Several tick-borne species of Rickettsia, broadly grouped under the heading “Spotted Fever Group Rickettsia (SFGR)” have been shown to cause human infections in the US and worldwide. An emerging tick-borne disease, Pacific Coast Tick Fever (PCTF, proposed name), is caused by the bacteria Rickettsia philipi (Type strain “Rickettsia species 364D”), long thought to be an agent of human disease since first isolated in 1966. It is transmitted to humans through the bite of an infected Pacific Coast tick (Dermacentor occidentalis). The Pacific Coast tick is found predominantly in shrublands, chaparral, and along trails from Oregon to northern Baja California and Mexico. A variety of small mammals are suspect as hosts for this bacteria.
As of 2016, fourteen cases of PCTF have been reported in California, mostly northern California; the first case was recognized in 2008. More than half of these reported cases have been in children. Findings presented in a recent article implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to humans, with peak transmission risk occurring in late summer, and that there is a higher risk of acquiring PCTF than Rocky Mountain spotted fever (RMSF) in California. Knowing that both larval and nymphal Pacific Coast Ticks bite people and can be infected with R. philipii, it is possible that a person could be exposed to multiple tick bites at one exposure without discovering the ticks, due to their small size.
The Pacific Coast tick (Dermacentor occidentalis) vector of PCTF is not known to occur in Colorado. The closely related Rickettsial infection, 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). Additional Rickettsial infections are also more prevalent along the coastal states, though expansion of disease occurrence through Rickettsia parkerii has just recently been noted in southern Arizona, well beyond the recognized geographic range of 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.
Though closely related to RMSF, the most common feature of PCTF is the presence of at least one necrotic lesion known as an eschar (100%).
- A distinctive “eschar” (blackened or crusted skin) at bite site, may have multiple eschars
- Rash (uncommon)
The observance of an eschar at the site of tick bite, and lack of petechial rash may help differentiate the infection from Rocky Mountain spotted fever, as eschars are rarely reported with that infection. The head, neck, forearm, back and shoulder are the most common sites for eschars in the patients identified.
The clinical presentations of RMSF and PCTF share certain characteristics. Both diseases typically present with fever, headache and lymphadenopathy. However, the presence of an eschar versus a petechial rash can be useful in the differential diagnosis. Though PCTF appears to be a less severe disease than RMSF, hospitalization has been required for some patients.
All suspect patients should be treated as if they have a Rickettsial infection. Prompt treatment with doxycycline is recommended if tick-borne Rickettsial infection is suspected, and should never be delayed pending the outcome of diagnostic tests.
Different Rickettsial pathogens cause immune reactions in humans that can be difficult to distinguish with antibody-based laboratory techniques. Serologic cross-reactivity with available Rickettsia tests does occur.
Physicians seeking confirmation of infection may elect to use commercially available serologic assays for Rickettsial species. Cross reactivity among pathogens does occur.
In a recent study for PCTF, “a confirmed case was considered one presenting with an eschar and a positive RT-PCR test result of the eschar with confirmatory sequencing, a probable case was one with a clinically documented eschar with no testing but with a documented four-fold antibody titer increase”.
Specific diagnosis of the PCTF relies upon comparative sequence analyses of Rickettsial polymerase chain reaction (PCR) products from an eschar swab, a scab, or a skin biopsy. 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.
Rickettsial 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