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Rocky Mountain
Spotted Fever Home > The Organism
The Organism
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| Gimenez
stain of tick hemolymph cells infected with R.
rickettsii |
Rocky Mountain spotted fever is caused by Rickettsia
rickettsii, a small bacterium that grows inside
the cells of its hosts. 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.
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.
Taxonomy
The genus Rickettsia is included in the bacterial
tribe Rickettsieae, family Rickettsiaceae,
and order Rickettsiales. This genus includes
many other species of bacteria associated with human
disease, including those in the spotted fever group
and in the typhus group. More than 20 species are currently
recognized in genus Rickettsia but not all are
known to cause disease in humans. Other genotypes are
also known but they have not been classified yet as
valid species and new agents are being discovered in
many areas of the world.
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