Pathophysiology
Entry into host
Rickettsia rickettsii can be transmitted to human hosts through the bite of an infected tick. As with other bacterium transmitted via ticks, the process generally requires a period of attachment of 4 to 6 hours. However, in some cases a Rickettsia rickettsii infection has been contracted by contact with tick tissues or fluids. Then, the bacteria induce their internalization into host cells via a receptor-mediated invasion mechanism.Researchers believe that this mechanism is similar to that of Rickettsia conorii. This species of Rickettsia uses an abundant cell surface protein called OmpB to attach to a host cell membrane protein called Ku70. It has previously been reported that Ku70 migrates to the host cell surface in the presence of "Rickettsia". Then, Ku70 is ubiquitinated by c-Cbl, an E3 ubiquitin ligase. This triggers a cascade of signal transduction events resulting in the recruitment of Arp2/3 complex. CDC42, protein tyrosine kinase, phosphoinositide 3-kinase, and Src-family kinases then activate Arp2/3. This causes the alteration of local host cytoskeletal actin at the entry site as part of a zipper mechanism. Then, the bacteria is phagocytosized by the host cell and enveloped by a phagosome.Studies have suggested that rOmpB is involved in this process of adhesion and invasion. Both rOmpA and rOmpB are members of a family of surface cell antigens (Sca) which are autotransporter proteins; they act as ligands for the Omp proteins and are found throughout the rickettsiae.
Rocky Mountain spotted fever was first identified in the 1800s in the Rocky Mountains. Signs and symptoms
Spotted fever can be very difficult to diagnose in its early stages, due to the similarity of symptoms with many different diseases.People infected with R. rickettsii usually notice symptoms following an incubation period of one to two weeks after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is nonspecific and may resemble a variety of other infectious and noninfectious diseases. Initial symptoms:
Fever
Nausea
Emesis (vomiting)
Severe headache
Muscle pain
MalaiseLater signs and symptoms:
Maculopapular rash
Petechial rash
Abdominal pain
Joint pain
Conjunctivitis
ForgetfulnessThe classic triad of findings for this disease are fever, rash, and history of tick bite. However, this combination is often not identified when patients initially present for care. The rash has a centripetal, or "inward" pattern of spread, meaning it begins at the extremities and courses towards the trunk. Rash
The rash first appears two to five days after the onset of fever, and it is often quite subtle. Younger patients usually develop the rash earlier than older patients. Most often the rash begins as small, flat, pink, nonitchy spots (macules) on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. The characteristic red, spotted (petechial) rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after onset of symptoms, but this type of rash occurs in only 35 to 60% of patients with Rocky Mountain spotted fever.