ATYPICAL PATHOGEN PNEUMONIA
Originally, the term atypical was used to describe the nonclassic clinical features of infection with certain organisms, but today the term has been retained to refer to a group of organisms that includes Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella spp. These organisms cannot be reliably eradicated bylactam therapy (penicillins and cephalosporins) but must be treated with a macrolide, tetracycline, or quinolone. The frequency of these organisms as com- munity-acquired pneumonia (CAP) pathogens has varied in studies, but they may be present in up to 60% of CAP episodes because they can serve as copathogens, along with bacteria, in up to 40% of patients. When mixed infection is present, particularly with C. pneumoniae and pneumococcus, it may lead to a more complex course than if a single pathogen is present. In patients with more severe forms of CAP, atypical pathogens can be present in almost 25% of all patients, but the responsible organism may vary over time. Although atypical pathogens have been thought to be most common in young and healthy individuals, some population data have shown that they are present in patients of all ages, including elderly people and those in nursing homes.
Studies reporting a high frequency of atypical
pathogens have made the diagnosis with serologic testing, which may not be as
accurate and specific as culture and antigen identification. The importance of
atypical pathogens has also been suggested by a number of studies of
inpatients, including those with bacteremic pneumococcal pneumonia, showing a
mortality benefit from therapies that include a macrolide or quinolone, agents
that would be active against these organisms.
MYCOPLASMA PNEUMONIA
M. pneumoniae is an organism
that closely resembles a bacterium, lacks a cell wall, and is surrounded by a
three-layered membrane (see Plate 4-67).
Most of the respiratory infections caused by M.
pneumoniae are minor and in the form of upper respiratory tract illness or
bronchitis. Although pneumonia occurs in only 3% to 10% of all Mycoplasma infections,
this organism is still a common cause of pneumonia, with a slight increase in
frequency in the fall and winter. All age groups are affected, and although it
is common in those younger than 20 years of age, it is also seen in older
adults.
Respiratory infection occurs after the organism is
inhaled and then binds via neuraminic acid receptors to the airway epithelium.
An inflammatory response with neutrophils, lymphocytes, and macrophages then
follows accompanied by the formation of IgM and then IgG antibody. Some of the
observed pneumonitis may be mediated by the host response to the organism
rather than by direct tissue injury by the organism. Up to 40% of infected
individuals have circulating immune complexes.
When pneumonia is present, it is usually characterized
by a dry cough, fever, chills, headache, and malaise after a 2- to 3-week
incubation period. Chest radiographs show interstitial infiltrates, which are
usually unilateral and in the lower lobe but can be bilateral and multilobar, although the patient usually does not
appear as ill as suggested by the radiographic picture. Rarely, patients have a
severe illness with respiratory failure or a necrotizing pneumonia, but most
cases resolve in 7 to 10 days in an uncomplicated fashion. Many patients also
have extrapulmonary manifestations such as upper respiratory tract symptoms
(present in up to 50% of all affected), including sore throat and earache (with
hemorrhagic or bullous myringitis). Pleural effusion is seen in at least 20% of patients, although it may be small.
Other manifestations include neurologic illness such as meningoencephalitis,
meningitis, transverse myelitis, and cranial nerve palsies. The most common
extrapulmonary finding is an IgM autoantibody that is directed against the I
antigen on the red blood cell and causes cold agglutination of the erythrocyte.
Although up to 75% of patients may have this antibody and a positive Coombs
test result, clinically significant autoimmune hemolytic anemia is uncommon. Other systemic complications
include myocarditis, pericarditis, hepatitis, gastroenteritis, erythema
multiforme, arthralgias, pancreatitis, generalized lymphadenopathy, and
glomerulonephritis. The extrapulmonary manifestations may follow the
respiratory symptoms by as long as 3 weeks. Diagnosis is suspected by finding a
compatible clinical picture and radiograph in a host with pneumonia and
possibly extrapulmonary findings.
Confirmation can be made by isolating the organism in culture from
respiratory tract secretions. Serologic diagnosis is made by documenting a
fourfold increase in specific antibody to M. pneumoniae by complement
fixation test, although a single titer of 1 : 64 is suggestive of infection. The
diagnosis is also suggested by a cold agglutinin titer of 1 : 64. Testing for
IgM antibody has also been used to define infection, and with this methodology,
some investigators have found that M. pneumoniae can be a copathogen
along with bacterial agents in patients with CAP. After the diagnosis has been
made, therapy is given for 10 to 14 days with a macrolide, quinolone, or
tetracycline, which can reduce the duration and severity of the illness.
Radiographic resolution is generally rapid, similar to Chlamydophila spp.,
and more rapid than with Legionella spp. or pneumococcal pneumonia.
CHLAMYDOPHILA INFECTIONS
As already mentioned, C. pneumoniae can be
found in patients of all ages with CAP, as either a primary or coinfecting
pathogen and is relatively common (see Plate 4-68). Chlamydophila species
can also cause a less common form of pneumonia, termed psittacosis, when a
patient is infected by Chlamydophila psittaci, an agent transmitted by
inhaling infected excrement from avian species; the infectious bird does not
need to be ill to transmit disease.
