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Wolbachia used to counter dengue fever The mosquito-borne viral disease dengue fever is an increasing problem in tropical and subtropical regions. Traditional control measures aimed at reducing populations of the main transmission vector, Aedes aegypti , have had little success. Two papers in this issue report an alternative approach to mosquito population control using the bacterium Wolbachia pipientis , natural insect symbionts that facilitate their own transmission through a process called cytoplasmic incompatibility. In the first paper, Scott O'Neill and colleagues describe a Wolbachia strain derived from fruitflies that significantly reduces dengue virus carriage in mosquitoes without imposing a fitness cost. In the second paper, they demonstrate in a controlled field trial that the release of colonized mosquitoes leads to successful invasion of natural mosquito populations. These results suggest a viable strategy to control dengue fever. Genetic manipulations of insect populations for pest control have been advocated for some time, but there are few cases where manipulated individuals have been released in the field and no cases where they have successfully invaded target populations 1 . Population transformation using the intracellular bacterium Wolbachia is particularly attractive because this maternally-inherited agent provides a powerful mechanism to invade natural populations through cytoplasmic incompatibility 2 . When Wolbachia are introduced into mosquitoes, they interfere with pathogen transmission and influence key life history traits such as lifespan 3 , 4 , 5 , 6 . Here we describe how the w Mel Wolbachia infection, introduced into the dengue vector Aedes aegypti from Drosophila melanogaster 7 , successfully invaded two natural A. aegypti populations in Australia, reaching near-fixation in a few months following releases of w Mel-infected A. aegypti adults. Models with plausible parameter values indicate that Wolbachia -infected mosquitoes suffered relatively small fitness costs, leading to an unstable equilibrium frequency <30% that must be exceeded for invasion. These findings demonstrate that Wolbachia -based strategies can be deployed as a practical approach to dengue suppression with potential for area-wide implementation.

Wolbachia used to counter dengue fever The mosquito-borne viral disease dengue fever is an increasing problem in tropical and subtropical regions. Traditional control measures aimed at reducing populations of the main transmission vector, Aedes aegypti , have had little success. Two papers in this issue report an alternative approach to mosquito population control using the bacterium Wolbachia pipientis , natural insect symbionts that facilitate their own transmission through a process called cytoplasmic incompatibility. In the first paper, Scott O'Neill and colleagues describe a Wolbachia strain derived from fruitflies that significantly reduces dengue virus carriage in mosquitoes without imposing a fitness cost. In the second paper, they demonstrate in a controlled field trial that the release of colonized mosquitoes leads to successful invasion of natural mosquito populations. These results suggest a viable strategy to control dengue fever. Dengue fever is the most important mosquito-borne viral disease of humans with more than 50 million cases estimated annually in more than 100 countries 1 , 2 . Disturbingly, the geographic range of dengue is currently expanding and the severity of outbreaks is increasing 2 , 3 , 4 . Control options for dengue are very limited and currently focus on reducing population abundance of the major mosquito vector, Aedes aegypti 5 , 6 . These strategies are failing to reduce dengue incidence in tropical communities and there is an urgent need for effective alternatives. It has been proposed that endosymbiotic bacterial Wolbachia infections of insects might be used in novel strategies for dengue control 7 , 8 , 9 . For example, the w MelPop-CLA Wolbachia strain reduces the lifespan of adult A. aegypti mosquitoes in stably transinfected lines 8 . This life-shortening phenotype was predicted to reduce the potential for dengue transmission. The recent discovery that several Wolbachia infections, including w MelPop-CLA, can also directly influence the susceptibility of insects to infection with a range of insect and human pathogens 9 , 10 , 11 has markedly changed the potential for Wolbachia infections to control human diseases. Here we describe the successful transinfection of A. aegypti with the avirulent w Mel strain of Wolbachia , which induces the reproductive phenotype cytoplasmic incompatibility with minimal apparent fitness costs and high maternal transmission, providing optimal phenotypic effects for invasion. Under semi-field conditions, the w Mel strain increased from an initial starting frequency of 0.65 to near fixation within a few generations, invading A. aegypti populations at an accelerated rate relative to trials with the w MelPop-CLA strain. We also show that w Mel and w MelPop-CLA strains block transmission of dengue serotype 2 (DENV-2) in A. aegypti , forming the basis of a practical approach to dengue suppression 12 .

