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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.

A serologic survey of swift fox (Vulpes velox) and kit fox (V. macrotis) from the western USA was conducted for 12 infectious diseases. Samples from swift fox were collected between 1987 and 1992 from Colorado (n = 44), Kansas (n = 10), and Wyoming (n = 9). Samples from kit fox were collected in California (n = 86), New Mexico (n = 18), Utah (n = 9), and Arizona (n = 6). Overall antibody prevalence rates were 33 of 110 (30%) for canine parvovirus (CPV), 9 of 72 (13%) for canine distemper virus (CDV), 23 of 117 (20%) for vesicular stomatitis New Jersey, 16 of 117 (14%) for vesicular stomatitis Indiana, six of 117 (5%) for Cache Valley virus, five of 117 (4%) for Jamestown Canyon virus, one of 97 (1%) for rabies virus, one of 117 (1%) for Colorado tick fever virus, and one of 117 (1%) for western equine encephalitis virus. In addition, antibodies were not found to Yersinia pestis, Francisella tularensis, and Borrelia burgdorferi in serum from 25 Colorado swift fox. Adult swift fox from Colorado had serologic evidence of exposure to CPV more often than juveniles. No juvenile swift fox from Colorado had serum antibodies to CDV. There were season-specific differences in serum antibody prevalence for CPV for swift fox from Colorado. No viruses were isolated from ectoparasites or fox from Colorado.

One hundred and twenty-four small mammals of six species were inoculated with three strains of Colorado tick fever (CTF) virus to define viremia and neutralizing (N) antibody responses. Adult Eutamias minimus and Eutamias embrinus, and juvenile Peromyscus maniculatus and Spermophilus lateralis, were highly susceptible to development of viremic infection. Adult S. lateralis and P. maniculatus were moderately susceptible (greater than or equal to 50% viremic). Five Sylvilagus nuttalli did not become viremic following experimental inoculation. Spermophilus richardsoni was also relatively resistant (less than or equal to 50% viremic). The longest duration of viremia (mean 15.8 days) and highest peak viremia levels (mean peak titer 10(3.9 plaque-forming units per ml) occurred in E. minimus. Adult E. umbrinus, juvenile S. lateralis, and juvenile P. maniculatus had moderate viremias. Adult S. lateralis and S. richardsoni often had short viremias during which virus was only intermittently detectable. N antibody production was most rapid in E. minimus in comparison with other species. In addition, N antibody persisted for 1 year in this species. In other species, many animals lost detectable antibody 5-11 months after infection. No significant differences were found in patterns of infection between three CTF virus strains. We conclude that of the six species inoculated, E. minimus is the best experimental host for CTF virus.

Ecologic studies of small mammals in Rocky Mountain National Park (RMNP) were conducted in 1974 in order to identify the specific habitats within the Lower Montane Forest that support Colorado tick fever (CTF) virus. Data was collected on the abundance and distribution of 4 primary rodent species, tick infestation, CTF virus, and neutralizing antibody prevalence. Rodents were captured along transects crossing different habitats. Open stands of ponderosa pine and shrubs on dry, rocky surfaces were found to be important for maintaining CTF virus.