Dual Infections of Mosquitoes with Strains of West Nile Virus1

Abstract

Culex pipiens molestus female mosquitoes were infected with strains of West Nile virus by parenteral injection of small amounts of the virus. A modified strain of WN virus (PL 44 + 45), produced on chick fibroblast agar plates, developed readily in the adult mosquitoes, and produced virus concentrations of about 106 mouse ic LD50 per mosquito. Modified PL 44 + 45 virus did not lose its characteristic nonpathogenicity to weanling white Swiss mice by ip inoculation after replication in C. molestus. An infection of C. molestus with PL 44 + 45 suppressed the subsequent development of the Egypt 101 strain of West Nile virus in these mosquitoes following a challenge inoculation 7 days after the initial infection. The mechanism of this acquired resistance in the mosquito is unknown; one possibility is competition between antigenically similar or idtentical viruses for cellular substrates.