Rocky Mountain spotted fever




The genus Rickettsia is included in the bacterial tribe Rickettsieae, family Rickettsiaceae, and order Rickettsiales. The genus Rickettsia includes many species of bacteria associated with human or canine disease, including those in the spotted fever group (SFG) and in the typhus group (TG). There are more than 20 valid species currently recognised in the spotted fever group.

The organism

R. rickettsii is a very small, gram-negative, rod-shaped bacterium that obligatory lives inside the cells of its hosts. These bacteria range in size from 0.2 x 0.5 µm to 0.3 x 2.0 µm. The three-layered cell wall evident in very high magnification electron micrographs consists of a bilayered inner membrane, a peptidoglycan layer, and a bilayered outer membrane. Surrounding the cell wall is a coat or slime layer, which is regarded as the potential site of the major group specific antigens, enabling the agent to attach to potential host cells.

Figure 2: Transmission electron micrograph showing the rod-like appearance and the trilaminar wall of R. rickettsii . Measurement bar 0.16 µm.

R. rickettsii is difficult to detect in tissues by using routine histologic staining and generally requires the use of special staining methods.

Figure 3: Gimenez stain of tick hemolymph cells infected with R. rickettsii

Rickettsiae are obligate intracellular bacteria. In humans, they live and multiply primarily within cells that line small- to medium-sized small arteries and venules. Spotted fever group rickettsiae can grow in the nucleus or in the cytoplasm of the host cell. Once inside the host the rickettsiae multiply, resulting in damage and death of these cells.


Further information

  • Shaw SE, Day MJ, Birtles RJ, et al.: Tick-borne infectious diseases of dogs. Trends Parasitol. 2001, 17, 74-80


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