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Coxiella burnetii is
an obligate intracellular pleomorphic gram-negative coccobacillus
that was originally named Rickettsia burnetii.
Extensive changes in the taxonomy of rickettsiae based on the sequencing
of the 16S rRNA has resulted in the removal of Cburnetii from
the order Rickettsiales and now finds it in the order Legionellales falling
into the gamma group of proteobacteria along with Legionella
pneumophila, and Francisella tularensis.1,2 The
reader should not become too comfortable with this new classification
because as new Coxiella-like organisms are found,
the phylogenetic relationships of this group of organisms will be
readdressed in the future.
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Cattle, sheep, and goats are the primary reservoirs for infections
resulting from Coxiella burnetii, although there
are an increasing number of cases that have been reported following
occasional contact with cats, rabbits, and dogs in an urban setting.3 Infection
in humans most often occurs after inhalation of aerosolized organisms or
with ingestion of raw milk or fresh goat cheese.4Reactivation
of infection can occur in female mammals during pregnancy where
high concentrations of C burnetii can be found
in the placenta. Animal-to-human transmission can occur during parturition
of such animals by direct aerosol transmission.5 Tick
vectors may be important in maintaining animal reservoirs, but are usually
not responsible for human disease.6 Q fever is
endemic in virtually every country in the world, especially those
areas where cattle are raised and sheep and goats are herded. Little
is known about the pathologic process associated with infection
because most patients recover from their illness. Evidence for human
intrauterine infection has also been reported.7
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Clinical Manifestations
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The incubation period for Q fever is usually between 14 and 22
days (range 2–6 weeks). The severity of illness in children
is varied and difficult to document because published data on infections in
children are limited.8,9 Acute illness in older patients
is usually manifested by an abrupt onset of fever, chills, weakness,
headache, and anorexia. Cough and chest pain should alert the clinician
to the possibility of pneumonia that occurs in approximately 50% of
patients. Symptoms are exacerbated during temperature spikes, whereas patients
frequently feel well during afebrile intervals. In patients younger
than age 3 years, the presentation is usually one of persistent
fever without respiratory manifestations. Although pneumonitis is
a hallmark of this illness, Q fever is a systemic illness. Hepatosplenomegaly
and gastrointestinal manifestations (eg, vomiting, abdominal pain)
are frequently noted; rash is unusual in adults but may be more
likely to develop in children. Most patients with Q fever improve with
or without specific antimicrobial therapy.
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A small number of patients (< 1% of adults) do not
clear the organism and develop a chronic illness.8 The
risk for developing chronic infection, however, is correlated with
advancing age. Children, therefore, are infrequently diagnosed with
chronic illness.8,9 Endocarditis is the major form
of ...