Dengue is a mosquito-borne disease of growing global health importance. Prevention efforts focus on mosquito control, with limited success. New insights into the spatiotemporal drivers of dengue dynamics are needed to design improved disease-prevention strategies. Given the restricted range of movement of the primary mosquito vector, Aedes aegypti , local human movements may be an important driver of dengue virus (DENV) amplification and spread. Using contact-site cluster investigations in a case-control design, we demonstrate that, at an individual level, risk for human infection is defined by visits to places where contact with infected mosquitoes is likely, independent of distance from the home. Our data indicate that house-to-house human movements underlie spatial patterns of DENV incidence, causing marked heterogeneity in transmission rates. At a collective level, transmission appears to be shaped by social connections because routine movements among the same places, such as the homes of family and friends, are often similar for the infected individual and their contacts. Thus, routine, house-to-house human movements do play a key role in spread of this vector-borne pathogen at fine spatial scales. This finding has important implications for dengue prevention, challenging the appropriateness of current approaches to vector control. We argue that reexamination of existing paradigms regarding the spatiotemporal dynamics of DENV and other vector-borne pathogens, especially the importance of human movement, will lead to improvements in disease prevention.

Malaria parasite transmission depends on the successful transition of Plasmodium through discrete developmental stages in the lumen of the mosquito midgut. Like the human intestinal tract, the mosquito midgut contains a diverse microbial flora, which may compromise the ability of Plasmodium to establish infection. We have identified an Enterobacter bacterium isolated from wild mosquito populations in Zambia that renders the mosquito resistant to infection with the human malaria parasite Plasmodium falciparum by interfering with parasite development before invasion of the midgut epithelium. Phenotypic analyses showed that the anti-Plasmodium mechanism requires small populations of replicating bacteria and is mediated through a mosquito-independent interaction with the malaria parasite. We show that this anti-Plasmodium effect is largely caused by bacterial generation of reactive oxygen species.

For RNA viruses, rapid viral evolution and the biological similarity of closely related host species have been proposed as key determinants of the occurrence and long-term outcome of cross-species transmission. Using a data set of hundreds of rabies viruses sampled from 23 North American bat species, we present a general framework to quantify per capita rates of cross-species transmission and reconstruct historical patterns of viral establishment in new host species using molecular sequence data. These estimates demonstrate diminishing frequencies of both cross-species transmission and host shifts with increasing phylogenetic distance between bat species. Evolutionary constraints on viral host range indicate that host species barriers may trump the intrinsic mutability of RNA viruses in determining the fate of emerging host-virus interactions.

Metarhizium anisopliae infects mosquitoes through the cuticle and proliferates in the hemolymph. To allow M. anisopliae to combat malaria in mosquitoes with advanced malaria infections, we produced recombinant strains expressing molecules that target sporozoites as they travel through the hemolymph to the salivary glands. Eleven days after a Plasmodium-infected blood meal, mosquitoes were treated with M. anisopliae expressing salivary gland and midgut peptide 1 (SM1), which blocks attachment of sporozoites to salivary glands; a single-chain antibody that agglutinates sporozoites; or scorpine, which is an antimicrobial toxin. These reduced sporozoite counts by 71%, 85%, and 90%, respectively. M. anisopliae expressing scorpine and an [SM1]₈:scorpine fusion protein reduced sporozoite counts by 98%, suggesting that Metarhizium-mediated inhibition of Plasmodium development could be a powerful weapon for combating malaria.

