Zika vector transmission risk in temperate Australia: a vector competence study

Abstract

Background: Zika virus is an emerging pathogen of global importance. It has been responsible for recent outbreaks in the Americas and in the Pacific region. This study assessed five different mosquito species from the temperate climatic zone in Australia and included Aedes albopictus as a potentially invasive species.

Methods: Mosquitoes were orally challenged by membrane feeding with Zika virus strain of Cambodia 2010 origin, belonging to the Asian clade. Virus infection and dissemination were assessed by quantitative PCR on midgut and carcass after dissection. Transmission was assessed by determination of cytopathogenic effect of saliva (CPE) on Vero cells, followed by determination of 50% tissue culture infectious dose (TCID₅₀) for CPE positive samples. Additionally, the presence of Wolbachia endosymbiont infection was assessed by qPCR and standard PCR.

Results: Culex mosquitoes were found unable to present Zika virus in saliva, as demonstrated by molecular as well as virological methods. Aedes aegypti, was used as a positive control for Zika infection and showed a high level of virus infection, dissemination and transmission. Local Aedes species, Ae. notoscriptus and, to a lesser degree, Ae. camptorhynchus were found to expel virus in their saliva and contained viral nucleic acid within the midgut. Molecular assessment identified low or no dissemination for these species, possibly due to low virus loads. Ae. albopictus from Torres Strait islands origin was shown as an efficient vector. Cx quinquefasciatus was shown to harbour Wolbachia endosymbionts at high prevalence, whilst no Wolbachia was found in Cx annulirostris. The Australian Ae. albopictus population was shown to harbour Wolbachia at high frequency.

Conclusions: The risk of local Aedes species triggering large Zika epidemics in the southern parts of Australia is low. The potentially invasive Ae. albopictus showed high prevalence of virus in the saliva and constitutes a potential threat if this mosquito species becomes established in mainland Australia. Complete risk analysis of Zika transmission in the temperate zone would require an assessment of the impact of temperature on Zika virus replication within local and invasive mosquito species.

Keywords: Aedes aegypti; Aedes albopictus; Aedes notoscriptus; Australia; Culex quinquefasciatus; Invasive; Vector competence; Zika virus.

Fig. 1

Map showing the origin of the mosquito populations. The Brisbane zone, as cited in the reference numbered 20, is indicated

Fig. 2

Quantitation of viral load in CPE positive saliva samples. Mosquito samples producing CPE and given a TCID₅₀ value were plotted, by species with number of tested samples. Horizontal bars are means with 95%CI. P-values of two-tailed Mann Whitney tests are presented (ns = not significant). For Ae. camptorhynchus, tests are not applicable due to the low number of values

Fig. 3

Viral RNA determination in mosquito organs. Real-time RT-PCR was performed on RNA collected from mosquito midgut (b) or carcass (c) using ZIKV NS5 primers. Horizontal bars as mean with 95% Confidence Interval. For comparison, a dilution curve for TCID₅₀ is given in (a), where real-time RT-PCR was performed on RNA collected from 10-fold dilutions of Zika virus of known titre. The average Ct value of 3 samples of freshly blood-fed Ae. camptorhynchus whole bodies, including the midgut is 30.03; it is equivalent to the viral nucleic acid loading input before any replication, approximatively 10^3.5 of TCID₅₀. Significant p values are denoted on the graph