Wolbachia-based emerging strategies for control of vector-transmitted disease

Abstract

Dengue fever is a mosquito-transmitted disease of great public health importance. Dengue lacks adequate vaccine protection and insecticide-based methods of mosquito control are proving increasingly ineffective. Here we review the emerging use of mosquitoes transinfected with the obligate intracellular bacterium Wolbachia pipientis for vector control. Wolbachia often induces cytoplasmic incompatibility in its mosquito hosts, resulting in infertile progeny between an infected male and an uninfected female. Wolbachia infection also suppresses the replication of pathogens in the mosquito, a process known as "pathogen blocking". Two strategies have emerged. The first one releases Wolbachia carriers (both male and female) to replace the wild mosquito population, a process driven by cytoplasmic incompatibility and that becomes irreversible once a threshold is reached. This suppresses disease transmission mainly by pathogen blocking and frequently requires a single intervention. The second strategy floods the field population with an exclusively male population of Wolbachia-carrying mosquitoes to generate infertile hybrid progeny. In this case, transmission suppression depends largely on decreasing the population density of mosquitoes driven by infertility and requires continued mosquito release. The efficacy of both Wolbachia-based approaches has been conclusively demonstrated by randomized and non-randomized studies of deployments across the world. However, results conducted in one setting cannot be directly or easily extrapolated to other settings because dengue incidence is highly affected by the conditions into which the mosquitoes are released. Compared to traditional vector control methods, Wolbachia-based approaches are much more environmentally friendly and can be effective in the medium/long term. On the flip side, they are much more complex and cost-intensive operations, requiring a substantial investment, infrastructure, trained personnel, coordination between agencies, and community engagement. Finally, we discuss recent evidence suggesting that the release of Wolbachia-transinfected mosquitoes has a moderate potential risk of spreading potentially dangerous genes in the environment.

Keywords: Arboviruses; Biocontrol; Environmental risk; Genetically modified organism; Genomics; Symbiosis; Wolbachia pipientis.

Fig 1.. Reproductive viability of A. aegypti depending on the presence of Wolbachia.

Female mosquitoes carrying Wolbachia produce viable offspring regardless of whether the male carries Wolbachia or not and the offspring is infected with Wolbachia. The outcome of a cross between a male mosquito carrying Wolbachia and a female mosquito depends on the status of the female (1-A, 2-A, 3-A). If the female is Wolbachia -free (1-B, 1-C) or if it carries a different strain of Wolbachia (2-C, 3-B), fertilization does not occur, resulting in infertile eggs. However, if the female harbors the same Wolbachia strain as the male (2-B, 3-C), offspring is produced, likely carrying Wolbachia. This figure has been adapted from (Wang et al., 2021).

Figure 2.. Distribution of Wolbachia genomes belonging to supergroups A and B by host.

Panel Cladogram showing the phylogenetic relatedness of 175 Wolbachia genomes belonging to supergroups A and B. Panel A. Genomes in the majority supergroup B clade (with majority supergroup A collapsed). Panel B. Genomes belonging to the majority supergroup A clade. The supergroup classification for each individual sequence is shown in the first column of the grid, with supergroup A highlighted in pink and supergroup B highlighted in blue. Columns 2-16 in the grid show the host from which Wolbachia was isolated, denoted by letters of the alphabet. The correspondence of the letters to the common name and icon are shown in the legend to the right. The complete genome assemblies of Wolbachia were downloaded from NCBI on June 30, 2023. From this set of 212 genomes, we selected those belonging to the two predominant supergroups (supergroups A and B, n=176). For each of the samples included, the GenBank accession number, host and date processed are listed in Supplemental Table 1. The phylogenetic analysis was performed using Validated Bacterial Core Genomes VBCG with default settings (Tian et al., 2023). The cladogram was generated and annotated using Interactive Tree Of Life (ITOL) v6.9 (https://itol.embl.de/ accessed on March 2024 (Letunic and Bork, 2021). Supergroups were labeled based on the NCBI annotation, and if no records were found on NCBI, a literature search was performed using the NCBI GenBank accession number (Benson et. al. 2013; Schoch et al., 2020). The NCBI Taxonomy label was used to identify the Blast common name for each genome (Sayers et al., 2022).

Fig 3.. Implementation sites for population replacement interventions.

The map shows the countries where pilot studies using the replacement strategy of Wolbachia-transinfected mosquito release have been implemented, highlighted in red. The box lists the country, the mosquito release period and, in cases where Wolbachia’s prevalence was demonstrated to have reached the stable introgression threshold, the associated decrease in Dengue prevalence is listed (in percentage). Studies that are still in process are labelled as “ongoing”.