Manipulating an insect vector Genetic approaches to manipulating or eradicating disease vectors have been proposed as alternatives to malaria eradication. The success of this approach depends on efficient spread of a genetic modification in field populations. Windbichler et al . show that a synthetic genetic element consisting of mosquito regulatory elements and the homing endonuclease gene I-SceI can spread from a small number of individual Anopheles gambiae mosquitoes into large receptive populations in just a few generations. This is the first demonstration of a synthetic gene drive system in the main human malaria vector — and a similar approach should be applicable to many other pest species. Genetic methods of manipulating or eradicating disease vector populations have long been discussed as an attractive alternative to existing control measures because of their potential advantages in terms of effectiveness and species specificity 1 , 2 , 3 . The development of genetically engineered malaria-resistant mosquitoes has shown, as a proof of principle, the possibility of targeting the mosquito’s ability to serve as a disease vector 4 , 5 , 6 , 7 . The translation of these achievements into control measures requires an effective technology to spread a genetic modification from laboratory mosquitoes to field populations 8 . We have suggested previously that homing endonuclease genes (HEGs), a class of simple selfish genetic elements, could be exploited for this purpose 9 . Here we demonstrate that a synthetic genetic element, consisting of mosquito regulatory regions 10 and the homing endonuclease gene I-SceI 11 , 12 , 13 , can substantially increase its transmission to the progeny in transgenic mosquitoes of the human malaria vector Anopheles gambiae . We show that the I-SceI element is able to invade receptive mosquito cage populations rapidly, validating mathematical models for the transmission dynamics of HEGs. Molecular analyses confirm that expression of I-SceI in the male germline induces high rates of site-specific chromosomal cleavage and gene conversion, which results in the gain of the I-SceI gene, and underlies the observed genetic drive. These findings demonstrate a new mechanism by which genetic control measures can be implemented. Our results also show in principle how sequence-specific genetic drive elements like HEGs could be used to take the step from the genetic engineering of individuals to the genetic engineering of populations.

The mosquito Anopheles gambiae is the major vector of malaria in sub-Saharan Africa. It locates its human hosts primarily through olfaction, but little is known about the molecular basis of this process. Here we functionally characterize the Anopheles gambiae odorant receptor (AgOr) repertoire. We identify receptors that respond strongly to components of human odour and that may act in the process of human recognition. Some of these receptors are narrowly tuned, and some salient odorants elicit strong responses from only one or a few receptors, suggesting a central role for specific transmission channels in human host-seeking behaviour. This analysis of the Anopheles gambiae receptors permits a comparison with the corresponding Drosophila melanogaster odorant receptor repertoire. We find that odorants are differentially encoded by the two species in ways consistent with their ecological needs. Our analysis of the Anopheles gambiae repertoire identifies receptors that may be useful targets for controlling the transmission of malaria.

Leucine-rich repeat-containing proteins are central to host defense in plants and animals. We show that in the mosquito Anopheles gambiae, two such proteins that antagonize malaria parasite infections, LRIM1 and APL1C, circulate in the hemolymph as a high-molecular-weight complex held together by disulfide bridges. The complex interacts with the complement C3-like protein, TEP1, promoting its cleavage or stabilization and its subsequent localization on the surface of midgut-invading Plasmodium berghei parasites, targeting them for destruction. LRIM1 and APL1C are members of a protein family with orthologs in other disease vector mosquitoes and appear to be important effectors in innate mosquito defenses against human pathogens.

Aim Recreational boating is arguably the largest unregulated vector for the introduction and spread of marine invasive species. Hull fouling communities have been recognized to harbour non-indigenous species (NIS), but presence should not be equated with transport. In this study, we characterize the presence of NIS in hull fouling communities, determine if host vessels transport these species and evaluate the importance of recreational boating as a vector for introduction and spread. Location Coastal British Columbia (BC), Canada. Methods Dive surveys in BC marinas were conducted to record the presence of NIS and to estimate their per cent cover. In addition, a boater questionnaire survey was used to determine common travel and maintenance practices. These results were combined to investigate the potential for recreational boats to transport NIS. Results Nine NIS, including the highly invasive ascidians Styela clava and Botrylloides violaceus y and the macroalga Sargassum muticum y were found in hull fouling communities on recreational boats. Overall, per cent cover was generally low; however, niche areas were commonly fouled, even on active and otherwise clean boats. Fouling of niche areas was not related to either antifouling paint age or travel frequency, and fouling levels were highly variable among individual boats both within marinas and across regions. Main conclusions Recreational boating is a major vector contributing to the spread of marine invasive species. Our results indicate that recreational boats represent a high-risk vector both for primary introduction and secondary spread of marine NIS and should be subject to vector management regulations.