Rickettsial infections occur worldwide and are associated with the patient having been bitten by an ectoparasite such as a louse, mite, flea, or tick.
Patients usually present with a fever and may have a rash (usually maculopapular) and an eschar (the bite site of the ectoparasite).
It is important to consider rickettsial infection in acutely unwell patients who recall having been “bitten by something” and/or have travelled to an endemic area.
Epidemic typhus ( Rickettsia prowazekii ) is now rare but may occur in situations of mass crowding, poverty, famine, and war where there is an increased risk of human body louse infestation.
Doxycycline is the drug of choice for treatment in patients of all ages.
Rickettsial infections (rickettsioses) of humans are caused by several related genera of bacteria. These include Rickettsia species, Orientia tsutsugamushi and Orientia chuto , Anaplasma species, Ehrlichia species, and Neoehrlichia species. Sometimes Coxiella burnetii (causing Q fever) is included as a rickettsiosis, but strictly speaking it is not as it belongs to a phylogenetically different clade of bacteria and is more closely related to Legionella species. Rickettsia have two common features: they are obligate intracellular bacteria and will only grow inside a host animal eukaryotic cell, and they live in the cells of both vertebrate (e.g., humans, rats) and invertebrate animals (e.g., lice, fleas, mites, and ticks). While the invertebrate is the main host animal for the rickettsia, it is also the vector for transmission of the rickettsia to humans and other vertebrate animals.
The genus Rickettsia is usually divided into the spotted fever group (SFG), where patients present with fever and spots, and the typhus group (TG). Some scholars also include an ancestral group but this is not yet widely accepted. The SFG consists of many species, about 20 of which can cause human infection. Different species occur in geographically different parts of the world: for example, Mediterranean spotted fever caused by Rickettsia conorii and Queensland tick typhus caused by Rickettsia australis . The SFG are mainly tick transmitted. The TG consists of two species: Rickettsia prowazekii (epidemic louse-borne typhus) which is human body louse transmitted, and Rickettsia typhi (murine typhus) which is rodent flea transmitted. The genus Orientia is mite transmitted. 
Professor of Medicine
Section on Infectious Diseases
Wake Forest University School of Medicine
CAO declares that he has no competing interests.
Professor Christopher A. Ohl would like to gratefully acknowledge Dr Stephen R. Graves, a previous contributor to this topic. SRG is the author of a reference cited in this topic.
Department of Medicine
University of Peradeniya
SAMK declares that he has no competing interests.
Professor of Medicine and Pathology
Albert Einstein College of Medicine
New York City
LW's laboratory research programme is supported by NIH (NIAID) grants. He performs infectious diseases consultation at Montefiore (Weiler Hospital).
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