mosquitoes transmit some of the most deadly infectious diseases of humans . although malaria is the best known
arthropod - transmitted viruses ( arboviruses ) include the causative agents of dengue , yellow fever , west nile virus , chikungunya , and many others .
the life cycle of these viruses typically depends on transmission from a suitable vertebrate host via a mosquito vector to another suitable vertebrate , and so on for ever . for some of these viruses , such as dengue ,
humans are the only suitable vertebrate species across most or all of their range ; others , such as west nile virus , can infect a wide range of vertebrates .
the mosquito is exposed to the pathogen when she ( only female mosquitoes bite ) takes a blood meal from an infectious vertebrate .
the virus infects the mosquito , typically first in the midgut and then disseminating through the body . when the salivary glands become infected ,
the next time she takes a blood meal , her food source is exposed to the virus . if this individual becomes infected , for a period of time it will become infectious to other mosquitoes that bite it , and so the virus continues to propagate and spread .
although insects lack the adaptive immune system of mammals , they are by no means merely passive hosts and vectors for these viruses ; rather , they have multiple innate immune defenses against the various microbial challenges they encounter .
rna interference ( rnai ) is one of the mosquito 's major defenses against arboviruses , and suppression of this pathway has previously been shown to increase viral load in infected mosquitoes .
two recent papers shed more light on the role of this system in insect antiviral innate immunity . writing in bmc microbiology ,
show that sindbis virus engineered to express a suppressor of rnai produces much more virus than normal in infected mosquitoes , and that this engineered virus is lethal to a range of mosquito species .
previous studies used transient knockdown of components of the rnai pathway ; cirimotich et al . use a protein that binds to double - stranded rna ( dsrna ) and presumably protects it from processing in the rnai pathway .
although either approach might have pleiotropic effects , both indicate a key role for the rnai pathway in reducing virus replication and titer . in this regard , in a recent paper in nature , saleh et al .  
show that drosophila can mount a systemic rnai - based response to viruses so that uninfected cells at distal locations can prepare a defense against infection .
this response was shown to depend on a dsrna uptake pathway ; mutant flies defective in this pathway are hypersensitive to infection with drosophila c virus and sindbis virus . targeted rna interference against dengue virus infection .
self - complementary rna with sequences from dengue virus is expressed from a promoter that expresses in the gut of the mosquito soon after a blood meal .
these fragments are bound by the risc complex of proteins and one strand is removed .
the risc complex is now primed to bind and cleave target sequences from an infecting dengue virus , preventing translation from the rna and replication of the virus .
as well as their interest in terms of basic immunology , the mosquito 's antiviral defenses are significant from an applied perspective .
if they could be artificially boosted to the point that infected mosquitoes do not themselves become infectious , mosquitoes that can not transmit a specific virus , or perhaps even a range of viruses , could be produced .
antiviral rnai has already been used to confer resistance to dengue virus in transgenic mosquitoes , by expressing a hairpin rna corresponding to part of the virus .
this long hairpin has the significant advantage of being relatively resistant to mutation of the virus target , as it presumably targets multiple viral sequences .
constitutive expression of a large hairpin rna may be deleterious , but this potential problem was minimized by using a promoter that expresses only in the midgut  the first cells to be infected  and only following a blood meal .
a virus - resistant strain of mosquitoes in the laboratory is , however , only a curiosity or a research tool . to have an impact on disease transmission , the virus - resistance gene(s ) must spread within the vector population in the wild . for diseases such as dengue , where remarkably few competent vectors are required to sustain epidemic transmission ,
such a resistance gene would have to spread to a high allele frequency , so that practically all mosquitoes in the target population carried at least one copy .
unfortunately , insertion and expression of a transgene imposes a fitness penalty ; this may be small , but will still tend to make the transgene decrease in frequency over time , even if a large number are initially introduced . if infection were itself highly deleterious , resistance might be a positive fitness trait , perhaps enough to cause the resistance gene to spread to fixation .
but the viruses carried seem to have remarkably little negative impact on the mosquito vector . an infected mosquito does not clear
so simply shortening the life expectancy of female mosquitoes is potentially an effective way to reduce transmission .
a first step towards a genetic control strategy using this principle was recently achieved , using a pathogenic mutant version of the intracellular bacterium wolbachia pipiens , which reduces the lifespan of mosquitoes that carry it .
