Wolbachia and the biological control of mosquito-borne disease

Abstract

Mosquito-borne diseases such as malaria, dengue fever and filariasis cause an enormous health burden to people living in tropical and subtropical regions of the world. Despite years of intense effort to control them, many of these diseases are increasing in prevalence, geographical distribution and severity, and options to control them are limited. The transinfection of mosquitos with the maternally inherited, endosymbiotic bacteria Wolbachia is a promising new biocontrol approach. Fruit fly Wolbachia strains can invade and sustain themselves in mosquito populations, reduce adult lifespan, affect mosquito reproduction and interfere with pathogen replication. Wolbachia-infected Aedes aegypti mosquitoes have been released in areas of Australia in which outbreaks of dengue fever occur, as a prelude to the application of this technology in dengue-endemic areas of south-east Asia.

Figure 1

From the generation of Wolbachia-infected mosquitoes to their release for the control of dengue fever in Australia. (1) wMelPop Wolbachia were extracted from their native D. melanogaster host and maintained in Aedes cell lines for 4 years. During this time, genetic changes and phenotypic adaptation occurred (McMeniman et al, 2008; I. Iturbe-Ormaetxe and J. Brownlie, unpublished data). (2) After injecting more than 10,000 A. aegypti embryos with purified Wolbachia, two stable transinfected mosquito lines were generated (McMeniman et al, 2009). (3) Transinfected mosquitoes were characterized for Wolbachia-induced phenotypes, including ability to block pathogen replication (Moreira et al, 2009a). (4) Studies were conducted in secure cages at James Cook University, Cairns, Australia, to determine the ability of transinfected mosquitoes to infect natural populations under semi-natural conditions. (5) Extensive community engagement and information campaigns (McNaughton et al, 2010), together with risk analysis (Murphy et al, 2010) preceded the granting of regulatory approval from the Australian government for the open release of the mosquitoes into the environment. (6) The release of A. aegypti mosquitoes transinfected with wMel Wolbachia started in January 2011 in two townships near to Cairns, Queensland, Australia and will take place for 12 weeks. This will be followed by extensive monitoring of the invasion of Wolbachia into field populations of A. aegypti, and the effect of this on dengue-fever transmission, before further releases in Vietnam (Jeffery et al, 2009) and Thailand. APVMA, Australian Pesticides and Veterinary Medicines Authority; CSIRO, Commonwealth Scientific and Industrial Research Organisation.

Figure 2

Immunofluorescence staining of Aedes aegypti mosquito compound eyes (ommatidia). wMelPop-CLA Wolbachia is shown in green, DENV in red and DNA in blue. (A) Ommatidia section of a control, uninfected mosquito (no DENV, no Wolbachia). (B) Ommatidia of a Wolbachia-free mosquito, 14 days post-infection with DENV (seen in red). (C) Ommatidia of a Wolbachia-transinfected A. aegypti mosquito. Scale bar, 50 μm. (D) Ommatidia of a Wolbachia-transinfected A. aegypti mosquito, 14 days post-infection with DENV; DENV levels are dramatically reduced by the presence of Wolbachia and no DENV signal is detectable (Figure modified from Moreira et al, 2009a, with permission). CLA, cell-line adapted; DENV, Dengue virus.

Figure 3

Phenotypes induced by the wMelPop-CLA strain in transinfected Aepes aegypti mosquitoes. (A) 100% cytoplasmic incompatibility. Crosses between Wolbachia-infected males (PGYP1) and uninfected females (JCU) produce no offspring, whereas every other crossing combination produces viable embryos (Figure taken from McMeniman et al, 2009, with permission). (B) Transmission to almost 100% of the mosquito offspring, as suggested by the high levels of Wolbachia (in red) in the oocytes of infected females, shown by FISH (McMeniman et al, 2009; Moreira et al, 2009a). (C) Dramatic reduction of A. aegypti egg viability (McMeniman et al, 2011). (D) Reduction of adult mosquito lifespan by 50%, compared with wild-type mosquitoes or uninfected counterparts after tetracycline curing of the Wolbachia infection (Figure taken from McMeniman et al, 2009, with permission). (E) Alteration of biting behaviour and reduction of blood-feeding success in ageing A. aegypti mosquitoes. ‘Shaky’ and ‘bendy’ proboscis phenotypes are commonly observed (Turley et al, 2009; Moreira et al, 2009b). (F) Inhibition of DENV replication, in both inbred (PGYP1) and outbred (PGYP1.out) mosquitoes. The bottom panel shows the cellular exclusion of Wolbachia and DENV in fat tissue of some Wolbachia-infected mosquitoes 14 days post-DENV injection (Figures taken from Moreira et al, 2009a, with permission). (G) Widespread tissue infection in A. aegypti, including Malpighian tubules, thoracic ganglia, thoracic muscle, ovaries, heads and salivary glands (Fig 1) as shown by FISH (Wolbachia stained in red, DNA stained in blue; Moreira et al, 2009a; T. Walker et al, unpublished data). CLA, cell-line adapted; DENV, Dengue virus; FISH, flourescence in situ hybridization.