(See also Overview of Rickettsial and Related Infections.)
Coxiella burnetii is a small, intracellular, pleomorphic bacillus that is no longer classified as Rickettsia. Molecular studies have reclassified it as Proteobacteria in the same group as Legionella species.
Q fever can be
Acute disease causes a febrile illness that often affects the respiratory system, although sometimes the liver is involved. Women infected during pregnancy have an increased risk of spontaneous abortion and preterm delivery.
Chronic Q fever occurs in < 5% of patients. It usually manifests as endocarditis or hepatitis; osteomyelitis may occur.
Worldwide in its distribution, Q fever is maintained as an inapparent infection in domestic or farm animals. Sheep, cattle, and goats are the principal reservoirs for human infection. C. burnetii persists in stool, urine, milk, and tissues (especially the placenta), so that fomites and infective aerosols form easily. C. burnetii is also maintained in nature through an animal-tick cycle, but arthropods are not involved in human infection.
Cases of Q fever occur among workers whose occupations bring them in close contact with farm animals (which are often asymptomatic) or their products. Transmission is usually by inhalation of infected aerosols that can travel long distances affecting people living downwind of an infected goat or sheep farm. The disease can also be contracted by ingesting infective raw milk.
C. burnetii is very virulent, resists inactivation, and remains viable in dust and stool for months; even a single organism can cause infection. Because of these characteristics, C. burnetii is a potential biological warfare agent.
Very rarely, the disease is transmitted from person to person.
The incubation period averages 18 to 21 days (range 9 to 28 days). Acute Q fever is often asymptomatic; in other patients, it begins abruptly with influenza-like symptoms: fever, severe headache, chills, severe malaise, myalgia, anorexia, and sweats. Fever may rise to 40° C and persist 1 to > 3 weeks.
Rarely, acute Q fever manifests as encephalitis or meningoencephalitis.
Respiratory symptoms (a dry nonproductive cough, pleuritic chest pain) appear 4 to 5 days after onset of illness. These symptoms may be particularly severe in older or debilitated patients. During examination, lung crackles are commonly noted, and findings suggesting consolidation may be present. Unlike rickettsial diseases, acute Q fever does not cause a rash.
Acute hepatic involvement, occurring in some patients, resembles viral hepatitis, with fever, malaise, hepatomegaly with right upper abdominal pain, and possibly jaundice. Headache and respiratory signs are frequently absent.
Chronic Q fever may manifest within a few weeks to many years after the initial infection. Hepatitis may manifest as fever of unknown origin. Liver biopsy may show granulomas, which should be differentiated from other causes of liver granulomas (eg, tuberculosis, sarcoidosis, histoplasmosis, brucellosis, tularemia, syphilis).
Endocarditis resembles viridans group subacute bacterial endocarditis; the aortic valve is most commonly affected, but vegetations may occur on any valve. Marked finger clubbing, arterial emboli, hepatomegaly, splenomegaly, and a purpuric rash may occur.
The case-fatality rate is only about 1% in untreated patients but is higher in those with endocarditis. Some patients with neurologic involvement have residual impairment.
Symptoms do not readily suggest the diagnosis of Q fever. Early on, Q fever resembles many infections (eg, influenza, other viral infections, salmonellosis, malaria, hepatitis, brucellosis). Later, it resembles many forms of bacterial, viral, and mycoplasmal and other atypical pneumonias. Contact with animals or animal products is an important clue.
IFA of infected tissue is the diagnostic method of choice; alternatively, enzyme-linked immunosorbent assay (ELISA) may be done. Acute and convalescent serum specimens (typically complement fixation) may be used. Antibodies to phase II antigen are used to diagnose acute disease, and antibodies to both phase I and phase II antigens are used to diagnose chronic disease.
PCR can identify the organism in biopsy specimens, but negative results do not rule out the diagnosis.
C. burnetii may be isolated from clinical specimens, but only by special research laboratories; routine blood and sputum cultures are negative.
Patients with respiratory symptoms or signs require chest x-ray; findings may include atelectasis, pleural-based opacities, pleural effusion, and lobar consolidation. The gross appearance of the lungs may resemble bacterial pneumonia but, histologically, more closely resembles psittacosis and some viral pneumonias.
In acute Q fever, complete blood count may be normal, but about 30% of patients have an elevated white blood cell count. Alkaline phosphatase, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) levels are mildly elevated to 2 to 3 times the normal level in typical cases. If obtained, liver biopsy specimens often show diffuse granulomatous changes.
For acute Q fever, primary treatment is doxycycline 200 mg orally once followed by 100 mg orally twice a day in adults until the patient improves, has been afebrile for about 5 days, and has received treatment for at least 7 days; typically, 2 to 3 weeks of treatment is required. Tetracycline resistance has not been documented.
For endocarditis, treatment needs to be prolonged (months to years to lifelong), typically for at least 18 months. Doxycycline 100 mg orally twice a day in adults plus hydroxychloroquine 200 mg orally every 8 hours is currently recommended. Owing to the cardiac adverse effects of hydroxychloroquine, patients on this drug should have their QTc interval monitored with repeated ECGs. Clinical signs, erythrocyte sedimentation rate, blood count, and antibody titers should be monitored to help determine when to stop treatment. Consultation with an infectious disease specialist may help with managing the complexities of the disease and its treatment. Frequently, antibiotic treatment is only partially effective, and damaged valves must be replaced surgically, although some cures have occurred without surgery.
For chronic granulomatous hepatitis, the optimal regimen has not been determined.
Vaccines are effective, and in Australia, where a Q fever vaccine is commercially available, vaccination is recommended to protect people with occupational risk (eg, slaughterhouse and dairy workers, rendering-plant workers, herders, woolsorters, farmers).
Prevaccination screening with skin and blood tests should be done to identify preexisting immunity to Q fever because vaccinating people who already have immunity can cause severe local reactions.
Sheep, cattle, and goats are the principal reservoirs for human Q fever infection, which occurs worldwide.
Transmission to humans is usually by inhalation of infected aerosols; arthropods are not involved.
Acute symptoms resemble influenza; respiratory symptoms may be particularly severe in older or debilitated patients.
Chronic Q fever occurs in < 5% of patients and usually manifests as endocarditis or hepatitis.
Diagnose using immunofluorescence assay or PCR testing of infected tissue.
Treat acute Q fever with doxycycline, typically for 2 to 3 weeks; endocarditis requires prolonged treatment (months to years to lifelong).
A vaccine to prevent Q fever is commercially available but only in Australia.