The genus Coxiella was originally classified with rickettsia because the bacteria showed features of a rickettsial pathogen. Like rick-ettsiae they (a) stain poorly with Gram staining, (b) multiply intracellularly in eukaryotic cells, and (c) are transmitted by arthro-pods. Coxiella differs from rickettsiae by the following features:
They are not transmitted by arthropod vectors but are transmitted by inhalation or ingestion.
They show relatively more resistance to actions of dry heat.
The genus Coxiella now has been separated from rickettsiae and is placed in the group Protobacteria along with other genera, such as Legionella and Francisella. This is because Coxiellawas found to be more closely related to these two genera. The genus Coxiella includes the species C. burnetii, which causes Q fever.
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.