ABSTRACT
Introduction Rickettsiae were not discovered until the twentieth century. These organisms were first described as short Gram-negative rods that retained fuchsine when stained by the method of Gimenez. They are obligate intracellular bacteria. The taxonomic classification of the genera of rickettsias has been subject to constant modification and recent developments in molecular taxonomic methods have resulted in reclassification within the Rickettsiales. However, there are two groups of diseases that are still usually called 'rickettsioses'. These include scrub typhus, due to Orientia tsutsugamushi, and disease due to bacteria of the genus Rickettsia, including the spotted fever group and the typhus group. The spotted fever group comprises more than 30 species and the typhus group consists of the two species R. prowazekii and R. typhi. These agents are associated with arthropods such as ticks, mites, fleas and lice, which may act as the vector and/or reservoir of the organism. These vectors play an important role in the transmission of rickettsial micro-organisms. They require specific optimal environmental conditions, and this determines the geographical distribution of the vector and consequently the areas where there is a risk of rickettsiosis. The reservoir (flying squirrels, for example) can play an important role in the spread of the disease and is responsible for sporadic cases of R. prowazekii in the US.
