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Chapter 34 : Colorado Tick Fever Virus and Other Arthropod-Borne Reoviridae
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The development of viral cultures in chicken embryos and mice by Koprowski and Cox in the 1940s and subsequent refinements in viral isolation and molecular biology permitted further description of Colorado tick fever virus (CTFV). Much is now known about its ecological niche and life cycle in vertebrate and invertebrate hosts as well as the epidemiology, pathogenesis, and clinical course of infection in humans. The current taxonomy recognizes three distinct species: (i) the type species, Banna virus (BAV); (ii) Kadipiro virus; and (iii) Liao Ning virus. The pathogenic potential of these viruses in humans remains to be determined, although associations of BAV infection with febrile illnesses and encephalitis are convincing. Seadornaviruses are serologically distinct from CTFV and Eyach virus (EYAV), have a lower G+C content than coltiviruses, and show genetic distances in the RNA-dependent RNA polymerase of more than 90% compared to coltiviruses. The genome of all members of the Reoviridae consists of segmented double-stranded RNA (dsRNA). Electron microscopic (EM) studies of cultured cells following infection reveal viral particles in association with granular matrices within the cytoplasm. The name Orbivirus derives from the appearance of virus particles as large, doughnut-shaped capsomeres by negative-contrast EM. Though the most-studied orbiviruses are agents of veterinary import, a number of serogroups within the genus cause human disease. They are predominantly transmitted by arthropod vectors.
Phylogenetic relationship between members of Coltivirus, Seadornavirus, and Orbivirus with other Reoviridae, based on analysis of partial polymerase sequence. Each forms a distinct sequence cluster. Seadornaviruses appear to be most closely related to rotaviruses. KDV, Kadipiro virus; LNV, Liao Ning virus; SCRV, St. Croix River virus; AHSV, African horse sickness virus; CHUV, Chuzan virus; BTV, bluetongue virus; BoRV, bovine rotavirus; SiRV, simian rotavirus; PoRV, porcine rotavirus; Hu/MuRV, human/murine rotavirus. (Reprinted from reference 7 with permission.)
Negative-contrast electron micrograph of CTFV virions. The bar represents 50 nm. (Reprinted from reference 76 with permission of Elsevier.)
Distribution of Dermacentor andersoni ticks (shaded area) and numbers of cases of Colorado tick fever from 1990 to 1996. (Modified from reference 98 with permission of Elsevier.)
Frequency distribution, by month, of confirmed illness due to CTFV in Colorado during 1973 and 1974. (Modified from reference 45 with permission.)
Coding organization of Coltivirus, Seadornavirus, and Orbivirus segments a,b
Symptom frequency in 320 patients suspected of having Colorado tick fever in Colorado for the years 1973 and 1974 a
Orbiviruses that naturally infect humans a