E n g l i s h fo r Medics
Pneumonianormal host
Introduction
Pneumonia is an acute or chronic infection involving the pulmonary parenchyma. Most
cases are caused by microbial pathogens, including bacteria, viruses, fungi, and parasites.
Pneumonia may also refer to inflammation involving the pulmonary parenchyma due to non-
microbial causes such as chemical pneumonia. Other modifying terms are used as follows:
pneumonia may be acute, subacute, or chronic, depending on the duration of symptoms; it
may be described as bronchopneumonia, consolidated (lobar) pneumonia, or interstitial
pneumonia based on chest radiography changes; or it may be named after the putative
agent, for example pneumococcal pneumonia, mycoplasma pneumonia, Pneumocystis
carinii pneumonia, etc. Pneumonia is also identified by the place of acquisitionas
community-acquired, nursing home-acquired, or hospital-acquired. This chapter will be
restricted to community-acquired pneumonia in the adult immunocompetent host.
Aetiology
Although the list of microbes that can cause pneumonia is legion, only a relatively small
number are frequent pathogens, for example: Streptococcus pneumoniae, Haemophilus
influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., anaerobic
bacteria, and viruses. Less common pathogens are Moraxella catarrhalis, Streptococcus
pyogenes, Acinetobacter spp., Chlamydia psittaci, Coxiella burnetii, Neisseria meningitidis,
Staphylococcus aureus, and enteric Gram-negative rods. In most reported series, each of
these generally accounts for less than 1 to 2 per cent of cases. The relative frequencies of
different pathogens causing community-acquired pneumonia in two large studies are
summarized in Table 1. However, important limitations of these studies should be
acknowledged: all the cases in the review conducted by the British Thoracic Society were
inpatients, as were the great majority of those reviewed in the meta-analysis. Most studies
of pneumonia show that only 20 to 30 per cent of patients are sufficiently sick to require
hospitalization. Furthermore, nearly all studies, including those that use extensive diagnostic
resources, only identify a likely aetiological agent in 40 to 60 per cent of cases. This
suggests that fastidious microbes are under-represented and that many cases of pneumonia
may be caused by, as yet, unidentified organisms.
Epidemiology
Pneumonia is the most important infectious disease in terms of morbidity and mortality. It is
estimated that in the United States there are four million cases of pneumonia per year (45
000 deaths), and worldwide there are 4400 million cases per year (4 million deaths). In the
United States, data suggest that between 20 and 30 per cent of all patients with a diagnosis
of pneumonia are hospitalized, and that the mortality rate for this subpopulation is about 14
per cent. The crude death rate from influenza and pneumonia in the United States for 1994
was 31.8 deaths per 100 000 of the population; this represents a 59 per cent increase over
the 20.0 deaths per 100 000 recorded in 1979, suggesting that the frequency of lethal
pneumonia in the United States is increasing. Those aged 65 or older accounted for 89 per
cent of the deaths in 1994, suggesting that increases in longevity account for most of this
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E n g l i s h fo r Medics
increase in mortality rate. Those pathogens associated with specific epidemiological and
underlying conditions are summarized in Table 2.
When an aetiological agent is identified, just three microbial agents account for the majority
of lethal cases of community-acquired pneumonia. Influenza accounts for an average of 20
000 deaths per year in the United States: the majority involve influenza A, occur in patients
over 65 years of age, and most deaths are due to complications of influenza rather than
influenza per se. The second common cause of lethal pneumonia is pneumococcal
pneumonia; risk factors for a fatal outcome include: bacteraemia, advanced age, and
concurrent alcoholism. Legionella is the third agent, with associated mortality rates
generally reported between 15 and 25 per cent for patients with community-acquired
infections.
Nearly all studies show that the risk of death with pneumonia is strongly associated with
age extremes. Concurrent conditions that contribute to increased mortality rates include
neoplastic disease, hepatic failure, congestive heart failure, cerebrovascular disease,
and renal disease.
Pathogenesis
As with nearly all infectious diseases, the probability of disease depends on the virulence
of the organism, the inoculum size, and the status of host defences. The normal
tracheobronchial tree and lung parenchyma is sterile below the level of the larynx, so that
agents of pneumonia must reach this site from external or adjacent sources, usually either
by aspiration or inhalation. Organisms may also reach the lung by haematogenous
seeding, direct extension from infection in a contiguous structure, or by activation of
dormant organisms in the lung. These mechanisms are pathogen-specific, as summarized
in Table 3.
Most pneumonias are probably caused by aspiration, which is defined as the abnormal
entry of endogenous secretions or exogenous substances into the lower airways. There
is a problem here with semantics because most cases of pneumonia are probably due to
aspiration as classically described, but 'aspiration pneumonia' probably accounts for only
5 to 10 per cent of cases. The explanation is presumably quantitative, 'aspiration'
generally referring to the abnormal entry of relatively large volumes in patients who are so
predisposed due to dysphagia or a compromised level of consciousness. The alternative
form is presumed to be 'microaspiration', involving the aspiration of very small numbers of
microbes, a process that commonly takes place in healthy patients during sleep and with
no apparent sequelae.
Clinical features
The classic presentation of pneumonia is of a cough and fever with the variable presence of
sputum production, dyspnoea, and pleurisy.
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