Comparison of the Potential for Different Genetic Forms in theCulex pipiensComplex in North America to Transmit Rift Valley Fever Virus¹

Abstract

Rift Valley fever virus (RVFV), a mosquito-borne virus, has been responsible for large outbreaks in Africa that have resulted in hundreds of thousands of human infections and major economic disruption due to loss of livestock and to trade restrictions. Culex pipiens was implicated as the principal vector of the Egyptian outbreak in 1977 that affected about 200,000 people. In the northern USA, Cx. pipiens occurs both as a mix of forms pipiens and molestus (i.e., US Culex pipiens) as well as pure Cx. pipiens form molestus, the latter mostly in underground locations such as sewers and basements. In order to understand the potential risk of spread of RVFV in the USA, we compared their relative abilities to transmit RVFV in the laboratory. After feeding on hamsters with high viremias, >10(9) plaque-forming units (PFU)/ml, both US Cx. pipiens and Cx. pipiens form molestus were highly susceptible to infection (∼80%) and about 20% of each form developed a disseminated infection. In contrast, when fed on a hamster with a moderate viremia, 10(7.5) PFU/ml, US Cx. pipiens were significantly (P < 0.001) more susceptible (84%) than were the pure form molestus (47%). Similarly, dissemination rates were significantly (P = 0.0261) higher in US Cx. pipiens (34%) than they were in pure Cx. pipiens form molestus (10%). These results underscore differences in vector competence between genetic forms in the Cx. pipiens complex but also indicate that if RVFV were to arrive in the USA, competent vectors abound in the highly urbanized Northeast.

Rift Valley fever virus and European mosquitoes: vector competence of Culex pipiens and Stegomyia albopicta (= Aedes albopictus)

“…A previous study, performed with the RVFV strain ZH501, suggested the presence of a midgut escape barrier (MEB) in Cx. pipiens form molestus (Turell et al ., ). This may explain the absence of disseminated infection in this species in the present study, but the low number of Cx.…”

“…pipiens from the U.S.A. were assayed using a similar viral load (10 7.5 PFU/mL) (Turell et al ., ). However, the authors of the U.S. study used an infected hamster as a blood source and specimens were maintained at a constant EIT of 26 °C (Turell et al ., ). As result, the infection and dissemination rates obtained were higher than those in the present study.…”

“…With regard to the influence of the viral load used, the findings of the present study can be compared with those of a previous study in which two forms of Cx. pipiens from the U.S.A. were assayed using a similar viral load (10 7.5 PFU/mL) (Turell et al, 2014). However, the authors of the U.S. study used an infected hamster as a blood source and specimens were maintained at a constant EIT of 26 ∘ C (Turell et al, 2014).…”

Peer Review #1 of "Genetic divergence between populations of feral and domestic forms of a mosquito disease vector assessed by transcriptomics (v0.1)"

“…Blooded females that had been biting local residents were allowed to lay egg rafts in the laboratory and henceforth the colony has been maintained without access to blood. Representative specimens of the NYC colony of f. molestus have been genotyped with a panel of 8 microsatellite loci and have a genetic signature that matches that of populations of f. molestus from southwestern Germany, as do other f. molestus specimens obtained from multiple locations around the world (Fonseca et al 2004b;Micieli et al 2013;Turell et al 2014). Once eggs hatched, larvae of both forms were reared in ceramic pans under a 16:8 L:D cycle on a diet of ground rat chow prior to emergence.…”

Identification and evaluation of antivirals for Rift Valley fever virus

“…In the natural environment, RVF breaks out after periods of heavy rainfall and flooding because a significant amount of RVFV infected mosquito eggs might hatch and transmit the virus to animals and humans (Davies, Linthicum, & James, 1985). Other species of mosquitoes such as Culex, Mansonia, and Anopheles can be mechanical vectors for disease transmission from infected animals to healthy animals or to humans through blood feeding (Turell, Dohm, & Fonseca, 2014). Sandflies, midge, and tick can also serve as vectors based on the published data (Fontenille et al, 1998).…”