Pneumonia - Atypical Organisms
At a Glance
"Atypical" pneumonia is a clinical syndrome with the following features:
The onset of illness is gradual, often migrating from the upper to lower respiratory tract.
The patient has a nonproductive cough, constitutional symptoms (e.g., fever, headache), and may have extrapulmonary manifestations of the infection.
Chest X-ray shows patchy (nonlobar) infiltrates, usually involving one or two segments.
The four bacterial agents conventionally associated with a typical pneumonia include Mycoplasma pneumoniae, Chlamydophila pneumoniae, Chlamydophila psittaci, and Legionella spp. Designating bacterial pathogens as agents of "typical" or "atypical" pneumonia is only marginally useful, however, as symptoms and clinical findings of community-acquired pneumonia (CAP) caused by agents from one group will overlap with those caused by the other. Nevertheless, certain associations can be made between the individual "atypical" agents and particular clinical or demographic features:
young >> old (infection rates highest among school-aged children and young adults)
involvement gradually progresses from upper to lower respiratory tract, with nonproductive cough and prominent constitutional symptoms (e.g., fever, malaise, headache)
extrapulmonary manifestations occur but are infrequent; disease is somewhat seasonal (most frequent in summer and fall)
may occur sporadically or in outbreaks (e.g., within families, dormitories, military barracks, or schools)
white blood cell count (WBC) is usually normal, but subclinical hemolytic anemia is common, with elevated cold agglutinin titers
may be sporadic or occur in outbreaks (e.g., within nursing homes)
old >> young (most common among the elderly, >65 years of age)
pneumonia or bronchitis is often preceded by pharyngitis and/or hoarseness
an associated sinusitis is common
other extra-pulmonary manifestations may also occur (e.g., otitis, reactive arthritis, cardiac involvement)
WBC is usually normal
recent history of exposure to birds, usually pet (caged) birds, but also poultry or wild birds
occurs more frequently in males than in females
usually manifests with dry cough and fever, headache, myalgias, and extrapulmonary manifestations such as diarrhea, pharyngitis, or mildly altered mental status
WBC is usually normal or slightly elevated
occurs more frequently in males than in females
may be sporadic or may occur in outbreaks associated with contaminated water reservoirs
sometimes travel-associated (hotel or cruise ship stay within previous 2 weeks) or nosocomial
risk is increased by cigarette smoking and/or chronic lung disease, and in elderly or those with compromised immunity, especially via transplantation (solid organ or bone marrow), glucocorticoid administration, or therapy with TNF-alpha inhibitors
What Tests Should I Request to Confirm My Clinical Dx?
If atypical pneumonia is suspected based on clinical findings, patient risk factors, and/or radiological findings, laboratory testing for specific agents can be useful. However, it can be difficult to detect the infectious agent in atypical pneumonia, and empirical therapy is often relied on (
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications that might affect the lab results?
Administration of antibiotics prior to specimen collection may reduce the sensitivity of culture or PCR.
Timing of specimen collection may affect the results of antibody testing. If serum is collected early in the course of disease, an antibody response may not be detectable. If results are negative on an acute-phase serum sample, it may be useful to retest the acute-phase serum along with a paired convalescent-phase serum specimen.
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