Work

West Nile Virus Viremia in Eastern Chipmunks (Tamias striatus) Sufficient for Infecting Different Mosquitoes

Year: 2007

Type: article

Abstract: In eastern chipmunks (Tamias striatus) inoculated intramuscularly with 10 1.5 to 10 5.7 PFU of West Nile virus (WNV), serum titers developed suffi cient to infect Aedes triseriatus (Say), Ae. vexans (Meigen), and Culex pipiens (L.).Mean titers (95% confi dence interval) of 8 chipmunks were 10 3.9(3.3-4.5), 10 6.7(6.4-7.0), and 10 5.8(4.1-7.5)PFU/mL on days 1-3 postinoculation (p.i.) and 10 5.8 PFU/mL in 1 chipmunk on day 4 p.i. Mean estimated days that WNV titers were >10 4.8 and >10 5.6 were 1.7 (1.1-2.3) and 1.4 (1.0-1.6).The longest period of viremia >10 4.8 PFU/mL was 3-4 days.WNV antigen was detected in the small intestine of 2 chipmunks and the kidneys of 4 chipmunks by immunohistochemistry. WNV also was detected in urine, saliva, and feces of some chipmunks.These data suggest chipmunks might play a role in enzootic WNV cycles and be an amplifying host for mosquitoes that could infect humans.T he role of mammals in the ecology of West Nile virus (WNV) has not been well defi ned.In many mammals, levels of viremia suffi cient to infect mosquito vectors do not develop (1).However, mosquito-infective viremia levels do occur in some mammals such as the golden hamster (Mesocricetus auratus), in which WNV serum titers can exceed 10 5.5 tissue culture infective dose (TCID) 50 /mL (2), and the fox squirrel (Sciurus niger), in which a maximum titer of 10 5.0 PFU/mL was reported (3).Mosquito-infective WNV serum titers with a mean duration of 2.2 ± 0.6 days also have been demonstrated in the eastern cottontail rabbit (Sylvilagus fl oridanus) (4).Minimum estimated infection rates of 12% and 21% occurred in Culex pipiens (L.) and Cx.salinarius (Coq.) after they fed on cottontail rabbits with WNV serum ti-ters of 10 4.3 -10 5.0 TCID 50 /mL.Infection rates increased to 21% and 25% at WNV titers of 10 5.0 -10 6.0 TCID 50 /mL.The magnitude and duration of WNV viremia levels in cottontail rabbits and fox squirrels, which are peridomestic, raise the question of whether other mammals can serve as WNVamplifying hosts for mosquitoes in enzootic WNV cycles, with potential for transmission to humans.This study describes the effect of needle inoculationinduced WNV infection in the eastern chipmunk (Tamias striatus).We also demonstrate the potential of chipmunks to serve as a WNV-amplifying host for Cx.pipiens, Aedes triseriatus (Say), and Ae.vexans (Meigen).These species were selected for the study because they share habitats in common with chipmunks in rural and suburban areas.Cx. pipiens is a major WNV enzootic and bridge vector in North America (5,6).It is ornithophilic but will feed on mammals (7-9), especially as it shifts its feeding preference during late summer and early fall (10).Ae. triseriatus and Ae.vexans are competent laboratory vectors of WNV (6), and this virus has been isolated from fi eld specimens (11,12).Both species feed primarily on mammals (6) but also will feed on avian species (9,13,14). Methods Cells, Media, Diluents, and SolutionsVero 76 cells were used for virus propagation, plaque assays, and virus isolation.Growth medium (GM) was Dulbecco modifi ed Eagle medium (GIBCO, Invitrogen Corp., Carlsbad, CA, USA) that contained 10% heat-inactivated fetal bovine serum (FBS) and 2.0 mmol/L L-glutamine.Maintenance medium (MM) was GM with 1% FBS.Overlay medium (OM) for virus assay was 1 part MM and 1 part 2% agarose (Difco, Becton, Dickinson and Co.,

Source: Emerging infectious diseases

DOI: 10.3201/eid1306.061008