Evolution and ecology of vesicular stomatitis virus in the southwestern United States: 1995 to 1998

Vesicular stomatitis (VS) is a disease of livestock and horses that is clinically undistinguishable from foot and mouth disease that causes considerable economic loss during outbreaks in the United States (US). The complete epidemiological relationship among agent, host, environment and time remains unknown. This dissertation was a multi-phase approach to understanding specific essential unsolved components of the epidemiology of the disease. The phylogenetic relationship of viruses circulating in the southwestern United States in 1995 with viruses in Mexico from 1984 to 1997 was investigated. Nucleotide changes were found with viruses from Mexico and viruses previously sequenced from the western United States. Overall sequence analysis indicated relatively homogenous and distinct viral lineages for each documented outbreak in the western US, with viruses from Mexico in each lineage of the major US outbreaks from 1982/83 to 1995, suggesting a common source for these viruses. A second study on the evolution of VSV was conducted in an in vitro model. The growth rate and rate of evolution of three genetically distinct VSV-NJ viruses from different ecological regions and different hosts in a natural (sand fly) versus non-natural (mosquito and mammalian) host cell lines was used to study its capacity to grow well and evolve in multiple cell lines. The virus originating from sand flies grew to higher titers in insect cells than did viruses of mammalian origin. Sequence analyses of the viruses identified no significant differences between rates of evolution by passage in homologous (sand fly) versus heterologous (mosquito or mammalian) cells were detected. A third study was conducted to identify spatial-temporal clustering of VS cases from the 1995, 1997, and 1998 VS outbreaks and investigate climatic and ecological conditions. Clusters were located in Arizona, Colorado, New Mexico, and Utah. Descriptive statistics indicated that positive premises were located dry ecological regions with a close proximity to perennial streams and canals. An increase in precipitation was observed from 1994 to 1999 from the 30-year average, but no substantial departures in temperature were identified.