In addition to Rickettsia rickettsii, the cause of Rocky Mountain spotted fever (RMSF), several other tickborne species of Rickettsia, broadly grouped under the heading "Spotted Fever group Rickettsia (SFGR)" have been shown to cause human infections. Tickborne SFGR are transmitted to humans by the bite of an infected tick, and may cause similar signs and symptoms to those observed for RMSF.
These pathogens include several species of Rickettsia found in the United States, including R. parkeri and Rickettsia species 364D. In addition, numerous tickborne SFGR pathogenic to humans have been described internationally, including but not limited to R. conorii and R. africae.
Travelers may be at risk for exposure to these pathogens when engaging in behaviors that place them at risk for tick exposure, and physicians should maintain awareness of these diseases when a patient presents with a febrile illness within 2 weeks of returning home. Rickettsial infections with R. africae have been reported as a common cause of fever in travelers returning from South Africa.
The following ticks are known to transmit Rickettsia in humans:
- Gulf Coast tick (Amblyomma maculatum) – Rickettsia parkeri
- Pacific Coast tick (Dermacentor occidentalis) – Rickettsia sp. 364D
Signs + Symptoms
The initial symptoms of most tickborne SFGR infections generally include:
- muscle aches
A petechial rash (red or purple spots) may be present, and frequently a distinctive eschar (blackened or crusted skin) may develop at the site of a tick bite.
Prompt treatment with doxycycline is recommended if tick-borne SFGR is suspected, and should never be delayed pending the outcome of diagnostic tests.