infectious diseases
Overview of Rickettsial and Related Infections
Introduction
Rickettsial diseases (rickettsioses) and related diseases (anaplasmosis, ehrlichiosis, Q fever, scrub typhus) are caused by a group of gram-negative, obligately intracellular coccobacilli. All, except forCoxiella burnetii, have an arthropod vector. Symptoms usually include sudden-onset fever with severe headache, malaise, prostration, and, in most cases, a characteristic rash. Diagnosis is clinical, confirmed by immunofluorescence assay or polymerase chain reaction (PCR). First-line treatment is withdoxycycline. Rickettsia,Orientia,Ehrlichia,Anaplasma, andCoxiellaspecies were once thought to belong to the same family but now, based on genetic analysis, are considered distinct entities. Although this group of organisms require living cells for growth, they are true bacteria because they have metabolic enzymes and cell walls, use oxygen, and are susceptible to antibiotics. These organisms typically have an animal reservoir and an arthropod vector; exceptions areR. prowazekii, for which humans are the primary reservoir, andC. burnetii, which does not require an arthropod vector. For some organisms (eg,R. rickettsiiand other spotted fever rickettsia, andR. akari,R. felis, andOrientia), the vector is also the reservoir, and the geographic distribution of these rickettsia is determined by that of the infected arthropod. Specific vectors, reservoirs, and endemic regions differ widely (see table). There are many rickettsial species, but 3 cause most human rickettsial infections: R. rickettsii R. prowazekii R. typhi 