Numerous prospective studies examining the etiology for community-acquired
pneumonia (CAP) have found that in a significant proportion of cases
there is no identifiable cause. This is due, in part, to incomplete
or inadequate diagnostic evaluation of all potential pathogens.
Although CAP is traditionally divided into typical and atypical
pneumonia based on clinical syndromes due to “typical” pathogens
(eg, Streptococcus pneumoniae) versus “atypical” organisms
(eg, Mycoplasma pneumoniae,Chlamydia pneumoniae, and Legionella pneumophila), it has been
repeatedly shown that this dichotomous classification lacks enough
predictive value and discriminatory ability to be reliable. As a
result, the initial treatment of CAP remains largely empiric. Nevertheless,
the term “atypical” is still pervasive in the
medical lexicon and useful for discussion. The three major atypical
organisms that account for pneumonia and upper respiratory infections
are resistant to β-lactam antibiotics: C pneumoniae and L
pneumophila because they are intracellular and M pneumoniae because it lacks a cell
wall. This chapter discusses these and also discusses some of the
important viral causes of CAP, specifically adenovirus, hantavirus,
influenza, and varicella pneumonias and Severe Acute Respiratory
Syndrome. The key clinical and laboratory features of each of these entities
are summarized in Table 37–1. Respiratory viruses
such as measles, respiratory syncytial virus, parainfluenza viruses,
coxsackievirus, enteroviruses, cytomegalovirus, human metapneumovirus,
and herpes simplex viruses are not discussed.
Table 37–1. Key
Features of Viral and Atypical Bacterial Pneumonias. |Favorite Table|Download (.pdf)
Table 37–1. Key
Features of Viral and Atypical Bacterial Pneumonias.
|Agent||Key Clinical and Laboratory Features|
- Adenovirus is highly contagious and may cause large
outbreaks in crowded facilities. It typically causes a self-limited
upper respiratory infection although necrotizing bronchitis, bronchiolitis,
and pneumonia may occur.
- Polymerase chain reaction is the best test to diagnose adenovirus
pneumonia and can be applied to various specimen types.
- Cidofovir or Ribavirin has in vitro activity
- Hantavirus is acquired via direct contact with
or inhalation of rodent excreta.
- Respiratory failure may be due to both noncardiogenic pulmonary
edema (from a capillary leak syndrome) and cardiogenic pulmonary
edema (from myocardial dysfunction).
- Hematological tetrad of neutrophilia with left shift, thrombocytopenia,
elevated hematocrit, and circulating immunoblasts is characteristic.
- Influenza typically occurs in the winter months,
although sporadic cases of influenza B can occur throughout the
- Primary influenza pneumonia is less common than secondary
- The inactivated influenza vaccine is recommended for at-risk
individuals. Oral antiviral agents may reduce the duration of illness
when begun early in the course of the disease.
- Respiratory disease due to varicella is most common
among adults, pregnant women, and immunocompromised individuals.
- It is highly contagious and spreads from person to person
- The live attenuated varicella vaccine is effective and is
recommended for at-risk individuals who are exposed, except for
- SARS is a severe respiratory syndrome associated
with high infectivity and high mortality.
- Laboratory findings of lymphopenia and an elevated LDH are
- Treatment with ...
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