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 have resulted in the removal of C burnetii from the order Rickettsiales.
PATHOGENESIS AND EPIDEMIOLOGY
Cattle, sheep, and goats are the primary reservoirs for infections resulting from C burnetii, although an increasing number of cases have been reported following occasional contact with cats, rabbits, and dogs in an urban setting. Infection in humans most often occurs after inhalation of aerosolized organisms or with ingestion of raw milk or fresh goat cheese. Reactivation of infection can occur in female mammals during pregnancy where high concentrations of C burnetii can be found in the placenta, resulting in animal-to-human transmission during parturition of such animals by direct aerosol transmission. Tick vectors may be important in maintaining animal reservoirs but are usually not responsible for human disease. Infection can also occur due to contact with contaminated wool, straw, bedding material, or laundry. 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.
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. 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, which 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 (50–80%); 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, although a relapsing illness has been described.
A small number of patients (< 1% of adults) do not clear the organism and develop a chronic illness. The risk for developing chronic infection, however, is correlated with advancing age. Therefore, children are infrequently diagnosed with chronic illness. Endocarditis is the major form of chronic Q fever, but chronic relapsing or multifocal osteomyelitis and chronic hepatitis have been described. Endocarditis occurs almost exclusively in patients who have had previous valvular heart disease or immunosuppression. Bone involvement can be demonstrated in patients with chronic Q fever and is more prevalent among children than it is among adults. Chronic Q fever is difficult to treat and often ends in death.
Q fever should be suspected in febrile patients who live in high-prevalence areas and who are in contact with domestic farm animals. In the United States, animal handlers and laboratory workers make up a significant portion of reported infection. Chest roentgenographic findings for Q fever pneumonia are nonspecific and are similar to those associated with pneumonia caused by viruses, Mycoplasma pneumoniae, or Chlamydophila pneumoniae. Multiple round opacities are commonly seen ...