West Nile virus epizootiology in the southeastern United States, 2001.

We investigated mosquito and bird involvement in West Nile virus (WNV) transmission in July 2001 in Jefferson County, FL, and Lowndes County, GA. We detected 16 WNV-infected pools from Culex quinquefasciatus, Cx. salinarius, Cx. nigripalpus, and Culiseta melanura. In Florida, 11% of 353 bird sera neutralized WNV. Antibody prevalence was greatest in northern cardinal (Cardinalis cardinalis, 75%), northern mockingbird (Mimus polyglottus, 50%), common ground-dove (Columbina passerina, 25%), common grackle (Quiscalus quiscula, 15%), domestic chicken (Gallus gallus, 16%), and house sparrow (Passer domesticus, 11%). Antibody-positive birds were detected in nine of 11 locations, among which prevalence in chickens ranged from 0% to 100%. Seropositive chickens were detected in Georgia as well. The primary transmission cycle of WNV in the southeastern United States apparently involves Culex mosquitoes and passerine birds. Chickens are frequently infected and may serve as effective sentinels in this region.

Seasonal abundance of Culex nigripalpus Theobald and Culex salinarius Coquillett in north Florida, USA.

North Florida is a transition zone between widespread Culex nigripalpus populations to the south and focal Culex salinarius populations to the north. Culex nigripalpus is a vector of St. Louis encephalitis (SLE) and eastern equine encephalitis (EEE) viruses in south Florida, while Cx. salinarius is a suspected New World vector of West Nile (WN) virus. Abundant vector populations are often a prerequisite for epidemic and epizootic transmission of arboviruses. Extensive SLE transmission has never been reported from north Florida, but sporadic WN transmission was reported there during the summer of 2001. The disparate flavivirus transmission patterns observed in north and south Florida may be due, in part, to the local geographical and seasonal distribution of Culex vectors. Here we report that from May 1991 to April 1994, Cx. salinarius was most commonly observed during the winter and spring in northeast Florida (Duval County), whereas Cx. nigripalpus was most abundant during the summer and autumn. An unusually mild spring in 1991 allowed Cx. nigripalpus to reproduce early in the year, resulting in a summer population that emerged more than 8 wks earlier than in 1992 and 1993. The 1991 Cx. nigripalpus population persisted through October, when SLE transmission was detected by sentinel chickens. Transmission of SLE was not detected in Duval County during 1992 or 1993. These data indicate that mild winter and spring conditions in north Florida may favor increased abundance and survival of Cx. nigripalpus in a region where this species is normally not abundant. A seasonal shift in population structure may increase the transmission risk of arboviruses for which Cx. nigripalpus is a competent vector, including SLE, WN, and EEE.