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Description:This photograph depicts a dorsal view of both a female (Lt), and male (Rt) Rocky Mountain wood tick, Dermacentor andersoni. This tick specie is a known North American vector of Rickettsia rickettsii, which is the etiologic agent of Rocky Mountain spotted fever. Note the smaller size of the female’s scutum compared to the male’s larger scutum. From the Latin word for “shield”, the scutum, i.e., dorsal shield, covers only a small part of the female’s dorsal surface, thereby, enabling her abdomen to expand, becoming engorged during feeding. The male’s scutum covers his entire dorsal surface, and is mottled with brown markings overall, while the female’s small scutum sports an almost entirely a cream-colored gray surface.
Rickettsia rickettsii usually infects members of the tick family Ixodidae (hard ticks), although a closely related rickettsia has been found in the soft bat tick, Carios kelleyi. These ticks have four stages in their life cycle: egg, larva, nymph, and adult. After the eggs hatch, each stage must feed once to develop into the next stage. Both male and female ticks will bite.

Rickettsiae are transmitted to a vertebrate host through saliva while a tick is feeding. It usually takes several hours of attachment and feeding, before the rickettsiae are transmitted to the host. The risk of exposure to a tick carrying R. rickettsii is low. In general, about 1%-3% of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported.

High Resolution: Click here for hi-resolution image (54.03 MB)
Content Providers(s):CDC/ Dr. Christopher Paddock
Creation Date:2008
Photo Credit:James Gathany
Links:CDC – Div. of Viral and Rickettsial Diseases National Center for Zoonotic, Vector-Borne, and Enteric Diseases; Rocky Mountain Spotted Fever
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Copyright Restrictions:None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.