Research

Background

 

A wide variety of arthropods are involved as vectors of arboviruses. To date, there are at least 600 arboviruses. They usually circulate within an enzootic cycle among wild animals, and many cause disease after spillover transmission to humans and domestic animals that are incidental or dead-end hosts.

 

Arboviruses mainly belong to three families: Togaviridae, Flaviviridae, Bunyaviridae. Most of them have a RNA genome. Many arboviruses become detrimental for human and animal health by using a variety of mechanisms. For example, Rift Valley Fever Virus (RVFV) emerges after heavy rainfalls in East Africa linked to the “El Niño” phenomenon and disseminates via livestock trade. However, the greatest threat comes from extensive urbanization in the tropics and the colonization of the domestic environment by anthropophilic mosquitoes, Aedes aegypti and Aedes albopictus. Dengue (DEN) and chikungunya (CHIK) viruses commonly transmitted by these two mosquito species have lost the requirement for enzootic amplification and now produce extensive epidemics in tropical urban centers with extension in northern latitudes.

 

To be able to transmit a virus, the mosquito must be a competent vector for the virus. Briefly, a mosquito female is naturally exposed to a virus while feeding on a viraemic host. Virus enters the midgut with the blood-meal, penetrates into the epithelial cells, replicates and is finally released into the hemocele. The virus disseminates and infects other organs including the salivary glands and can be secreted with the saliva. At that stage, the mosquito is able to transmit the virus to a vertebrate host that first acts as blood source. The period between feeding upon a viremic host and the presence of the virus in the saliva is designated as the “extrinsic incubation period”. Of note for some viruses, in addition to horizontal transmission between vertebrate and arthropod hosts, vertical transmission can occur reflecting the virus persistence in desiccated resistant eggs.

 

The main objective of our laboratory is to dissect the changes in mosquito population structure leading to the selection of viral variants causing new arboviral epidemic outbreaks or emergence. For doing so, different combinations of viruses/blood-feeding mosquitoes have been addressed.

 

 

The Rift Valley fever virus (RVFV)

 

Rift Valley fever virus (RVFV) belongs to the genus Phlebovirus in the family Bunyaviridae and causes intermittent epizootics and sporadic epidemics. The disease leads to substantial die-offs of young animals (especially lambs) with occasional spill over to include other domestic animals and humans. Contrary to most arboviruses, RVFV can be transmitted through direct contacts with body fluids or aborted foetuses of animals. It causes recurrent outbreaks predominantly in sub-Saharan Africa.

 

The most likely routes of RVFV dissemination are the movement of viremic animals or humans. It has been proposed that a single infected person or animal (live or dead) that enters a naive country is sufficient for the initiation of a major outbreak. We have assessed the potential to transmit RVFV of European mosquitoes and more particularly, those collected in southern France. We demonstrated that most were able to transmit the virus (Moutailler et al. Am J Trop Med Hyg 2007; 76(5): 827-829; Moutailler et al. Vector Borne Zoonotic Dis 2008; 8(6): 749-753). Potential RVF vectors are already present in North Africa where border traffic of livestock connects different countries in the region. We found that the predominant mosquito Culex (Culex) pipiens implicated as the primarily vector of RVFV in Egyptian outbreaks were susceptible to RVFV (Amraoui et al. PLoS ONE 2012; 7(5): e36757).

 

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Contrary to most arboviruses, RVFV can be transmitted by direct contacts of animals/humans with infected tissues. We demonstrated that freeing RVFV from replicating in alternation between insects and vertebrates results in deletions of the non-structural gene encoding for the virulence factor. Virulence is likely to be restored when alternation between animals and vectors is initiated again through the acquisition of a complete NSs gene consecutive to gene reassortments in vectors (Moutailler S et al. PLoS Neglect Trop Dis 2011; 5(5): e1156). This suggests that in nature, virulence must be maintained by continuous alternating passages between vertebrates and insects. Indeed, whereas in vertebrates, short term infectious processes deploy with clearance of the virus being rapidly triggered by the immune system, sustained viral replication does occur in competent vectors. Therefore, following the co-ingestion of the deleted-NSs variant and the wild type RVFV, in the course of virus progeny production, the competent vector becomes the site of genetic changes such as RNA segment re-assortments.

