Another step in determining if a virulent genotype had an increased transmission fitness phenotype was to study differences in replication and dissemination (i.e., the possibility of transmitting virus by bite) in the natural mosquito vector, Aedes aegypti. Laboratory-reared colonies of mosquitoes (e.g., Rexville or Rockefeller strains) seem to have lost their selectivity for infection and it is recommended that mosquitoes used in these experiments be from the F4 generation or lower (F0 ¼ field-collected eggs). The virulent, Southeast Asian strains of serotype 2 were shown to infect a larger proportion of mosquitoes than the less virulent, American genotype strains, after feeding mosquitoes on blood containing the same titer of virus (Armstrong and Rico-Hesse 2001) ; also, a greater proportion of mosquitoes develop disseminated infections with the virulent genotype (Armstrong and Rico-Hesse 2003) . If mosquitoes were fed both genotypes simultaneously, they were much more likely (sevenfold) to develop an infection with the virulent strains (Cologna et al. 2005) . When the dynamics of virus replication and dissemination were compared for both genotypes, the virulent strains had reached the salivary glands up to 7 days earlier than the less virulent viruses (Anderson and Rico-Hesse 2006) . This means that virulent strains may replicate and be transmitted much sooner to human hosts, outcompeting the less virulent viruses and causing many more cases of disease, thus ecologically displacing those that cause less severe dengue. This efficiency of transmission by the vector could explain how certain genotypes have displaced others, shifting the evolution of dengue disease towards more virulence (i.e., more DHF).