Assessment of Local Mosquito Species Incriminates Aedes aegypti as the Potential Vector of Zika Virus in Australia

Abstract

Background: Within the last 10 years Zika virus (ZIKV) has caused unprecedented epidemics of human disease in the nations and territories of the western Pacific and South America, and continues to escalate in both endemic and non-endemic regions. We evaluated the vector competence of Australian mosquitoes for ZIKV to assess their potential role in virus transmission.

Methodology/principal findings: Mosquitoes were exposed to infectious blood meals containing the prototype African ZIKV strain. After 14 days incubation at 28°C and high relative humidity, infection, dissemination and transmission rates were assessed. Infection in Culex annulirostris and Cx. sitiens could not be detected. 8% of Cx. quinquefasciatus were infected, but the virus did not disseminate in this species. Despite having infection rates > 50%, Aedes notoscriptus and Ae. vigilax did not transmit ZIKV. In contrast, Ae. aegypti had infection and transmission rates of 57% and 27%, respectively. In susceptibility trials, the virus dose required to infect 50% (ID50) of Ae. aegypti was106.4 tissue culture infectious dose50 (TCID50)/mL. Additionally, a threshold viral load within the mosquito of at least 105.1 TCID50 equivalents/mL had to be reached before virus transmission occurred.

Conclusions/significance: We confirmed Ae. aegypti to be the most likely mosquito vector of ZIKV in Australia, although the restricted distribution of this species will limit the receptive zone to northern Queensland where this species occurs. Importantly, the role in ZIKV transmission of Culex and other Aedes spp. tested will be negligible. Despite being the implicated vector, the relatively high ID50 and need for a high titer disseminated infection in Ae. aegypti suggest that high mosquito population densities will be required to facilitate epidemic ZIKV transmission among the currently immunologically naïve human population in Australia.

Fig 1. Susceptibility of Ae. aegypti and Ae. notoscriptus to ZIKV infection.

Percent infection rates in Ae. aegypti (circles) and Ae. notoscriptus (triangles) exposed to serial dilutions of ZIKV and tested at 14 d post-exposure.

Fig 2. Replication of ZIKV in Australian species of Aedes.

The titer in TCID₅₀ equivalents per mL of ZIKV in the bodies (A) and legs+wings (B) of four species of Aedes tested 14 days after ingesting an infectious blood meal containing 10^{6.7± 0.2} TCID₅₀/mL of ZIKV. Ae. notoscriptus A and B represents the two different blood feeds for this species. Each point on the plot represents an individual mosquito, and horizontal lines denote medians. Ae. aegypti had significantly higher (P < 0.001) legs+wings titer than Ae. notoscriptus B. The horizontal dashed line represents the threshold titer above which virus transmission occurred.

Fig 3. Replication of ZIKV in Ae. aegypti on different days following exposure to an infectious blood meal.

The titer in TCID₅₀ equivalents/mL of ZIKV in the bodies (A), legs+wings (B) and saliva (C) of Ae. aegypti and tested on different days after ingesting an infectious blood meal containing 10^{6.7± 0.2} TCID₅₀/mL of ZIKV. Each point on the plot represents an individual mosquito, and bars denote medians. Solid lines represent significant differences P < 0.001 (**) between days for legs+wings and saliva titers. The horizontal dashed lines on days 10 and 14 represent the threshold titer above which virus transmission occurred.