if the resistance transgene will not spread through a population on its own , then further genetic tricks are needed to make it spread .
natural self - spreading genetic systems include obligate bacterial endosymbionts such as wolbachia and selfish dna elements such as active transposons .
however , artificial versions of self - spreading systems have proved remarkably difficult to construct , although a demonstration of spreading in drosophila of an artificial dna element based on the tribolium castaneum selfish dna system medea ( maternal - effect dominant embryonic arrest )   is a very promising development .
can we get them to spread ? ' is important , but so is ' can we get them to stop ? '
both wolbachia and medea are extremely difficult to remove from a target population after release  probably impossible in the case of wolbachia  and also difficult or impossible to stop from spreading beyond the target population , perhaps even to all populations of the species worldwide .
this is new territory for genetic engineering and such use or outcomes may well be controversial .
however , it is not an entirely new concept  analogies can be drawn with the introduction of exotic biocontrol agents , for which some of the same issues arise .
( a ) in population replacement strategies , the wild population is invaded by a heritable modification ( e.g. transgene   or pathogenic wolbachia ) that reduces the vector competence of the mosquitoes that carry it .
the number of competent vectors therefore declines , but the total number of ( female ) mosquitoes remains relatively constant , though possibly with some transient change during the invasion .
( b ) in contrast , a population suppression strategy aims to reduce the total number of vector mosquitoes .
the two panels illustrate the changes in female population number and type over time for the two strategies . in both cases
the situation will eventually reverse due to various pressures such as resistance , mutation , immigration , etc , unless some maintenance activities are undertaken .
the strategy outlined above is commonly known as ' population replacement ' : a wild vector population is converted to a modified one in which the mosquitoes have reduced vectorial capacity . the other main strategy for genetic control of mosquitoes is ' population suppression ' . here
the objective is not to change the properties of the vector mosquitoes but to reduce their number , as in the case of the increased mortality induced by cirimotich et al . .
this is a more familiar objective , in that it is also the aim of most source - reduction and chemical insecticide programs .
the major current strategy in this area is based on the use of genetically sterile mosquitoes . in principle , large numbers of sterile male mosquitoes are released so that a wild female has a good chance of mating with a sterile male and so produces no or fewer progeny than usual .
the population therefore tends to decline , and if enough sterile males can be released for long enough , the population collapses .
this sterile insect technique ( sit ) has been used for decades to control some major agricultural pests , sterilizing the insects by irradiating them before release . applying conventional
sit to mosquitoes has proved problematic , but genetic modifications should be able to overcome many of the key difficulties and limitations .
the leading genetically modified sterile release system , known as ridl(release of insects carrying a dominant lethal ) , is ready to enter field trials for aedes aegypti .
they tend to be extremely species - specific , as the modified insects will mate only with their own species .
the self - spreading systems are hard to develop but may be relatively cheap to deploy , as the genetic system does much of the work .
sterile - release methods such as ridlare relatively cheap to develop , but need regular releases of sterile insects to maintain sufficient sterile males in the field . this self - limiting nature 
stop releasing and the transgene will rapidly disappear from the field population  may , however , be better accepted by the public and regulators , and these systems are likely to be the first ones used in the field . none of these systems should be seen as a ' magic bullet ' .
self - spreading systems will undoubtedly fail over time , due to mutation and pathogen evolution , and replacement versions will be required .
sterile - release methods will be much more effective in the context of an integrated vector management program than on their own .
all of these methods will have to be tested in the context of different health systems , cultures and ecosystems ; experience will determine where each is more or less valuable .
nonetheless , these powerful genetics - based vector - control tools , about to emerge from the laboratory into the field , provide rare new hope for the control , and perhaps one day elimination , of some of the world 's major infectious diseases .
this work is funded in part by a grant to the regents of the university of california from the foundation for the national institutes of health through the grand challenges in global health initiative .
thanks to derric nimmo for creating the figures and to neil morrison for comments on the manuscript .
 a recent paper in bmc microbiology shows how suppression of mosquito innate immunity against a virus that the mosquito can normally tolerate increases mosquito mortality . 
 this is just one of several approaches that may soon bring genetics - based mosquito control methods from the laboratory into the field . 
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Boosting mosquito immunity
Spreading a new immunity gene through a wild population
Population suppression using genetically engineered mosquitoes
Acknowledgements