Infection and transmission of West Nile virus by Ochlerotatus triseriatus (Diptera: Culicidae) in the laboratory

Abstract

<p>West Nile virus (WNV) is a newly recognized arbovirus in North America. The WNV transmission cycle involves mosquitoes in the genus Culex, and avian species as the amplification hosts. Mosquitoes that feed on both birds and mammals are known as bridge vectors, which transmit WNV from avian amplification hosts to other vertebrates susceptible to WNV infection. These secondary hosts do not develop viremias high enough to continue the infection cycle, and are known as dead end hosts. The primary objective of this study was to compare the susceptibility of Ochlerotatus triseriatus (Say) to Culex p. pipiens (L.), a primary amplification vector of WNV. Ochlerotatus triseriatus is the primary vector of La Crosse encephalitis virus (LAC). It feeds primarily on mammalian hosts, and is a possible bridge vector of WNV. West Nile virus infection rates of Oc. triseriatus and Cx. p. pipiens were determined following WNV viremic blood meals ranging from 10⁴·⁰ to 10⁸·² PFU/ml. The minimum blood meal titer to infect Oc. triseriatus was 10⁴·³ PFU/ml, and it was 10⁴·⁰ PFU/ml for Cx. p. pipiens. West Nile virus infection rates in both species significantly increased as the viremic blood meal titers increased. Maximum infection of 98 and 100% was observed at 10[super script greater than or equal to 8.0<8.5] and 10[super script greater than or equal to 7.0<7.5] PFU/ml for Oc. triseriatus and Cx. p. pipiens, respectively. West Nile virus titers predicted to infect Cx. p. pipiens at ID₁₀, ID₅₀, and ID₉₀ are 10³·³, 10⁴·², and 10⁶·⁵ PFU/ml. Predicted titers to infect Oc. triseriatus at ID₁₀, ID₅₀, and ID₉₀ are 10⁴·⁷, 10⁶·⁵, and 10⁸·³ PFU/ml. Observed and predicted infection rates suggest that Oc. triseriatus is significantly less susceptible than Cx. p. pipiens to WNV. Ochlerotatus triseriatus orally infected from viremic chicken blood meals with a WNV titer>10⁸·⁰ PFU/ml were able to transmit virus by bite to susceptible chickens on days 14 and 18 post infection. The infection, dissemination, and transmission rates of Oc. triseriatus were 94, 91, and 60 percent respectively. Ochlerotatus triseriatus is less susceptible to WNV infection then Cx. p. pipiens. It is a competent vector with a 55% estimated transmission rate among individuals infected at high titer WNV viremic blood meals. Ochlerotatus triseriatus is a potential bridge vector of WNV and may play a role in transmission among susceptible mammalian hosts. Characterization of WNV infection within primary hosts of Oc. triseriatus, i.e. squirrels, chipmunks, rabbits, and deer, is needed to assess the possibility of a mammalian WNV transmission cycle.</p>