 

 

The chikungunya virus (CHIKV)

 

Chikungunya virus (CHIKV) is an arbovirus (Alphavirus genus, Togaviridae family) endemic to Africa, India and South-East Asia. In Africa, the virus is maintained within a sylvatic cycle with wild mosquitoes feeding preferentially on primates. In Asia, CHIKV is mainly transmitted within an urban cycle in an inter-human transmission achieved essentially by human-biting mosquitoes, Aedes aegypti, and Aedes albopictus. The disease is characterized in humans by an acute illness with high fever, rash, headache, myalgia and invalidating arthralgia.

 

albopictus-distribution-europe-2012.jpg 

In 2004, chikungunya (CHIK) emerged in Kenya and spread to different islands of the Indian Ocean including La Réunion where Ae. albopictus is predominant (Delatte et al. Parasite 2008; 15(1) : 3-13). During the 2005-2006 outbreak, Ae. albopictus was able to very efficiently transmit a new variant of CHIKV (Schuffenecker et al. PLoS Med 2006; 3(7): e263). A single substitution at the position 226 in the E1 glycoprotein in a region predicted to interact with the target membrane has enhanced its ability to infect Ae. albopictus (Vazeille et al. PLoS ONE 2007; 2(11): e1168). CHIKV was detectable in the salivary glands and the excreted saliva from day 2 after infection (Dubrulle et al. PLoS ONE 2009; 4(6): e5895). This species was able to sustain a high level of replication with almost 109 viral particles in females from day 3 after ingestion of the infectious blood-meal. Is there any negative effect of viral replication on the mosquito’s life traits? When infected, Ae. albopictus from La Réunion died 6-9 days earlier than non-infected females but not soon enough to interrupt transmission as the extrinsic incubation period is shortened (Martin et al. BMC Ecol 2010; 10: 8).

 

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In Europe, Ae. albopictus was first recorded in Albania in 1979 then in Italy in 1990 introduced from the United States. Now, the species is present in at least 17 European countries. In 2007, a CHIK outbreak occurred in northern Italy where populations of Ae. albopictus were found to be highly susceptible to CHIKV (Talbalaghi et al. Med Vet Entomol 2010; 24(1): 83-87). In France, Ae. albopictus succeeded to become established in the five southern departments since 2004. In laboratory, Ae. albopictus from Alpes maritimes (Vazeille et al. Acta Trop 2008; 105(2): 200-2002)and Corsica (Moutailler et al. Trop Med Int Hlth 2009; 14(9): 1105-1109) showed high susceptibility to CHIKV. This fear was confirmed in the department of Var where two autochthonous CHIK cases were detected at the end of summer 2010 (Grandadam, Emerg Infect Dis 2011; 17(5): 910-913).

 

Because CHIKV circulates in DEN-endemic regions where Ae. albopictus can transmit both viruses, reports of co-infection in humans are increasing. Ae. albopictus orally infected with the two viruses in a single blood-meal is able to deliver concomitantly infectious particles of CHIKV and DENV in saliva (Vazeille et al. PLoS Neglect Trop Dis 2010; 4(6): e706).

 

The role of bacteria in modulating vector competence

 

Pathogens transmitted by arthropods coexist with bacteria, including the endosymbiotic bacterium Wolbachia which can affect the pathogen transmission process to humans. Wolbachia is an obligatory intracellular α-proteobacteria mainly transmitted maternally. They are present in 66% of all arthropod species and in 20% of insect species. In mosquitoes, Wolbachia induces various distortions of host reproduction via a form of sterility known as cytoplasmic incompatibility (CI) promoting its spread of infections into host populations. While Ae. aegypti is free of Wolbachia, Ae. albopictus is superinfected with two Wolbachia strains, wAlbA and wAlbB.

