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The NY99 genotype of West Nile virus (WNV) introduced into North America has demonstrated high virulence for American crows (AMCRs), whilst a closely related WNV strain (KEN-3829) from Kenya exhibits substantially reduced virulence in AMCRs [Brault, A. C., Langevin, S. A., Bowen, R. A., Panella, N. A., Biggerstaff, B. J., Miller, B. R. & Nicholas, K. (2004). Emerg Infect Dis 10, 2161-2168]. Viruses rescued from infectious cDNA clones of both the NY99 and KEN-3829 strains demonstrated virulence comparable to that of their parental strains in AMCRs. To begin to define parameters that might explain the different virulence phenotypes between these two viruses, temperature-sensitivity assays were performed for both viruses at the high temperatures experienced in viraemic AMCRs. Growth curves of the two WNV strains were performed in African green monkey kidney (Vero; 37-42 degrees C) and duck embryonic fibroblast (DEF; 37-45 degrees C) cells cultured at temperatures that were tolerated by the cell line. Unlike the NY99 virus, marked decreases in KEN-3829 viral titres were detected between 36 and 120 h post-infection (p.i.) at temperatures above 43 degrees C. Replication of KEN-3829 viral RNA was reduced 6500-fold at 72 h p.i. in DEF cells incubated at 44 degrees C relative to levels of intracellular virus-specific RNA measured at 37 degrees C. In contrast, replication of virus derived from the NY99 infectious cDNA at 44 degrees C demonstrated only a 17-fold reduction in RNA level. These results indicated that the ability of WNV NY99 to replicate at the high temperatures measured in infected AMCRs could be an important factor leading to the increased avian virulence and emergence of this strain of WNV.