The susceptibility of Aedes triseriatus to oral infection with La Crosse (LAC) virus resulting from feeding on chipmunks with viremia titers of 0.6 to 4.6 log10SMICLD50/0.025 ml was determined. Results indicated that viremia titers must exceed 3.2 log10SMICLD50/0.025 ml before a significant proportion (≥50%) of mosquitoes are infected and capable of transmitting LAC virus. Mosquitoes which fed on chipmunk blood-LAC virus mixtures through a membrane feeder had significantly lower infection rates at virus titers of 1.8 to 4.4 log10SMICLD50/0.025 ml and transmission was also significantly reduced. Application of these data to LAC viremia titers measured in chipmunks in an earlier study indicate that viremias sufficiently high to ensure transmission by the mosquitoes becoming orally infected average only about 1 day per infective bite delivered to the susceptible portion of the amplifier population
Oral infection and transmission rates were also determined for Ae. triseriatus feeding on chipmunk blood containing LAC virus neutralizing (N) antibodies and for Ae. triseriatus feeding on deer blood containing Jamestown Canyon (JC) virus N antibodies. Infection rates were similar to those observed in mosquitoes imbibing blood free of N antibody at the virus titers tested, but, oral transmission was reduced in females feeding on chipmunk blood-LAC virus mixtures containing LAC N antibodies and there was no transmission by females feeding on deer blood-LAC virus mixtures containing JC N antibodies. These data suggest that high LAC antibody prevalences in chipmunk populations and high LAC or JC antibody prevalences in deer populations may be antagonistic to horizontal LAC virus transmission.