Infectious agent
The infectious agent is Chlamydophila pneumoniae, an obligate intracellular bacterium (previously
named Chlamydia pneumoniae).
Identification
Clinical features
Chlamydophila pneumoniae infection is often mild. The initial
infection appears to be the most severe with reinfection often asymptomatic. A
spectrum of illness from pharyngitis and sinusitis to pneumonia and bronchitis
may occur. Sometimes there is a biphasic illness
with initial upper respiratory
tract infection symptoms which
resolve and then a dry cough and low grade fever. The organism may be an
infectious precipitant of asthma and is implicated in about 5% of episodes of
acute bronchitis. Cough occasionally persists for some
weeks despite appropriate antibiotic therapy.
Method of diagnosis
Chest X-ray may show small infiltrates. Most cases of pneumonia
are mild but the illness can be severe in otherwise debilitated patients. Laboratory
diagnosis is made with serology or culture:
• Serological diagnosis is made by detecting a four fold rise in
antibody
titre using microimmunofluorescence (MIF). MIF is the only
serological test
that can reliably differentiate chlamydial species. A single
antibody titre is of little diagnostic value on its own as the seroprevalence
of antibodies to C. pneumoniae approaches 50% in the adult
population. Seroconversion may take up to eight weeks in an
initial infection but it tends to occur much more quickly in reinfection (one
to two weeks). False positive antibody tests
can occur in the presence of a positive rheumatoid factor.
• Culture of nasopharyngeal aspirates, throat swabs or bronchial
lavage fluid is possible. Swabs should be placed in chlamydia transport medium
whilst other specimens can be collected in the usual containers. All samples
should be kept refrigerated. Diagnosis by PCR is available through the Victorian
Infectious Diseases Reference Laboratory (VIDRL) but it is currently only being
used in investigation of outbreaks
of respiratory illness where conventional testing has not revealed
the cause ofinfection.
