Rickettsioses are caused by obligate intracellular bacteria belonging to the genus Rickettsia. These bacteria are associated with arthropods that may act as vectors. They represent some of the oldest recognized infectious diseases. Epidemic typhus is suspected to be the cause of the Athens plague during the fifth century BC, and was differentiated from typhoid in the sixteenth century AD [1]. At the beginning of the twentieth century, ticks were implicated as reservoirs and vectors of rickettsiae. Ricketts proved that the wood tick, Dermacentor andersoni, was involved in the transmission of Rickettsia rickettsii, the agent of Rocky Mountain spotted fever [2]. Furthermore, he demonstrated that ticks were infective during all feeding stages and that rickettsia was maintained in ticks by transovarial transmission [2]. In 1910, the first cases of Mediterranean spotted fever were reported in Tunis by Conor and Brush [3]. The role of Rhipicephalus sanguineus, the brown dog tick, in the transmission of the disease was established in 1930. Rickettsioses are also some of the most recently recognized infectious diseases. Prior to 1984, only eight rickettsioses were clinically recognized (Table 1) [4] and in the subsequent 13 years a further seven new rickettsial diseases have been described [4, 5]. The recent discoveries of new rickettsioses have not been confined to countries with relatively low levels of medical research; for example, Japanese spotted fever was described in Japan in 1984 (Fig. 1) [6]. The main clinical symptoms of rickettsioses include fever, headache, rash that sometimes includes an inoculation eschar, and local lymphadenopathy (Table 2). Careful clinical ex-

TABLE 1 Old and New Rickettsial Diseases

Year of Rickettsia Disease Vector isolation

Rickettsia prowazekii Epidemic Pediculus humanus corporis 1916 typhus

Rickettsia rickettsii Rocky Moun-Dermacentor andersoni, 1919 tain spotted Dermacentor variabilis fever

Rickettsia typhi Murine typhus Xenopsylla cheopis 1920 Rickettsia conorii Mediterranean Rhipicephalus sanguineus 1932

spotted fever Rickettsia akari Rickettsial pox Allodermanyssus sanguineus 1946 Rickettsia sibircia Siberian tick Dermacentor nuttali, 1949

typhus Dermacentor marginatus, North Asian Haemophysalis concinna

tick typhus Rickettsia australis Queensland tick Ixodes holocyclus 1950

typhus Israeli tick typhus Israeli spotted Rhipicephalus sanguineus 1974

rickettsia fever Rickettsia honei Flinders Island Unknown 1991

spotted fever Astrakhan fever Astrakhan fever Rhipicephalus pumilio 1991

rickettsia Rickettsia africae African tick bite Amblyomma hebraeum, 1992

fever Amblyomma variegatum Rickettsia japonica Japanese or Dermacentor taiwanensis, 1992

Oriental Haemaphysalis flava, spotted Haemaphysalis formosensis, fever Haemaphysalis hystricis,

Haemaphysalis longicornis, Ixodes ovatus

Rickettsia felis Pseudotyphus of Ctenophtalides felis 1994 California

Rickettsia Spotted fever Haemaphysalis asiaticum 1996 mongolotimonae

Rickettsia slovaca Fever Dermacentor marginatus 1997

FIGURE 1 Patient with Japanese spotted fever. (Courtesy of Dr. Mahara).