TABLE 1191 Common Causes of Pneumonia

NEWBORNS: BIRTH TO 1 MONTH The newborn age group is the only developmental period when bacterial infections are more common than viral agents as the leading cause of pneumonia. The majority of infections in this age group are caused by aspiration of the maternal genital organisms present during labor and delivery. The predominant pathogen is group B streptococcus, followed by Escherichia coli, Klebsiella species, and other gram-negative enteric bacilli from the Enterobacteriaceae family. Other, less commonly encountered organisms include nontypeable H. influenzae, other streptococci (group A and a-hemolytic species), enterococci, Listeria monocytogenes, Bordetella pertussis, and anaerobic bacteria.11

INFANTS: 1 TO 24 MONTHS During the first 2 years of life, viruses are the most common etiologic agent of pneumonia. RSV, parainfluenzavirus, influenzavirus, and adenovirus account for most lower respiratory tract infections, including pneumonia, in this age group. I2,,4,7 There are at least 14 other viral agents isolated in children with pneumonia, which include rhinoviruses, enteroviruses, coronavirus, measles, varicella, rubella, herpes simplex virus, and Epstein-Barr virus. 7812 Bacterial pneumonia due to Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, and H. influenzae should be considered in infants and toddlers who are severely ill, have rapid onset and progression of symptoms, or have lobar or diffuse infiltrates, large effusions, or abscesses on radiograph. 13

Very young infants (1 to 3 months) may present with what is often referred to as afebrile pneumonitis, or atypical pneumonia. This syndrome is typified by cough, tachypnea, and sometimes progressive respiratory distress in the absence of fever. Apneic episodes can occur with RSV, chlamydia, and pertussis. There is often radiographic evidence of bilateral diffuse pulmonic infiltrates with air trapping. The viruses listed above are the most common etiologic agents. 14 Chlamydia trachomatis is also often identified in this scenario.1 l4 Ureaplasma urealyticum, Mycoplasma hominis, Pneumocystis carinii, and B. pertussis have also been implicated in this syndrome, but the extent of their role is not as well defined.1 l5

PRESCHOOL: 2 TO 5 YEARS As age increases, the overall incidence of pneumonia decreases, but the relative frequency of bacterial pathogens, particularly S. pneumoniae, as etiologic agents increases. Despite this, respiratory viruses, particularly influenzavirus A and B and adenovirus, remain the most common cause of pneumonia in this age group. The most common bacterial pathogen encountered is S. pneumoniae. In the recent past, H. influenzae type B (HIB) was encountered nearly as frequently; however, since implementation of widespread immunization against this agent, its incidence as an agent of invasive disease is thought to be much less.1 H However, non-type-B H. influenzae (NTHI), which is not protected against by the vaccine, may be an increasingly common agent of bacterial pneumonia.18 Other bacteria that are isolated less commonly include of S. aureus, group A streptococcus, and Moraxella catarrhalisMZ Mycoplasma pneumoniae has been found more frequently in this age group in recent studies.1920

SCHOOL AGE AND ADOLESCENCE Once children reach school age, M. pneumoniae is the most frequent bacterial cause of pneumonia.12 21 The peak incidence is between 10 and 15 years of age.22 Streptococcus pneumoniae also remains a common pathogen in this age group.1 Chlamydia pneumoniae is estimated to be the cause of up to 19 percent of pneumonias in school-aged and adolescent patients. "H,0,! 20 These infections are usually mild or asymptomatic.20 Staphylococcus aureus pneumonia can occur at any age but tends to be most frequent in older children.23 Respiratory viruses, especially adenovirus, can also cause pneumonia in this age group.

ALL AGES: SPECIAL CONSIDERATIONS The most common etiologic agents causing severe pneumonia (requiring admission to an intensive care unit) in all age groups beyond the neonatal period include S. pneumoniae, S. aureus, group A streptococcus, HIB, adenovirus, and M. pneumoniae. Staphylococcus aureus is notorious for causing rapidly progressive disease, often with pulmonic abscesses. A resurgence of virulent group A streptococci has been associated with sporadic cases of invasive disease, including necrotizing fasciitis with pneumonia and empyema in children. 24 Increased severity of M. pneumoniae infections in children with sickle cell disease has been described.

Gram-negative bacilli, including Pseudomonas, should be considered in patients who have recently been hospitalized. Anaerobic infections should be considered in children with neurologic or anatomic defects that predispose them to aspiration. Unusual causes of bacterial pneumonia in children include Mycobacterium tuberculosis, Legionella pneumophila, Chlamydia psittaci, Francisella tularensis, and rickettsial infections. Children with progressive or unresponsive pneumonia should be evaluated for evidence of these microorganiams. An immunocompromised host is susceptible to all of the infections listed above as well as to opportunistic infections, such as P. carinii, cytomegalovirus, and fungal diseases.

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