Tick-borne rickettsial diseases

Bacteria of the genus Rickettsia belong to the family Rickettsiaceae in the order Rickettsiales and have for long been described simply as short, Gram-negative, strict intracellular rods that retain basic fuchsin when stained by the method of Gimenez (Raoult and Roux 1997). These bacteria are associated with ticks, lice, fleas or mites. To date the Rickettsia genus contains 24 recognized species classified into three groups based on their antigenic, morphological, and ecologic patterns: 1) the typhus group, 2) the spotted fever group and 3) Rickettsia bellii (Fournier and Raoult 2007). Most spotted fever group (SFG) rickettsiae are closely associated with ticks belonging to the family Ixodidae (also called “hard” ticks) (Parola et al. 2005). Ticks can act as vectors, reservoirs, and/or amplifiers of SFG rickettsiae and require optimal environmental conditions which determine the geographic distribution of the vectors and consequently the risk areas for rickettsioses. Many Rickettsia species are strictly associated with one genus of ticks and the transmittion to people is made through the tick bite, which generally implies that the Rickettsia can localize to their salivary glands. Therefore, since larvae, nymphs, and adults may all be infective for susceptible vertebrate hosts, the ticks must be regarded as the main reservoir host of rickettsiae. Humans are not considered as good reservoirs for Rickettsiae, as they are seldom infested with ticks for long periods and rickettsiaemia has normally short duration, especially with antibiotic intervention.