Tickborne Rickettsial Diseases: Epidemiological studies in China

Rickettsial diseases are vector-borne zoonoses caused by obligate intracellular bacteria within the order Rickettsiales, which was previously described as short, Gram-negative rod bacteria that retained basic fuchsin when stained by the method of Gimenez. As development in molecular technologies, the taxonomy of the fastidious bacterial species in the order Rickettsiales has been modifi ed (Dumler et al. 2001), and certain agents such as Coxiella burnetii, the pathogen of Q fever have recently been removed from this order (Raoult & Roux 1997). Although specialists in the fi eld of rickettsiology frequently disagree over species defi nitions, the taxa as well as names of species or subspecies based on polyphasic taxonomic studies by integrating phenotypic and phylogenetic data (Fournier et al. 2003) are currently accepted and used in this thesis. Three groups of diseases are usually classifi ed as rickettsial diseases. These include (i) rickettsioses caused by the spotted fever group (SFG) and the typhus group rickettsiae of the genus Rickettsia within the family Rickettsiaceae, (ii) ehrlichioses and anaplasmoses due to microorganisms within the family Anaplasmataceae, and (iii) scrub typhus caused by Orientia tsutsugamushi (Raoult & Roux 1997; Dumler et al. 2001; Hechemy et al. 2003; Watt & Parola 2003). Among these rickettsial diseases, scrub typhus is transmitted by trombiculid mites (Watt & Parola 2003), and cat fl ea typhus (also called fl ea-borne spotted fever) due to Rickettsia felis is transmitted by fl ea (Adams et al. 1990; Higgins et al. 1996). Tickborne rickettsial diseases are caused by two groups of intracellular bacteria belonging to the order Rickettsiales, i.e. the SFG of the genus Rickettsia within the family Rickettsiaceae and several genera of Anaplasma and Ehrlichia groups within the family Anaplasmataceae. These pathogens infect and proliferate in the organs of ticks, particularly in the salivary glands, and can be transmitted to animal hosts during feeding (Parola & Raoult 2001). Because each tick species favours particular optimal environmental conditions and biotopes, the geographic distribution of the ticks is usually restricted to a specifi c area (small or large) and tickborne rickettsial diseases are natural focus diseases. This is particularly true for most of the spotted fever rickettsiae, which are maintained in ticks through transstadial (from larvae to nymphs to adults) and transovarial (from one generation to the next through ovaries) transmissions (Raoult & Roux 1997). Ticks are not insects but Arachnids, a class of Arthropods, which also includes