Epidemiology and etiology Etiology and Mortality Rates

The etiology of bacterial pneumonia varies in accordance with the type of pneumonia. Table 71-1 lists the common pathogens associated with the various types or classifications of pneumonia. S. pneumoniae colonizes the nasopharyngeal flora in up to 50% of healthy adults and may colonize the lower airways in individuals with chronic bronchitis.6"7 It possesses many virulence factors enhancing its ability to cause infection in the respiratory tract. Therefore, it is not surprising that S. pneumoniae is the predominant bacterial pathogen associated with CAP. The second most common pathogen is one of the atypical organisms, Mycoplasma pneumoniae. Nontypeable Haemophilus influenzae intermittently colonizes about 80% of the population and the incidence of permanent colonization increases in chronic obstructive pulmonary disease (COPD) patients and those with cystic fibrosis. Therefore the likelihood of nontypeable H. influenzae causing pneumonia increases in COPD patients. Moraxella catarrhalis is a more common cause of pneumonia in the young children and the elderly. Chlamydia pneumoniae and Legionella pneumophila are less frequent causes than the other bacterial and atypical organisms. Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) is associated with necrotizing and severe pneumonia in healthy children and young adults. Less than 2% of all CA-MRSA infections are pneumonia (most are skin and soft tissue); however, the number of reports of pneumonia are increasing.8

Table 71-1 Common Pathogens by Type of Pneumonia

Type of

Pneumonia

Common Pathogens

Community

Aerobic bacteria: 5. pneumoniae

H. influenzae, M. cararrhalis

Atypical: M. pneumoniae, C. pneumoniae,

Lpneumophila, respiratory viruses

Aspiration

Oral contents; Anaerobesr Vsridops

streptococci

Gl contents wich pH increase

Tnterk: gram-negaiive bacilli

Hospital

(Early onset no risk factors for resistant

pathogens)

Ventilator

5. pneumoniae, .MSSA, f. coli„ K. pneumoniae,

health care

(M. pneumoniae, C pneumoniae are rare)

Hospital

(I. ale onset and/Of risk factors for resfeiani

pathogens)

Ventilator

MRSA. extended'ipectruin ß lactamase-

health care

producing K. pneumoniae, P. aeruginosa,

Acinerobaaer spp,

M RSA, m ethicil lin-resistant Staphylococcus awe us; MSSAr methiclllin susceptible 5. aureus.

Viruses are a common cause of CAP in children (about 65%) and much less common in adults (about 15%).9 Viruses often associated with pneumonia in adults include influenza A and B, adenoviruses while less common causes include rhinovir-uses, enteroviruses, cytomegalovirus, varicella-zoster, herpes simplex, and others. In children, viral pneumonia is more commonly caused by respiratory syncyntial virus, influenza A, and parainfluenza, and less commonly the viruses are similar to those listed above for adults.

Mortality associated with CAP is dependent upon the severity of the illness and the age of the patient. In elderly patients admitted to the hospital with severe pneumonia the mortality rate is up to 40%.10-13 In the outpatient setting (mild to moderate disease) the mortality rate is less than 5%.14 Mortality among case reports of CA-MRSA necrotizing pneumonia is 42%.8 Pneumonia owing to aspiration of oral contents is caused by a variety of anaerobes (Bacteroides spp ., Fusobacterium spp ., Prevotella spp., and anaerobic gram-positive cocci) as well as Streptococcus spp. M. catarrhalis and Eikenella corrodens may be involved but much less frequently ,16 When gastric contents are aspirated enteric gram-negative bacilli and S. aureus are more commonly the pathogens. 6

HAP, VAP, and HCAP may be caused by a wide spectrum of organisms. HCAP, early-onset HAP, and VAP commonly can be caused by enteric gram-negative bacilli in addition to the bacteria listed above for CAP. Late-onset HAP and VAP are more likely to be caused by more resistant enteric gram-negative bacilli or , Pseudomonas aeruginosa, or Acinetobacter spp., or S. aureus. Rarely are viruses or fungi a cause of HAP, VAP, or HCAP. The number of infections caused by multidrug-resistant (MDR) bacteria is increasing significantly in hospitalized patients. 17-22

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