Rickettsiae comprise a group of microorganisms that phylogenetically occupy a position between bacteria and viruses. The genus Rickettsia is included in the bacterial tribe Rickettsiae, family Rickettsiaceae, and order Rickettsiales. They are obligate intracellular gram-negative coccobacillary forms that multiply within eukaryotic cells. Rickettsiae do not stain well with Gram stain, but they take on a characteristic red color when stained by the Giemsa or Gimenez stain. They have typical gram-negative cell walls and lack flagella. Their genome is very small, composed of 1-1.5 million bases.
Rickettsiae are a rather diverse collection of organisms with several differences; this prohibits their description as a single homogenous group. A general characteristic of rickettsiae is that mammals and arthropods are natural hosts. Rickettsioses are usually transmitted to humans by arthropods. Q fever, traditionally described among the rickettsial illnesses until recently, is primarily acquired by inhalation of contaminated airborne droplets.
The epidemiology of human diseases caused by rickettsiae is intimately related to the biology of the vector that transmits it. Rickettsial diseases widely vary in severity from self-limited mild illnesses to fulminating life-threatening infections.
Rickettsial illnesses, caused by organisms within the genus of rickettsiae, are recognized and can be divided into the following 3 biogroups:
Spotted fever biogroup (15 rickettsioses)
See the list below:
Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii
Rickettsialpox, caused by Rickettsia akari
Boutonneuse fever (ie, Kenya tick-bite fever, African tick typhus, Mediterranean spotted fever, Israeli spotted fever, Indian tick typhus, Marseilles fever)
These are similar diseases that differ epidemiologically. The causative organisms (Rickettsia prowazekii and Rickettsia typhi) are similar to those of the spotted fever group but are antigenically distinct.
Louse-borne (epidemic) typhus
Brill-Zinsser disease (ie, relapsing louse-borne typhus)
Murine (endemic or flea-borne) typhus
Scrub typhus biogroup (Tsutsugamushi disease)
The rickettsial agents of scrub typhus have a single taxonomic name: Orientia tsutsugamushi. However, these organisms represent a heterogeneous group that strikingly differs from Rickettsial species of the spotted fever and typhus groups. The 3 major serotypes are Karp, Gilliam, and Kato.
Other rickettsioses and closely related illnesses
See the list below:
New or reemerging rickettsioses have been described in the last few decades, including tickborne lymphadenopathy (TIBOLA) and Dermacentor -borne-necrosis-eschar-lymphadenopathy (DEBONEL) related to Rickettsia slovaca infection, as well as lymphangitis-associated rickettsiosis attributed to Rickettsia sibricia infection.
Recently, a new Rickettsia species, 364D, that causes an eschar-associated illness was identified in California.
Ehrlichia organisms (the cause of human monocytic ehrlichiosis and Ehrlichia ewingii infection), Anaplasmaphagocytophilum (the cause of human granulocytic anaplasmosis), and Bartonella species (the cause of Catscratch disease, relapsing fever, and Trench fever) are organisms related to the rickettsiae. They are discussed in separate articles.
Q fever is a disease caused by Coxiella burnetii, which has recently been removed from the Rickettsiales.
The disease is described here for comparison with other rickettsioses.
The environmental stability, small size, aerosol transmission, persistence in infected hosts, low infectious dose and high associated morbidity and mortality have made pathogenic rickettsiae desirable bioterrorism agents. In fact, R prowazekii and C burnetii have been weaponized. However, developing rickettsial pathogens as biological weapons has many drawbacks, such as the lack of direct host-to-host transmission and availability of therapeutic countermeasures against them.