The term "Rickettsial diseases" is loosely applied to a variety of infectious diseases caused by gramnegative fastidious bacteria belonging to the genera Rickettsia, Orientia, Ehrlichia, and Coxiella. This grouping is justified by historical aspects of the discovery of each microorganism, by similarities in their microbiological and ecological characteristics, and by their association with arthropod vectors (lice, fleas, ticks, and mites). Rickettsial diseases described in this section include both classical
Table 1
Rickettsial Diseases of Humans
Disease1
Organism
Invertebrate vector
Reservoir/Mammalian host
Typical mode of transmission to humans
Natural cycle
Geographic distribution
1 Newly emerging diseases are indicated with a star (*)
2 More details on epidemic typhus and R. prowazekii-associated infections can be found in the section: "Typhus."
and newly emerging rickettsioses and ehrlichioses (see Table 1).
RICKETTSIOSES
The classical rickettsioses have been divided traditionally into several groups: louse- and flea-borne typhus group rickettsial diseases; tick- and mite-borne spotted fever group (SFG) rickettsial diseases, and chigger-borne scrub typhus. Epidemic typhus, murine typhus, spotted fevers, and scrub typhus share one dominant feature: widespread microvascular injury, which develops as a result of the invasion of, and multiplication of, rickettsiae in the cytoplasm of endothelial cells. Rickettsioses typically begin with an acute onset of symptoms one to two weeks after exposure to rickettsiae. Common symptoms include fever, severe headache, malaise, and myalgia. A rash often appears a few days after the onset of fever, the appearance of
Figure 1
which varies depending upon the type of disease. In scrub typhus, tick typhus, and rickettsialpox a specific skin lesion develops at the site of the infecting arthropod bite (see Figure 1).
Rocky Mountain spotted fever (RMSF), epidemic typhus, and scrub typhus are frequently life-threatening illnesses when left untreated. Murine typhus and the other spotted fever infections are typically milder, but they may have fatal outcomes in weakened patients.
Epidemic and recrudescent typhus, caused by R. prowazekii, are fatal to their infective louse hosts. Other forms, however, do not kill their arthropod hosts, increasing the possibilities of transmission. Pathogenic species of Rickettsia are also transmitted through their mammalian hosts. Humans get infected when they enter areas infested with infected arthropods. RMSF, endemic typhus, rickettsialpox, sylvatic epidemic typhus, and cat-flea transmitted R. felis infection are indigenous to the United States. Epidemic typhus
and endemic typhus occur worldwide. Mediterranean spotted fever, African tick bite fever, North Asian tick typhus, Flinders Island spotted fever, Oriental spotted fever, Queensland tick typhus and scrub typhus infections all have a specific geographical distribution, which is determined by the distribution of the specific arthropod that carries each disease. Travelers are often infected in one country but exhibit symptoms in another because of the lengthy incubation period for rickettsial diseases.
The specific diagnosis of rickettsioses is most often based on the indirect detection of specific antibodies in the patient's serum, or by detection of rickettsiae in skin biopsy samples. A history of travel, camping, and arthropod bites is particularly important for diagnostic purposes. Doxycycline, tetracycline, and chloramphenicol are the recommended drugs used in the treatment of rickettsioses. Preventive measures include avoiding contact with infected ticks, mites, fleas, lice, and chiggers, and their removal from skin and clothing before transmission of the rickettsia occurs. There is no effective commercial vaccine to prevent these diseases, but several promising experimental vaccines have been developed.
EHRLICHIOSES
Intracellular bacteria in the genus Ehrlichia are the cause of four human diseases that emerged in the later half of the twentieth century (see Table 1). Ehrlichia chaffeensis infects mainly macrophage and monocyte cells, and it causes human monocytic ehrlichiosis (HME). Another isolate, which is not yet officially named, invades human granulocytes and causes human granulocytic ehrlichiosis (HGE). It is closely related to E. ewingii and E. phagocytophila. E ewingii, an organism first associated with illness in dogs, has recently been found to also cause disease in humans. E. sennetsu, the agent of sennetsu ehrlichiosis in humans, is unusual because it is not transmitted by ticks. It appears to be associated with the consumption of raw fish. Several other closely related species of Ehrlichia cause illnesses in dogs, horses, and other animals. In contrast to Rickettsia and Orientia, which grow directly in the cytoplasm of their host cells, ehrlichiae grow in phagosomes where they form clustered, mulberry-like microcolonies called morulae.
Monocytic ehrlichiosis occurs widely in the southern United States. Human granulocytic ehrlichiosis occurs in the United States, Europe, and Africa. Ehrlichiae are transmitted by tick bites, and ticks acquire the bacteria by feeding on infected animals. The American lone star tick is the vector of E. chaffeensis, while the blacklegged tick (deer tick) and western blacklegged ticks are vectors for HGE. The incidence of ehrlichioses correlates with the season of greatest tick activity, peaking from May to July, but the diseases may occur throughout the year in conjunction with human exposure to ticks.
Clinical manifestations of HME and HGE appear after a one- to three-week incubation period and may persist for three to eleven weeks if the diseases are untreated. Ehrlichioses present as a wide range of nonspecific symptoms, including fever, headache, myalgia, gastroenteric dysfunctions, and inflammation of the lymph nodes. Severe cases may be complicated by respiratory insufficiency, neurological symptoms, renal failure, gastrointestinal hemorrhage, and opportunistic viral or fungal infections. Fifty-six to 62 percent of patients require hospitalization, and 2 to 5 percent of patients die.
Clinical diagnoses of ehrlichioses are based on the presence of persisting fever and exposure to ticks in endemic areas. Laboratory diagnosis during the acute stage is achieved by identification of ehrlichial DNA from patient blood samples or detection of cells containing mulberry-like clusters of organisms in peripheral blood smears.
Ehrlichial infections generally respond well to treatment with doxycycline. Ehrlichioses may be prevented by avoidance of tick bites, wearing protective clothing, use of repellents, and prompt removal of attached ticks. Vaccines are not yet available.
