Prevalence of Rickettsia species phylotype G022 and Rickettsia tillamookensis in Ixodes pacificus nymphs and adults from Northern California

Abstract

Ticks are known vectors of various pathogenic bacteria, including species of Rickettsia. Two novel Rickettsia species have been identified in adult Ixodes pacificus: Rickettsia species phylotype G022 in 2011 and R. tillamookensis in 2021. Currently, the pathogenic potential of these species found in I. pacificus remains unknown. This study aimed to determine the prevalence of phylotype G022 and R. tillamookensis in I. pacificus nymphs across different mean annual temperature and relative humidity zones in California. Adult ticks were also tested for phylotype G022. Ticks were collected from multiple locations in seven northern California counties and tested by real-time PCR. The overall prevalence of phylotype G022 and R. tillamookensis in nymphs (n = 550) was 5.3 % (95 % CI = 3.7 %-7.5 %) and 1.6 % (95 % CI=0.8 %-3.3 %), respectively. The overall prevalence of phylotype G022 in adult I. pacificus (n = 720) was 9.0 % (95 % CI = 7.2 %-11.3 %). Phylotype G022 infects nymphal I. pacificus across a broad geographic range. The prevalence of phylotype G022 was higher in the 11.7-13.3 °C (53-56°F) temperature zone, at 6.4 % (95 % CI = 4.5 %-9.2 %), compared to the 13.9-15 °C (57-59°F) zone, where the prevalence was 0.8 % (95 % CI = 0.2 %-4.6 %). In contrast, the prevalence of R. tillamookensis did not show a statistically significant difference between the two temperature zones, with 1.9 % (95 % CI = 0.9 %-4.1 % in the 11.7-13.3 °C (53-56°F) zone and 0.9 % (95 % CI = 0.2 %-4.9 %) in the 13.9-15 °C (57-59°F) zone. The detection of phylotype G022 in both questing nymphs and adults of I. pacificus suggests that it is transmitted transstadially. qPCR testing revealed no coinfections of G022 and R. tillamookensis in any of the nymphs. Although R. tillamookensis exhibited a lower overall prevalence in nymphs compared to phylotype G022, both bacteria exhibited a similar geographic distribution.

Keywords: Ixodes pacificus; Nymphal ticks; Rickettsia species phylotype G022; Rickettsia tillamookensis.

Fig. 1.

Collection-site map grouped by mean annual temperature in degrees Fahrenheit (1A) and collection map grouped by mean annual relative humidity, values expressed as percentages (1B). The samples spanned four different temperature zones: the 10–11.1 °C (50–52°F) zone, 11.7–13.3 °C (53–56°F) zone, 13.9–15 °C (57–59°F) zone, and 15.5–16.7 °C (60–62°F) zone. The samples spanned four different relative humidity zones: the 56–59 % range, 59–63 % range, 63–66 % range, and 66–70 % range. The most southern county sampled for ticks was Santa Cruz, with the most northern being Humboldt. Red stars designate the adult collection sites while yellow triangles indicate nymphal collection sites. The collection site in Napa County was the same for both adults and nymphs, so the icons overlap.