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Colorado tick fever (CTF) virus, family Reoviridae, genus Orbivirus, contains 12 genes distinguishable by polyacrylamide gel electrophoresis (PAGE). Multiple genotypes of CTF virus were isolated at 3 field sites in Colorado in 1985. Five genotypes were found at Campos Cabin, 2 at Drake, and 6 at Rocky Mountain National Park. Virus isolations were made in 1985 from 6 patients with CTF. These isolates were distinct from each other and the field isolates. Although the CTF isolates were different by PAGE profile, the majority of the 12 genes were highly conserved among the 1985 isolates and a Florio isolate (FMA). Only genes 4 and 6 were variant among the 1985 CTF isolates and FMA, and no unique genes were identified. In 1986, a follow-up field survey was done at the Campos Cabin site. Of the 3 CTF PAGE genotypes obtained, 2 exhibited PAGE profiles which were different from the 1985 isolates. One isolate may have resulted from the reassortment of genes from 2 of the isolates circulating at Campos Cabin in 1985.

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.

Two strains of Colorado tick fever virus grew in Singh's Aedes albopictus cells. In one of three experiments, virus growth continued for 7 weeks.