Chapter 290

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.

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

### Clinical Manifestations

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.

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 chronic Q fever. Endocarditis occurs almost exclusively in patients who have had previous valvular ...

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