Vector-borne pathogens are important global (re)-emerging pathogens (more than 17% of infectious diseases) making them a major concern for public health. They are transmitted by a vector, generally an haematophagous arthropods such as mosquitoes or ticks. Vectors are living organisms that can transmit infectious diseases between humans or from animals to humans. Many of these vectors are bloodsucking insects, which ingest disease-producing microorganisms during a blood meal from an infected host (human or animal) and later inject it into a new host during their subsequent blood meal. Mosquitoes are the most important and the best known disease vectors. .
Pathogens responsible for vector-borne diseases can be parasite (Plasmodium falciparum, dirofilaria immitis), and viruses (i.e. arboviruses). 100 out of 500 estimated unique arboviruses are pathogen for humans. Arboviruses do not represent a single family of viruses but several viruses from different families: Flaviviridae (Dengue virus DENV, Zika virus ZIKV, Yellow Fever virus YFV, West Nile virus WNV, etc), Togaviridae (Chikungunya virus CHIKV, Ross River virus RRV, etc), Bunyaviridae (Rift Valley virus RVV, etc), which explain why specific treatments or vaccines need to be adapted for each family . By infecting more than 400 million people per year, DENV is the most important arbovirus. In recent decades, the global incidence of dengue has risen sharply and the transmission has increased in urban and suburban areas. More recently, Zika virus has been in the news : outbreaks occurred in French Polynesia (2013) and Brazil (2016), causing serious neurological complications such as Guillain-Barré syndrome and microcephaly. ZIKV epidemic has become since the begining of 2016 a Public Health Emergency of International Concern, according to WHO. One of the particularities of the arboviruses is that they infect two kinds of host : a mammalian host, which ensures the maintenance and the amplification of the virus, and an arthropod vector (mosquitoes) that ensures its dissemination. In fact, arboviruses originally circulate in a sylvatic cycle in which the virus is transmitted between non-human primates by zoophilic mosquitoes. During its repeated intrusions into forests (hunting, deforestation…), men are accidentally contaminated by contact with anthropo-zoophilic vectors. Then, anthropophilic vectors ensure the dissemination of arboviruses between humans in urban areas, during the urban cycle and epidemic cycle.
The lifetime of a mosquito is about 50 days. Its lifestyle changes from the aquatic mode to the terrestrial mode and is divided into 4 stages: egg, larvae, pupae, adult. The mean duration of this developmental cycle is comprised between 8 and 12 days. First, a female lays between 50-300 eggs where standing water is present : a small amount of water is sufficient and breeding sites can be natural (flooded grasslands, waste water) or artificial (tires, bowls, cups, fountains, tires, barrels, vases). These eggs stick to container walls like glue, and hatch between 3 to 7 days after egg-laying according to e adult mosquitonvironmental conditions. In case of unsuitable conditions, some eggs can resist desiccation for several years or enter diapause. Second, when water completely covers the eggs, larvae emerge. Often called “wigglers”, larvae are active in water, where respiration occurs thanks to a siphon on the 8th segment of the abdomen of the larvae. Hung upside-down, larvae eat organic matter such as algae, microscopic organisms in order to ensure its development to the third stage: pupae. Unlike the larval stage, pupae are not active and do not feed. A pair of respiratory trumpets allows pupae to live 2-3 days. Then, an adult mosquito emerges and flies away. Male and female adult mosquitoes are physically distinguishable thanks to a very bushy antennae of the male, allowing it to sense females. While males feed on nectar and sources of sugar, only females are haematophagous because they need supplemental proteins to develop eggs. During blood-feeding, adult female mosquitoes bite people and animals and pump blood. We don’t feel the bite of the mosquito because of the anesthetic agents injected with saliva (that contains also anticoagulants and anti-inflammatory molecules).
Fortunately, not all mosquitoes are able to transmit arboviruses! The competence of a mosquito is the ability of a vector to be infected by and to transmit a pathogen in natural conditions. For example, competent vectors of malaria are Anopheles mosquitoes. Competent vectors for DENV, CHIKV, ZIKV, YFV, RRV are mostly mosquitoes Aedes genus.
An arbovirus-related outbreak arises when the virus is introduced into a permissive environment where anthropophilic competent vectors and susceptible human population coexist. The mosquito becomes infected when taking a blood meal from a vertebrate host in a phase of viremia. In the mosquito, the virus can replicate itself, transit across various anatomical barriers of the mosquito and reach the salivary glands. Once in the saliva, the virus is transmitted to the host during blood-feeding. The period between infection of the mosquito by a human and infection of a human by the same mosquito changes according to the temperature. At 20°C, this extrinsic incubation period (EIP) is about 2 weeks but at 35°C, in endemic areas, this EIP can be achieved in less than one week! Since only yellow fever is prevented by vaccination, the single efficient method to fight against arboviruses is vector control. Several methods exist but the most common is insecticide spraying (particularly pyrethroids and organophosphates). Unfortunately, mosquitoes become more and more resistant and more and more classes of insecticides are developed, increasing multiresistances and high environmental impact. Moreover, since we are unable to maintain effective control of mosquito populations, the fact that resistance contributes to the re-emergence of arboviruses cannot be excluded . That’s why insecticide resistance is now regarded by the WHO as a major obstacle to the control of diseases transmitted by mosquitoes. Many organizations have been created and act together. For example, in accordance with the Research and Training Program on Tropical Diseases (TDR), supported by WHO, the Department of Neglected Tropical Diseases (NTDs) and the WIN network met for the first time. WIN network is comprised of fifteen international institutions recognized for research on vectors to counter the resistance to insecticides globally. Additionally, the IRD, CNRS and the Institut Pasteur of French Guiana are highly mobilized in France. Concerning France, insecticides are sprayed when human infected cases are identified and where competent mosquitoes are present. Considering asymptomatic carriers, symptomatic carriers who think they have flu, and physicians who don’t report identified cases, the majority of disease importations are not detected. It seems obvious that it would be better to anticipate disease importations and outbreak (no s) occurrences by acting before infected cases are identified . Finally, competent vectors distribution all over the world is well known but that of infected vectors is not. By mapping the infected vectors’ distribution, we could introduce a more rational use of insecticides and focus our efforts on concerned regions. In this context, our Mos(kit)O project consists in the development of the infected mosquitoes mapping, the indispensable tool that vector control is lacking. Let us present to you our Mos(kit)O project in OVERVIEW section!