Rickettsial diseases

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. [1] Parola P, Paddock CD, Socolovschi C, et al. Update on tick-borne rickettsioses around the world: a geographic approach. Clin Microbiol Rev. 2013 Oct;26(4):657-702. http://cmr.asm.org/content/26/4/657.long http://www.ncbi.nlm.nih.gov/pubmed/24092850?tool=bestpractice.com [Figure caption and citation for the preceding image starts]: Male yellow dog tick (Amblyomma aureolatum) CDC/Dr Christopher Paddock and James Gathany (photographer) [Citation ends]. [Figure caption and citation for the preceding image starts]: Female body louse (Pediculus humanis corporis) obtaining a blood meal from a human host CDC/Frank Collins, Ph.D. and James Gathany (photographer) [Citation ends].