Rickettsia and Orientia

Rickettsia and Orientia are obligately intracellular bacteria that are transmitted by ticks, mites, fleas, and lice. Disseminated endothelial infection results in febrile illness, often manifesting as headache, myalgia, and rash and, in severe cases, interstitial pneumonia and encephalitis. Rocky Mountain spotted fever, Rickettsia parkeri infection, murine typhus, and rickettsialpox are endemic in the United States. Scrub typhus, caused by Orientia tsutsugamushi, is a highly prevalent disease in southern and eastern Asia and islands of the western Pacific and Indian Oceans. African tick bite fever occurs frequently in travelers returning from South Africa. Laboratory-confirmed diagnosis is seldom achieved during the acute stage of illness, when therapeutic decisions are crucial, although effective direct detection of rickettsiae by immunohistochemical and molecular methods is possible. Serologic methods, the mainstay of diagnosis, do not usually detect antibodies until the second week of illness. Seroconversion or a 4-fold rise in titer during convalescence emphasizes that this approach provides a retrospective diagnosis. Cultivation of rickettsiae requires antibiotic-free cell culture and a biosafety level 3 biocontainment facility and procedures and does not provide a timely diagnosis.