Colorado Tick Fever Virus in the Far West: Forgotten, but Not Gone

Abstract

In the past few decades, reported human cases of Colorado tick fever in the western United States have decreased dramatically. The goal of this study was to conduct surveillance for Colorado tick fever virus (CTFV) in Dermacentor ticks in recreational sites in Colorado, Wyoming, and California to determine whether the virus is still present in Dermacentor ticks from these states. Surveillance focused on regions where surveys had been conducted in the 1950s, 1960s, and 1970s. Adult Rocky Mountain wood ticks ( Dermacentor andersoni ), Pacific Coast ticks ( Dermacentor occidentalis ), and winter ticks ( Dermacentor albipictus ) were tested by PCR. A subset of PCR-positive D. andersoni ticks ( n = 7) were cultured in Vero cells. CTFV-positive Rocky Mountain wood ticks were found in all states: Colorado (58% prevalence), Wyoming (21%), and California (4%). Although no winter ticks tested positive, Pacific Coast ticks tested positive in one county (Siskiyou County, 15% prevalence) and were positive only in a location that also maintained Rocky Mountain wood ticks and golden mantled ground squirrels, a known CTFV host. In summary, CTFV is prevalent in D. andersoni and D. occidentalis in regions where they are sympatric in California and in D. andersoni in Colorado and Wyoming. Although the number of human CTFV cases has declined dramatically, this decrease in reported disease does not appear to be due to the disappearance or even the decline in prevalence of this virus in ticks in historically endemic regions of the country.

Colorado tick fever virus: a review of historical literature and research emphasis for a modern era

“…In South Dakota, approximately 3.5% of adult ticks were positive (Burgdorfer 1977). Prevalence in ticks collected from Wyoming, California, and Colorado from 2010 to 2011 was approximately 4%, 21%, and 58%, respectively (Padgett et al 2022). A study of D. andersoni in the Bitterroot Valley of Montana conducted over 5 yrs found 6.6% ( n = 921) of ticks were infected with CTFV (Williamson et al 2019).…”

“…Possible alternative vector associations are largely based on historical data collected throughout the 1940s and 1970s. This includes tick species shown to be positive for the presence of CTFV: Dermacentor albopictus , Dermacentor parumapertus , Otobius lagophilus , Haemophysalis leporispalustris , Ixodes spinipalpis , and Ixodes sculptus (Eklund et al 1955, Emmons 1988, Padgett et al 2022). Notably, detection of CTFV in many of these species occurred in a very small number of ticks and appears to have been incidental and no current studies demonstrate competence beyond exposure during blood feeding.…”

“…Over 700 D. variabilis from Long Island, NY, and surrounding areas of the Northeast and 1,000 ticks collected in Montana were all negative for CTFV (Eklund et al 1955). Detection of CTFV in Dermacentor occidentalis collected in California demonstrated a 15% (8/54) infection prevalence, but further experimentation is needed to confirm this tick species is a competent vector (Padgett et al 2022). It should also be noted that historical data regarding viral prevalence in ticks was reliant on intracranial virus passage through either hamsters or suckling mice.…”

“…The incidence of CTF reported in the United States is stable or declining, but these observed trends may be confounded by low reporting and changing reporting requirements over time (Geissler et al 2014, Yendell et al 2015). For instance, California has reported no cases of CTFV since 2010 (Padgett et al 2022). Approximately 6 states account for 80% of reported cases (Wyoming, Montana, Utah, Oregon, Colorado, and Idaho) (Yendell et al 2015).…”

“…These case numbers were 4 times higher than previous years, largely due to active enrollment of individuals by the authors in an attempt to better define human clinical course (Goodpasture et al 1978). Contemporary reporting of CTF from Colorado alone suggests the 10-yr incidence was approximately 9.9 cases per one million population for the years 1985–1995, indicating a decrease in CTF or reporting (Padgett et al 2022). Overall, Yendell et al reported a 90% decrease in CTF case reports across states where the virus had previously been detected, with 864 cases reported from 1985 to 1995, 75 from 2002 to 2012, and 59 from 2010 to 2019 (Yendell et al 2015, Padgett et al 2022).…”