Frequency of Saint Louis encephalitis virus in humans from Florida, USA: 1990-1999

Four general frequencies of human St. Louis encephalitis (SLE) virus (epide mic, focal. sporadic. and no transmission) occurred in Florida between 1980 and 1999. An epidemic with 226 clinical cases and 11 deaths was reported f rom 28 Florida counties between July 1990 and January 1991. During the autu mn of 1993. a focal outbreak was reported from Lee (5 cases) and Collier (3 ) Counties in southwest Florida. During the autumn of 1997. sporadic transm ission to nine humans was reported from five Florida counties (Brevard [1 c ase \ Polk \3\, Charlotte \1\, Lee \2\, and Palm Beach \2\. Human infection with SLE virus depends on a number of variables that drive virus transmiss ion, These include vector, virus, and avian host abundance, and meteorologi cal events, especially rainfall. We monitored the abundance and serological status of wild avian amplification hosts. virus isolations from Culex nigr ipalpus Theobald females, and SLE virus transmission to sentinel chickens d uring 1990. 1993. and 1997. The epidemic of 1990 was characterized by condi tions that produced an unusual abundance of vector mosquitoes and avian amp lification hosts early in the year. We propose that epidemics may result wh en a specific combination of biotic and abiotic conditions favor SLE virus minimum field infection rates that approach 1:1.000 in Ct. nigripalpus vect ors.