The Eco-epidemiology of Pacific Coast Tick Fever in California

Abstract

Rickettsia philipii (type strain "Rickettsia 364D"), the etiologic agent of Pacific Coast tick fever (PCTF), is transmitted to people by the Pacific Coast tick, Dermacentor occidentalis. Following the first confirmed human case of PCTF in 2008, 13 additional human cases have been reported in California, more than half of which were pediatric cases. The most common features of PCTF are the presence of at least one necrotic lesion known as an eschar (100%), fever (85%), and headache (79%); four case-patients required hospitalization and four had multiple eschars. Findings presented here implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to people. Peak transmission risk from ticks to people occurs in late summer. Rickettsia philipii DNA was detected in D. occidentalis ticks from 15 of 37 California counties. Similarly, non-pathogenic Rickettsia rhipicephali DNA was detected in D. occidentalis in 29 of 38 counties with an average prevalence of 12.0% in adult ticks. In total, 5,601 ticks tested from 2009 through 2015 yielded an overall R. philipii infection prevalence of 2.1% in adults, 0.9% in nymphs and a minimum infection prevalence of 0.4% in larval pools. Although most human cases of PCTF have been reported from northern California, acarological surveillance suggests that R. philipii may occur throughout the distribution range of D. occidentalis.

Fig 1. Dermacentor occidentalis collection records in California by month; adults were collected from 1948–2014, nymphs 1949–2014, and larvae 1969–2014.

Red Arrows indicate illness onset month for human cases of Pacific Coast tick fever (numbers inside arrows represent onset of symptoms for human cases per month).

Fig 2. Counties where Dermacentor occidentalis ticks have tested positive (green) and negative (blue) for Rickettsia philipii (Lane et al., 1981 and Philip et al., 1981; present study).

Counties with no ticks tested (white). Counties where Pacific Coast tick fever confirmed cases were acquired (red dots, number indicates number of cases), California, 2005–2014.

Fig 3. Eschars identified on the eyebrow, shoulder, and neck of three laboratory-confirmed Pacific Coast tick fever patients, California.

Fig 4. Number of nymphal D. occidentalis collected biweekly at Jack London State Park, Sonoma County, California, May-October, 2009–2010.