|Description:||This photomicrograph of a Gimenez-stained yolk sac smear revealed the presence of Rickettsia rickettsii bacteria, which are the cause of Rocky Mountain spotted fever (RMSF). These bacteria range in size from 0.2 x 0.5 micrometers to 0.3 x 2.0 micrometers. They are difficult to see in tissues by using routine histologic stains, and generally require the use of special staining methods, such as the Gimenez stain used in this case.|
In the human body, rickettsiae live and multiply primarily within cells that line small- to medium-sized blood vessels. Spotted fever group rickettsiae can grow in the cytoplasm or in the nucleus of the host cell. Once inside the host the rickettsiae multiply, resulting in damage and death to these cells. This causes blood to leak through tiny holes in vessel walls into adjacent tissues. This process causes the rash that is traditionally associated with Rocky Mountain spotted fever, and also causes damage to organs and tissues.