Coxiella burnetii is a pleomorphic, Gram-negative obligate intracellular coccobacillus measuring approximately 0.2 μm x 0.7 μm. The spore-like form, produced in infected host cells, is resistant to drying and environmental influences and can survive for months in water and food. It is extremely infective to humans.
The zoonotic pathogen exists in a wide range of animal hosts, including domesticated livestock (especially cattle, sheep, goats) cats, dogs, rodents, baboons and wild birds. The enzootic cycle includes numerous species of ixodid and argasid ticks. Arthropod vectors, however, do not play a significant role in transmission to humans.
Mode of transmission
Transmission to humans occurs primarily by inhalation of dust, droplets or aerosols from parturient fluids and excreta of infected livestock. Contaminated droplets and dust may also infect the conjunctivae and abraded skin. Inhalation of only a few organisms is sufficient to cause infection. Contaminated aerosols released to the atmosphere may cause infection at distances up to several kilometres from their source.
Sporadic human infections may also result from ingestion of unpas- teurized dairy products. High-temperature pasteurization is sufficient to kill the organism. Person-to-person transmission has been reported but is rare.
Incubation period The incubation period is usually 18–21 days, but can be less if large doses of the organism are inhaled.
The onset may be sudden, with chills, fever, sweating, headache, loss of appetite, malaise, and muscle and chest pains. There may also be nausea, vomiting and diarrhoea. In severe cases the disease progresses to extreme stiffness of the neck and back, disorientation and pneumonia. The fatality rate is usually less than 1%, although somewhat higher rates have been reported in some outbreaks. Weakness and fever may continue for months. Long-term complications are uncommon but may include endocarditis. Asymptomatic infections routinely occur and may be revealed by serology.
Isolation and microbiological identification of the organism from blood or other clinical materials is a valid diagnostic test but is hazardous to personnel. Specific and relatively rapid identification of the organism in blood or paraffin-embedded tissue may be accomplished by PCR assays. Serological diagnosis may be performed by complement fixation, indirect immunofluorescent antibody test or ELISA.
Biosafety Level 2 practices, equipment and facilities are recommended for activities not involving propagation of the pathogen and involving only limited manipulation of infected materials, such as microscopic and serological examinations. Biosafety Level 3 is recommended for activities involving the handling of infected human or animal tissues or isolation of the pathogen.
Patient isolation is not required. Patient materials and contaminated articles should be autoclaved, incinerated or disinfected with solutions containing hypochlorite, peroxide, 70% ethanol, phenol or a quaternary ammonium compound.
Prophylaxis and therapy
A formalin-inactivated vaccine, commercially available in Australia, has been developed for laboratory workers and others at high risk. Tetracyclines, particularly doxycycline, are effective if given early and may abort the infection if administered before symptoms appear.