Patients with psittacosis commonly have headache, high
fever, splenomegaly and dry cough, all of insidious onset after a 1- to 2-week
incubation period. A macular rash similar to that of typhoid fever may also be
seen along with relative bradycardia. Other extrapulmonary findings may occur,
including hepatitis, encephalitis, hemolytic anemia, and renal failure.
Diagnosis is on the basis of a compatible contact history and can be confirmed
serologically. Treatment is with a tetracycline (2-3 g/d) for 14 to 21 days.
Although C. pneumoniae has been recognized as a
common cause of CAP and may be a cause of sporadic pneumonia, it has also led
to epidemics of respiratory infection, including pneumonia in patients residing
in nursing homes. The disease has no specific features but is commonly seen with
laryngitis and pharyngitis. Patients have fever, chills, pleuritic chest pain,
headache, and cough and occasionally have respiratory failure. Therapy can be
with tetracycline, the newer macrolides, or the fluoroquinolones, but the
duration of therapy is uncertain. However, this form of pneumonia tends to
resolve more rapidly than other forms of CAP unless it is part of a mixed
infection with pneumococcus. More than 85% of patients have complete
radiographic resolution by 6 weeks and 100% by 12 weeks.
LEGIONELLA PNEUMOPHILA
This small, weakly staining, gram-negative bacillus
was first characterized after an epidemic in Philadelphia in 1976, which
infected primarily the attendees at an American Legion convention (see Plate
4-69). The pathogen is not really new, and after it was identified, evidence was
obtained of infection involving this pathogen before 1976. Infection may occur either sporadically or in
epidemic form, with the organism being transmitted via the aerosol route from
an infected water source such as air conditioning equipment, drinking water,
lakes and river banks, water faucets, saunas, and shower heads. Infection is
more common in the summer and early fall. When a water system becomes infected
in an institution, nosocomial outbreaks may occur, as has been the case in some nursing homes and hospitals.
After the organism enters the lungs, the incubation
period is 2 to 10 days. Initially, the organism localizes intracellularly to
the alveolar macrophage and multiplies, generating an inflammatory response that
involves neutrophils, lymphocytes, and antibody. Because cell-mediated immunity
is needed to contain infection, the disease may occur in compromised hosts,
particularly hospitalized individuals who are receiving corticosteroids, and
may relapse if not treated long enough.
At present, there are more than 46 species of Legionella
and at least 68 serotypes, but serogroup 1 of Legionella pneumophila is
the most commonly diagnosed, and it can be identified by a urinary antigen test.
The other species that commonly causes human illness is Legionella micdadei.
In its sporadic form, Legionella spp. may account for 7% to 15% of all
cases of CAP, being a particular concern in patients with severe forms of
illness.
The varying incidence of Legionella infection
among admitted patients is a reflection of geographic and seasonal variability
in infection rates, as well as the extent of diagnostic testing. For a
serologic diagnosis, it is necessary to collect both acute and convalescent
titers, and this can take at least 8 to 9 weeks. A diagnosis is made if the
indirect fluorescent antibody titer increases fourfold to a level of at least 1
: 64 or if a single convalescent titer is above 1 : 128; however, the latter
finding is not always specific for acute infection. The organism can be recovered
in culture of infected secretions grown on charcoal yeast extract agar, but
this requires patients to produce sputum, and the results are positive in 10%
to 80% of patients. Direct fluorescent antibody staining for the organism in
infected secretions is another method of diagnosis. Urinary antigen test is the
single most accurate acute diagnostic test for Legionella spp. but is
specific to serogroup 1 infection only. In recent years, most cases have been
diagnosed with urinary antigen, and with reliance on urinary antigen testing
the case fatality rate of Legionella has fallen, possibly reflecting
diagnosis of less severe illness than in the past. In the future, real-time
polymerase chain reaction testing on respiratory secretions may become
available.
Patients with Legionella pneumonia commonly
have high fever, chills, headache, myalgias, and leukocytosis. Features that
can suggest the diagnosis are the presence of pneumonia with preceding diarrhea
and mental confusion, hyponatremia, relative bradycardia, and liver function abnormalities,
but this classic Legionella syndrome is not usually present. Symptoms
are rapidly progressive, and the patient may appear to be quite toxic. The
patient may have purulent sputum, pleuritic chest pain, and dyspnea, but the
sputum Gram stain generally shows inflammatory cells but no identifiable
organisms. The chest radiograph is not specific and may show bronchopneumonia,
unilateral or bilateral disease, lobar consolidation, or rounded densities with
cavitation. Up to 15% have pleural effusion, but empyema is uncommon.
Proteinuria is common, and some patients have developed glomerulonephritis and
acute tubular necrosis. Myocarditis and cerebellar dysfunction have been reported as rare complications of Legionella pneumonia.
When the diagnosis is suspected, therapy should be
with either a macrolide or quinolone, with recent reports showing excellent
results with the use of levofloxacin or moxifloxacin. Macrolide therapy has
generally been given for 14 to 21 days; quinolones have shown efficacy with
shorter durations of therapy. With effective therapy, the decline in fever may be slow, and high spikes in
temperature may continue for 1 week after starting appropriate therapy.
Radiographic resolution is much slower than for other forms of atypical
pathogen pneumonia and nonbacteremic pneumococcal pneumonia. The mortality rate
is less than 5% in normal hosts but may be as high as 25% in compromised hosts.
