Susceptibility of Ochlerotatus trivittatus (Coq.),Aedes albopictus (Skuse),and Culex pipiens (L.) to West Nile Virus Infection

The susceptibility of Ochlerotatus trivittatus (Coq.) to West Nile virus (WNV) was assessed by comparing it to the susceptibility of Aedes albopictus (Skuse), a likely bridge vector, and Culex pipiens (L.), a primary WNV amplifying species. The three species were infected with WNV (NY crow–1999) by feeding on 2–3-day-old chickens with serum virus titers ranging from 10 ^2.5 to 10 ^9.5 cell culture infective dose (CID) _50s/mL. The lowest infective titer for Oc. trivittatus and Cx. pipiens was 10 ^4.5 CID₅₀s/mL. Thirteen percent (4/32) and 2% (1/45) of each species became infected postprandially. Infection rates of the two species increased to 43% (6/14) and 15% (6/40) after blood meals with a titer of 10 ^5.5 CID₅₀s/mL. In contrast no infection was observed in nine Ae. albopictus that fed among three chickens with titers of 10 ^4.5 CID₅₀s/mL nor in 41 Ae. albopictus that fed among three chickens with titers of 10 ^5.0 CID₅₀s/mL. The infective dose ₅₀s for Oc. trivittatus, Cx. pipiens and Ae. albopictus were 10 ^6.0, 10 ^6.2, and 10 ^6.6 CID₅₀s/mL, respectively. Collectively these observations suggest that Oc. trivittatus and Cx. pipiens are more susceptible than Ae. albopictus to WNV when they feed on hosts with WNV titers of <10 _7.5 CID_50s/mL, but nearly as susceptible with blood meal titers of ≥10 ^7.5 CID₅₀s/mL. Unpublished studies in our laboratory showed that cottontail rabbits fed on by WNV-infected Oc. trivittatus developed viremias as high as 10 ^5.5 CID₅₀s/mL serum which exceeds 10 ^{4.2 (3.4-4.6)} CID₅₀s/mL, the predicted ID₁₀ ± 95% CI of Oc. trivittatus. Consequently this mosquito, which also feeds on humans and birds has the potential to serve as a bridge vector and as a maintenance vector among mammals.