Several tick-borne species of Rickettsia, broadly grouped under the heading “Spotted Fever Group Rickettsia (SFGR)” have been shown to cause human infections.
These pathogens include several species of Rickettsia found in the United States, including:
- R. parkeri, transmitted by the bite of the Gulf coast (Amblyomma maculatum) in the Eastern and southern US, primarily along the coast.
- Pacific Coast tick fever (PCTF), proposed name; (Type strain “Rickettsia species 364D”), transmitted by the Pacific Coast tick (Dermacentor occidentalis). Reported cases primarily from Northern California, the tick vector ranges from coastal Oregon to Baja California and Mexico.
- Rocky Mountain spotted fever (RMSF), Rickettsia rickettsii, transmitted by the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (Dermacentor andersoni), and brown dog tick (Rhipicephalus sanguineus) in the US.
Numerous additional tick-borne Rickettsial species which are pathogenic to humans occur internationally, including but not limited to R. conorii and R. africae. Rickettsial infections with R. africae have been reported as a common cause of fever in travelers returning from South Africa. The brown dog tick and other tick species are associated with RMSF in Central and South America.
Rocky Mountain Spotted Fever (R. rickettsia) is known to occur in Colorado and is transmitted through the bite of the Rocky Mountain wood tick (Dermacentor andersoni), the American Dog tick (Dermacentor variabilis) and the Brown dog tick (Rhipicephalus sanguineus). Contrary to what the name suggests, this disease is actually more prevalent outside of the Rocky Mountains region. Other Rickettsial infections are also more prevalent along the coastal states, though expansion of disease occurrence through Rickettsia parkeri has just recently been noted in southern Arizona, well beyond the recognized geographic range of Gulf Coast Ticks (A. maculatum) ticks. The likely vector for these infections was identified as the Amblyomma triste tick, a Neotropical species only recently recognized in the United States.
The severity of illness may vary depending on the species. For example, human infections with R. parkeri and R. africae are generally considered mild and to some extent self-limiting infections. In contrast, R. conorii and some other infections may be more severe.
- Muscle aches
- Maculopapular rash or petechial rash
- A distinctive “eschar” (blackened or crusted skin) at bite site
Although the severity of infections attributable to SFGR vary greatly depending on the causative species, all suspect patients should be treated as if they have R. rickettsii (Rocky Mountain Spotted Fever) infection. Treatment should not be delayed pending diagnostic testing if Rickettsia is suspected based on clinical presentation, and recent tick bite or exposure to tick habitat.
Multiple eschars may be present if more than one tick bite has occurred. The observance of an eschar at the site of tick bite may help differentiate the infection from Rocky Mountain spotted fever, as eschars are rarely reported with that infection. Prompt treatment is recommended if tick-borne SFGR is suspected, and should never be delayed pending the outcome of diagnostic tests.
Physicians seeking confirmation of infection with a SFGR pathogen may elect to use commercially available serologic assays that diagnose RMSF, and should report it as a case of “Spotted Fever Rickettsiosis” to their state health department.
Different SFGR pathogens cause immune reactions in humans that can be difficult to distinguish with antibody-based laboratory techniques. Serologic cross-reactivity with available tests for R. rickettsii does occur, therefore some human illnesses currently being attributed to Rocky Mountain Spotted Fever in the United States may actually be caused by other SFGR, such as R. parkeri, Rickettsia 364D, or imported spotted fevers.
Polymerase chain reaction (PCR) assay, immunohistochemistry (IHC), and culture isolation of a swab or biopsy from an eschar or rash site may also be used to identify the pathogen. Specimens should be submitted to the state health department in the state where the patient resides, who can submit the samples to CDC for testing.
Tick-borne SFGR infections respond well to treatment with doxycycline, and this is considered the antibiotic of choice for patients of all ages. Treatment should never be delayed pending confirmation of laboratory tests.
Padgett KA, Bonilla D, Eremeeva ME, Glaser C, Lane RS, Porse CC, et al. (2016) The Eco-epidemiology of Pacific Coast Tick Fever in California. PLoS Negl Trop Dis 10(10): e0005020. doi:10.1371/journal.pntd.0005020