Q fever is caused by infection with Coxiella burnetii, a bacterial pathogen that historically has been grouped with rickettsial organisms. Contemporary genetic studies, however, have revealed that the microbe is actually more closely related to Legionella species. Q fever has been a reportable disease in the United States since 1999, but because of chronic underreporting its true incidence is not known.
Humans contract Q fever primarily by inhalation or ingestion, although C. burnetii can be vectored by ticks as well. In nature, the organism is maintained in mammals, birds and ticks. (Thirty-five species of ticks have been identified as natural hosts of the microbe.)
C. burnetii is commonly found in livestock on farms; as a result, infection in humans is strongly associated with exposure to farm animals -- primarily cows, sheep and goats. These animals usually do not manifest disease when infected, although spontaneous abortion occurs with some frequency. Interestingly, the organism appears to become more active in pregnant animals. Most commonly, humans acquire the pathogen by Coxiella burnetii organisms. Photo courtesy of Rocky Mountain Laboratories, NIAID, NIH.
inhaling air contaminated with digestive waste products, birth fluids and/or placental remains of farm livestock. Thus, farms and food processing establishments are the most frequent sites of Q fever outbreaks.
C. burnetii is highly infectious – susceptible humans can be infected by a single organism - and is also resistant to heat, drying and many disinfectants. Because of this, it is considered a significant threat for biowarfare and is classified as a Category B agent of bioterrorism.
Signs and Symptoms
Roughly 50% people infected with Q fever will exhibit signs of clinical illness. Manifestations range widely, and vary somewhat with geographical location. There are no findings specific to the disease, but common initial signs include high fever, headache, sore throat, malaise, nausea and diarrhea. Chest pain and non-productive cough are also common. At least one-third of symptomatic patients develop pneumonia after exposure to C. burnetii. In addition, more than half of patients with symptoms will have abnormal liver function tests, with some developing granulomatous hepatitis.
Although the vast majority of patients with acute Q fever recover even without treatment, a chronic form of the disease can develop anywhere from one to twenty years after initial exposure. The most serious manifestation of chronic Q fever is endocarditis, the risk of which is significantly higher in patients with pre-existing valvular disease. Infected patients with prior kidney disease or cancer are more likely to develop chronic Q fever, as are transplant recipients. The mortality rate for chronic Q fever is high, approaching 65%.
About 1% of patients with Q fever develop neurological Fibrin ring granuloma of the liver due to Q fever. Photo courtesy of Yale Rosen MD.
manifestations such as meningitis, encephalitis, myelitis and/or peripheral neuropathy. These manifestations apparently can occur in either
the acute or chronic stage of the illness.
Because the signs and symptoms of Q fever are non-specific, diagnosis is based on a combination of clinical presentation, history and serologic tests. Chest radiographs can be useful in patients with respiratory manifestations; an atypical pneumonia pattern is the most common finding.
The most frequently used and dependable serologic method for Q fever diagnosis is indirect immunofluorescent antibody (IFA) testing. Enzyme-linked immunosorbent assays (ELISA), complement fixation (CF) analyses and immunohistochemical staining tests are also available. Polymerase chain reaction (PCR) tests exist, but are generally used only as a research tool. Culturing of C. burnetii is also possible, but is rarely performed, as laboratory-transmitted cases of Q fever have been reported.
The preferred treatment for Q fever is doxycycline, ideally initiated within 3 days of illness onset. Quinolone antibiotics are also active against C. burnetii, and are sometime used as an alternative. A common doxycycline regimen for acute Q fever is two to three weeks of 200-300 milligrams per day. If signs and symptoms persist or if the patient relapses, treatment is extended or reinitiated.
Chronic Q fever is significantly more difficult to treat. In patients with endocarditis, open ended treatment with doxycycline in combination with either a quinolone or hydroxychloroquine (Plaquenil) is frequently employed. The treatment period for the doxycycline/quinolone combination is usually at least 4 years; for the doxycycline/hydroxychloroquine combination, periods of 1.5 – 3 years have been studied. The latter combination seems to be associated with fewer relapses, although it requires regular eye exams to detect possible chloroquine accumulation.
| Top |