Impact of West Nile Virus on the Natural History of St. Louis Encephalitis Virus in Florida

The emergence of West Nile virus (WNV) has raised important questions about the capacity of the public health infrastructure to implement surveillance and control programs for WNV and other emerging or re-emerging arboviruses in the United States. Florida\u27s mild climate supports year round enzootic transmission of WNV, St. Louis encephalitis virus (SLEV), and Eastern Equine Encephalitis virus (EEEV). It is unknown what effect the establishment of WNV (in 2001) will have on SLEV transmission in Florida, where these closely related flaviviruses share amplifying hosts, habitats, and vectors. An Arbovirus Isolation Network was formed to obtain and characterize arbovirus strains collected from a large population of naturally exposed birds, including sentinel chickens and wild birds admitted to rehabilitation centers in Florida. Weekly sentinel seroconversion data was used to target sampling of chicken flocks at 37 active sites (17 WNV, 7 EEEV, and 13 SLEV) in eight counties from 224 birds during 2005-2006. Sampling of wild birds occurred following admittance at rehabilitation centers in 2006, based on symptoms and known amplifying host species (n=64), but virus was not detected. We report the isolation of St. Louis encephalitis virus, West Nile virus and detection of Eastern Equine Encephalitis viral RNA from cloacal swabs of naturally exposed adult sentinel chickens. We also report the first known dual infection and isolation of St. Louis encephalitis and West Nile viruses from one chicken. In addition, a novel flavivirus strain was detected in two chickens. Early season transmission of WNV appears to limit subsequent infection and amplification of SLEV late in the year. Phylogenetic analysis revealed that the introduction (and re-introduction) of South American (Brazil) SLEV occurred in 1972 and 2006 in Florida. These strains represent the first reported isolation of South American strains of SLEV in the United States, with placement in Lineage VA and VB, as proposed by Kramer and Chandler (2001). Arbovirus isolation remains an effective tool for surveillance programs and a targeted strategy is most cost-effective to capture arboviruses in their natural settings for molecular epidemiology analysis that can elucidate genetic variations impacting virulence, mosquito infectivity, and disease potential of these pathogens