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.http://repub.eur.nl/res/pub/19763/100602_Cao%2C%20Wu-Chun.pdf
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