Our study aimed to measure whether VT was less likely to occur in wMel-infected Ae. aegypti mosquitoes versus their wild type counterparts. The estimation of VT was conducted in 480 wMel- Ae. aegypti females, alongside 480 Wt females. Ten days after intrathoracic inoculation with DENV-1, 420 Wt and 389 wMel surviving females were given a non-infectious blood meal. Between five to seven days later, ~ 10,328 eggs were collected from 343 virus-infected Wt F0 females and ~ 12,027 F1 eggs from 304 virus-infected wMel F0 females (Fig. 1). After egg collection, the F0 females were collected and tested for DENV-1 RNA concentrations. DENV-1 RNA concentrations in F0 Wt and wMel-infected Ae. aegypti whole bodies were comparable and high: 7.7 (95% CI = 7.63–7.72) and 7.6 (95% CI = 7.57–7.67) log10 copies/mL, respectively (Fig. 2).
From the ~ 10,328 eggs collected from 343 virus-infected Wt F0 females, a total of 6,047 F1 adults emerged. Similarly, the ~ 12,027 F1 eggs from 304 virus-infected wMel F0 completed their development, providing 5,541 wMel F1 adults (Fig. 1). All Wt F1 adult mosquitoes were grouped into 785 pools, and 5,500 wMel F1 adult mosquitoes were grouped into 712 pools (Additional file 1: Table S1 shows this in more detail). These pools were tested for DENV-1 and nine positive pools (9/785 (1.14%)) were detected from the Wt group, while no positive pools were found in the wMel group. The MLE for VT of DENV-1 in Wt mosquitoes was 1.49% (95%CI = 0.73–2.74) per 1,000 adults, while this rate was 0% in wMel mosquitoes. Thirteen DENV-1 infected Wt F1 individuals were identified from the nine positive pools, of which ten were females. According to MLE of 1.49% for VT of DENV-1 per 1,000 Wt mosquitoes, we can expect that approximately 90 out of the 6,047 observed F1 adults have acquired the virus vertically from their infected mothers. The higher VT rates in Wt Ae. aegypti (1.49%) compared to the VT frequency identified in field-reared Ae. aegypti (0.23%) using patient-derived blood meals [9] might be attributed to the bypassing of the midgut barrier, which plays an important role in virus replication and dissemination [34].
The MLE of DENV-1 infection rate observed in wMel-infected Ae. aegypti was comparable to that found in wMel-infected Ae. aegypti from Brazil [35]. In our study, we optimized the conditions to increase the probability of a VT event in wild-type Ae. aegypti, by employing intrathoracic inoculation of virus and a long extrinsic incubation period. Furthermore, our study was conducted with a large sample size, ∼11,547 total F1 Ae. aegypti (6,047 Wt and 5,500 wMel). Despite the advantages, the VT event was not recorded in wMel-infected Ae. aegypti. The absence of DENV infection in wMel-infected F1 supports the hypothesis that wMel could help reduce VT. The ability of wMel to decrease DENV replication in wMel-carrying mosquitoes has been used to reduce the incidence of dengue through the deployment of Wolbachia mosquitoes in endemic areas. The ability of wMel to reduce the vertical transmission of DENV can be attributed to various mechanisms, including resource competition, immune system activation and interference with viral replication in the reproductive tissues of mosquitoes [36–39]. These mechanisms are recognized for their ability to reduce the replication of dengue virus and could therefore limit its transmission from one generation to the next generation. The capacity of wMel to diminish dengue transmission in both horizontal and vertical modes of transmission is critical in mitigating the incidence of dengue, ultimately decreasing the overall disease burden.