 

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In mosquitoes, the Wolbachia load decreased from day 2 to day 5 after infection with CHIKV at a time when viral replication was increasing (Mousson et al. Mol Ecol 2010; 19(9): 1953-1964). The Wolbachia decrease might result from competition for resources with replicating CHIKV in mosquito cells. This profile was also found when examining the two key organs for viral transmission, midgut and salivary glands where both CHIKV and Wolbachia coexist (Zouache et a., PLoS ONE 2009; 4(7): e6388).

 

Whereas Ae. aegypti is the main vector of dengue viruses, Ae. albopictus is known as a weak dengue vector. We found that Wolbachia was able to limit viral replication in salivary glands of Ae. albopictus leading to lower potential for transmission of DENV (Mousson et al., submitted).

 

After removing wAlbA and wAlbB from naturally infected Ae. albopictus and inoculating a Wolbachia from Drosophila melanogaster (wMel), Ae. albopictus is able to strongly inhibit dengue virus (DENV) and CHIKV suggesting that inhibition depends on the Wolbachia strain (Blagrove et al. Proc Natl Acad Sci USA;2012 ; 109(1): 255-60).

 

Our beloved mosquitoes

 

aedes-albopictus.jpg- Aedes albopictus Skuse, 1894

The Asian Tiger mosquito, Aedes albopictus (Culicidae, Aedes) is native to Southeast Asia and is commonly found in peri-urban, rural and forested areas. Ae. albopictus has no ecological specialisation, has succeeded in colonising temperate zones such as the United States and Europe and is currently invading African countries. Ae. albopictus is a competent laboratory vector for numerous arboviruses. It has been involved in the recent chikungunya outbreaks.

 

 

 

 

aedes-aegypti-1.jpg- Aedes aegypti Linnaeus, 1762

Aedes aegypti (Culicidae, Aedesis) a day-biting mosquito described under two forms: Ae. aegypti formosus and Ae. aegypti aegypti - differing in bio-ecology, behaviour, genetic variations and susceptibility to viruses. The taxonomic status of these two forms is debatable and no absolute diagnostic character is yet available. The pale domestic and anthropophilic form, Ae. ae. aegypti, breeds essentially in man-made sites whereas the dark, peri-domestic and less anthropophilic form, Ae. ae. formosus, is found mostly in Africa. Ae. ae. aegypti is implicated in dengue epidemics worldwide.

 

 

 



aedes-vexans-1.jpg- Aedes vexans Meigen, 1830
Aedes vexans (Culicidae, Aedesis) a species complex composed of two subspecies, Aedes vexans vexans in northern latitudes and Aedes vexans arabiensis in tropical regions. Aedes vexans vexans is the most abundant floodwater mosquito in the Northern hemisphere where it is a serious blood-feeding pest of humans and animals. Ae. vexans arabiensis was among the main mosquitoes found infected with RVFV in West Africa. This wide distribution is of major concern because of the potential for RVFV to invade new geographic areas, such as seen during the epidemic/epizootic in Saudi Arabia in 2000.

 

 

 

culex-pipiens-2.jpg- Culex pipiens Linnaeus, 1758

Cx. pipiens pipiens (Culicidae, Culex) has two distinct forms or biotypes: form pipiens and form molestus which are morphologically indistinguishable and differ in physiology and behavior. Cx. pipiens form pipiens is subjected to diapause (heterodynamic), is anautogeneous (only lays eggs after a blood-meal), and eurygamous (unable to mate in confined spaces).On the other hand, Cx. pipiens form molestus does not diapause (homodynamic), is autogeneous (lays first batch of eggs without taking a blood-meal) and stenogamous (mates in confined spaces). The two forms have different trophic preferences: pipiens biting mainly birds and molestus mammals/humans. Cx. p. pipiens is a competent vector of several pathogens infecting animals and humans including West Nile virus, Rift Valley Fever virus and filarial worms.

Updated on 20/01/2014

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