C. burnetii is the causative agent of Q fever, a zoonotic diseasetransmitted from animals to humans. Q fever was first studied in an experimental infection of guinea pigs by inoculation of blood from patients suffering from typhus-like fever to guinea pigs by Derrick in 1935. As the etiological agent of the disease was not known, the condition was referred to as query or Q fever. Subsequently, Burnett identified the causative agent as a rickettsial, after which the pathogen was named as Rickettsiaburnetii. Cox in the United States demonstrated the agent inticks and named it Rickettsia diaphorica. Later, both the rickett-sial strains were shown to be identical. The organism is now named as C. burnetii and reclassified in the group Protobacteria.
burnetii shows following features:
· C. burnetii is an obligate intracellular bacterium.
· It is pleomorphic and measures 1 3 0.3 mm in size.
· The bacterium is Gram negative, but stains poorly with Gram staining. These bacteria like other rickettsiae stain best with Giemsa or Gimenez stains.
C. burnetii primarily infects the monocytic macrophage cell. Itmultiplies in phagolysosomes of the infected cell. It grows well in various cell lines and in the yolk sac.
Susceptibility to physical and chemical agents: Coxiellaaremost resistant pathogens to heat and dryness. They are not completely killed at 608C or by 1% phenol in 60 minutes. Hence, pasteurization by holding method is not effective method for killing C. burnetii, but the flash method is effective. The bacte-ria survive for a month in meat and for a year or more in dried feces at 48C.