Tickborne viruses in western North America. III. Viruses from man-biting ticks (Acari: Ixodidae) in Oregon.

Abstract

Numerous isolations of a new, serologically unique, filterable agent, Cascade virus, were made from adult Dermacentor occidentalis ticks in southwestern Oregon during 1979 and 1980. The virus was not found in Ixodes pacificus collected the same general areas and at the same times. Dermacentor andersoni ticks collected in eastern Oregon were occasionally infected with Colorado tick fever virus, but not with the new agent. Cascade virus could readily be isolated in Xenopus laevis (XTC-2) or Dermacentor variabilis (RML-15) cells incubated temperatures of 34 or 27 °C, respectively, but could not be isolated when tick material was inoculated into laboratory animals or homeothermic cell lines. The virion contains RNA and essential lipids and is sensitive to exposure to heat and acid; its diameter is between 100 and 220 nm. Virus growth in XTC-2 cells is accompanied by the production of cytopathic effects and plaques, but growth in other cell lines (RML-15, Vero, PS, BHK-21) is demonstrable only upon secondary assay. The virus caused cytopathic effects and plaques in XTC-2 cells incubated at 27 or 34 °C, but failed to grow in or visibly affect these cells when incubated at 37 °C. Vero, PS and BHK-21 cell lines permitted the virus to grow at an incubation temperature of 34 °C, but growth was inhibited at 37 °C in at least 2 of these lines (Vero, PS). Cascade virus failed to replicate in mouse fibroblasts (L cells) or Aedes albopictus (ATC-15) mosquito cells. Intracerebral (ic) inoculation of virus in suckling Microtus pennsylvanicus caused illness and death only sporadically. Suckling white mice, hamsters and wet chicks showed no signs of infection after ic or peripheral inoculation, and the virus failed to grow in the yolk sac of embryonated chicken eggs. Although no evidence of human infection with Cascade virus was obtained from a small sample of sera selected from Californians who became sick after tick exposure, recent evidence in Oregon and California of tick-associated disorders of undetermined cause (erythema chronicum migrans, Lyme disease) suggests further study.