Table 1.

Disease pathogenesis of selected obligate intracellular bacteria

	Coxiella burnetii	Chlamydia Trachomatis	Anaplasma phagocytophilum	Ehrlichia chaffeensis	Rickettsia spp.	Orientia tsutsugamushi
Disease	Q fever97	Genital infection and trachoma58	Anaplasmosis69	Ehrlichiosis69	Spotted fever and typhus66	Scrub typhus98
Clinical presentation	Mild-to-severe pneumonia and hepatitis; it may progress to chronic infection97	Genital: 70% have no symptoms99 Trachoma: conjunctival inflammation100	Asymptomatic-to-severe fever, malaise, leukopenia and increased liver enzymes69	Mild-to-severe fever, malaise, leukopenia, increased liver enzymes69 and occasional CNS symptoms101	Fever, respiratory and CNS symptoms, and organ failure66	Fever, disseminated intravascular coagulation and organ failure98
Distribution	Global97	Global58	The Americas, Europe and Asia102	The Americas and Asia69	Global103	Asia, Oceania and Chile98,104
Epidemiology	Ubiquitous in animals; potential for outbreaks among agricultural workers97	130 million new genital infections annually105 and 40 million people with active trachoma (230 million at risk)106	2,600 reported cases annually in the United States107 with increasing incidence108 but limited global estimates	3 cases per million people annually in the United States; increasing incidence108 but limited global estimates	Historically devastating outbreaks, global estimates limited and new species constantly emerging109,110	1 million infections per year; 1 billion people at risk109
Transmission	Inhalation38	Contact with infected fluids58	Tick vector69	Tick vector69	Arthropod vector66	Mite vector98
Major mammalian host cell	Alveolar macrophage38	Epithelial cells58	Granulocytes and endothelial cells69	Monocytes and macrophages69	Endothelial cells66	multiple98
Organs affected	Acute: the lungs Chronic: the heart and liver97	Genital tract and eyes58	Inflammatory lesions in organs and liver damage72	Multiple101	Multiple66	Multiple98
Notes	Highly virulent97; lung endothelium and epithelium are minor targets of infection97	Serovars A–C cause trachoma and serovars D–K cause genital infection58	Lacks LPS and peptidoglycan69; macrophages are minor targets of infection72; antigenic variation69	Lacks LPS and peptidoglycan; antigenic variation69	SFG, typhus group, transitional and ancestral groups66	~40% of genome contains repeated sequences85,111
Genetic tractability	Genetically tractable; axenic medium7	Serovar L2 is genetically tractable24	Transposon library13	Transposon library15	Mutagenesis41,42,49,53 and shuttle vectors29,30	No reports

CNS, central nervous system; LPS, lipopolysaccharide; SFG, spotted fever group.