Vector competence of Aedes albopictus field populations from Reunion Island exposed to local epidemic dengue viruses

Abstract

Dengue virus (DENV) is the most prevalent mosquito-borne Flavivirus that affects humans worldwide. Aedes albopictus, which is naturally infected with the bacteria Wolbachia, is considered to be a secondary vector of DENV. However, it was responsible for a recent DENV outbreak of unprecedented magnitude in Reunion Island, a French island in the South West Indian Ocean. Moreover, the distribution of the cases during this epidemic showed a spatially heterogeneous pattern across the island, leading to questions about the differential vector competence of mosquito populations from different geographic areas. The aim of this study was to gain a better understanding of the vector competence of the Ae. albopictus populations from Reunion Island for local DENV epidemic strains, while considering their infection by Wolbachia. Experimental infections were conducted using ten populations of Ae. albopictus sampled across Reunion Island and exposed to three DENV strains: one strain of DENV serotype 1 (DENV-1) and two strains of DENV serotype 2 (DENV-2). We analyzed three vector competence parameters including infection rate, dissemination efficiency and transmission efficiency, at different days post-exposition (dpe). We also assessed whether there was a correlation between the density of Wolbachia and viral load/vector competence parameters. Our results show that the Ae. albopictus populations tested were not able to transmit the two DENV-2 strains, while transmission efficiencies up to 40.79% were observed for the DENV-1 strain, probably due to difference in viral titres. Statistical analyses showed that the parameters mosquito population, generation, dpe and area of sampling significantly affect the transmission efficiencies of DENV-1. Although the density of Wolbachia varied according to mosquito population, no significant correlation was found between Wolbachia density and either viral load or vector competence parameters for DENV-1. Our results highlight the importance of using natural mosquito populations for a better understanding of transmission patterns of dengue.

Fig 1. Map of sampling sites of Aedes albopictus and Aedes aegypti in Reunion Island.

For Ae. albopictus, the sampling sites are colored according to geographic regions: Orange, South; blue, West; purple, North; and green, East. The unique Ae. aegypti population is colored in gray. Population codes and the generation (i.e. F₀, F₁, F₂, F₃₁, or F₃₇ generation) at which mosquitoes were used for the vector competence experiments are given in brackets. The satellite image was extracted from the NASA Earth Observatory public domain image database [42].

Fig 2. Vector competence parameters of Aedes albopictus and Aedes aegypti populations from Reunion Island exposed to the DENV-1 strain.

(A.) Infection rates (IR), (B.) dissemination efficiencies (DE), and (C.) transmission efficiencies (TE) of mosquito populations according to geographic areas (orange: South; blue: West; purple: North; and green: East). Data obtained with Ae. aegypti are shown in gray. Vector competence parameters were examined at 14, 21, and 28 days post-exposure (dpe) to the DENV-1 strain via an infectious blood meal. Error bars correspond to the 95% confidence intervals. ND = not determined.

Fig 3. Correlation between Wolbachia density and viral load of the DENV-1 strain in Aedes albopictus mosquitoes from Reunion Island.

The mosquitoes tested belong to the populations of F₀ generation from Sainte-Marie (F0_SM), Saint-Gilles les Hauts (F0_SG), Saint-Philippe (F0_SPh), or Saint-André (F0_SA), and were collected 21 or 28 days post-exposure (dpe) to the DENV-1 local strain. (A.) The total Wolbachia density (wAlbTot i.e. wAlbA + wAlbB) is given based on the ratio between the Wolbachia genomes (wAlbA and wAlbB) and Ae. albopictus genomes (RSP7 concentrations) in the bodies of mosquitoes according to their population of origin. (B.) Number of DENV-1 RNA copies in the body of the mosquitoes according to their population of origin. In graphs A and B, the line inside each boxplot represents the median and the diamond corresponds to the mean of each population. (C.) Number of DENV-1 RNA copies according to the total Wolbachia density in the body of each mosquito tested (N = 43). *p < 0.05, **p < 0.01, Dunn’s tests.