| emergence of attenuated west nile virus variants in texas, 2003. | in order to understand how west nile virus (wnv) has evolved since its introduction into north america, we have studied the genetic and phenotypic variation among wnv isolates collected in various areas during consecutive transmission seasons. the present report describes for the first time phenotypic changes occurring in the north american wnv population. several isolates collected in texas during 2003 display a small plaque (sp) and temperature sensitive (ts) phenotype, as well as reduced repl ... | 2004 | 15527859 |