without comorbidity are more likely to receive macrolide therapy in North America, whereas in Europe these patients would probably receive a beta-lactam agent. Major issues that are fundamental to this difference include the importance of providing therapy for atypical pathogens and the clinical significance of macrolide-resistant S pneumoniae. Prospective data are required to evaluate which of these two approaches offers clinical superiority.

Current Management Issues Associated with Community-Acquired Pneumonia

Community-acquired pneumonia (CAP) is a significant cause of morbidity, mortality, and increased cost. Despite numerous managementguidelines, CAP continues to existas a challenge to the learned clinician. Due to a lack of sensitive diagnostic testing, causative pathogens are often not identified, making most therapy empiric. Increasing levels of bacterial resistance to available antimicrobials worldwide has been implicated in driving up the costs of treatment and adversely effecting clinical outcomes. Pharmacists can be part of the solution by encouraging appropriate antimicrobial selection based on resistance patterns in their communities and ensuring appropriate vaccines are employed to prevent CAP.

Macrolides: structures and microbial targets

The macrolide class of antibiotics is well established and often recommended for use in the treatment of community-acquired respiratory tract infections. A number of agents with varying antimicrobial activity have been developed via chemical modification of the core macrolide structure, a macrocyclic lactam ring. Although structurally diverse, the macrolides share a common ability to bind to the bacterial 50S ribosome subunit and inhibit protein synthesis, thereby preventing bacterial multiplication. Resistance in the clinic is due to modification of the 50S subunit in the area of the peptidyl transferase center or to an efflux pump. The newer macrolides, and in particular azithromycin, with their broad-spectrum microbiological profile have extended the therapeutic uses of this class of antibiotics and ensured that they remain an integral part of the clinician's armamentarium.