Transmission of pathogens

Culex quinquefasciatus (Pipiens subgroup) has a nocturnal activity with an endophilic tendency (see Figure 1.2). It is the main vector of Bancroftian lymphatic filariasis in urban settings in tropical regions throughout the world,3 while this parasite is transmitted by Anopheles in rural areas and Aedes in most Pacific Islands, a curious situation, as it is usually accepted that co-evolution occurs between the parasite and its vector.

Culex tritaeniorhynchus (Vishnui subgroup) is an important vector of Japanese encephalitis in India and South-East Asia and develops in clear water, in particular rice plantations, but also fish farm basins where manures are added. The Japanese encephalitis virus is also transmitted by Cx. pseudovishnui, Cx. vishnui and Cx. gelidus. Many other arboviruses, such as West Nile, are transmitted by species of the Culex pipiens complex in the USA, by Cx. univittatus and Cx. theileri in Africa, Cx. modestus and Cx. molestus in the Western and Eastern zones of the Mediterranean basin, and Cx. vishnui in India, etc. The virus of Saint Louis encephalitis is also transmitted by species of the Cx. pipiens complex and Cx. nigripalpus in the USA. The Murray Valley encephalitis virus is transmitted by Cx. annulirostris in the USA. In South America, the Bunyavirus of the groups "C", "Guana" and "Nyando" are also transmitted by various Culex species of which Cx. portesi and Cx. vomerifer are examples. The Rift Valley fever is a Phlebovirus transmitted by Cx. pipiens in Egypt and by other Culicidae elsewhere.

1.2.3.1.2 Aedes. Aedes mosquitoes are widespread throughout the World, even in Arctic zones where they represent an important nuisance for the human populations and cattle. There are some 870 species divided into 36 subspecies.

The eggs, generally black and ovoid, are laid separately on a wet substrate. The eggs are resistant to desiccation for several months, then, when they are covered with water, the eggs hatch quickly. Many Aedes larvae develop in small temporary habitats like tree holes (e.g. Ae. pseudoscutellaris), rock holes (e.g. Ae. Togoi and Ae. vittatus), bamboo stumps, coconut shells (e.g. Ae. polynesiensis) and crab holes. Aedes larvae are also well adapted to colonising anthropogenic habitats, such as worn and abandoned tyres (e.g. Ae. albopictus), funeral urns, cans, domestic containers to preserve water (e.g. Ae. aegypti), or any other small domestic containers. Their ability to develop in a large variety of habitats, with both freshwater and brackish water, and to resist desiccation, confers great advantages on Aedes mosquitoes in terms of adaptation and colonization of new sites. As a consequence, larval control of Aedes mosquitoes is quite problematic. In addition, as the larvae develop in water containers reserved for drinking, it is necessary to pay great attention in the choice of the larvicides employed. The majority of Aedes adults have a particular and quite visible ornamentation of white and black scales on the thorax and legs, which often allows for their rapid identification (see Figure 1.3). Aedes albopictus has characteristic black and white scale stripes, at the origin of his name ''Tiger mosquito'', as well as a central line of white scales on the thorax (see Figure 1.3B). Aedes aegypti has a characteristic shape of white scales drawing a ''lyre'' on the thorax (see Figure 1.3A). The normal duration of the cycle, egg to adult, is from one week to 10-12 days.

Aedes tends to be a rural insect which flees the urban areas and pullulates everywhere else, especially in natural sites; except Aedes aegypti which is an urban mosquito that reproduces in all domestic and peridomestic containers (e.g. cans, gutters, dustbins, wheelbarrows, flower pots, drums, dugouts, tyres, concrete pits, grease-boxes, etc.). It bites only during the day, with a peak of activity in the early morning and at sundown. Their behaviour is especially exophagic and exophilic, hence the use of insecticides in indoor spraying (inside residual spraying or IRS) for adult control is inefficient. Control is therefore often based on the elimination of larval habitats, with the participation of local communities. During epidemics, spatial and focal pulverizations must be repeated to eliminate transmission. Epidemics of dengue (in Cayenne) have

Figure 1.3 Adults of Aedes aegypti (A) (Photo © IRD/Jean-Pierre Hervy) and Aedes albopictus (B) (Photo © IRD/Michel Dukhan).

been analysed through geospatial studies, for monitoring the expansion of the disease and in order to adapt the vector control program according to the local context.4

Some species are involved in the transmission of pathogens, including: Aedes aegypti, the main vector of yellow fever of urban type; along with Ae. africanus and Ae. simpsoni, the vectors of the sylvatic yellow fever virus; and Ae. albopictus, a vector of dengue and chikungunya viruses which is spreading across the World (e.g. epidemics in Reunion Island in 2005-2006).5 Aedes polynesiensis and Ae. pseudoscutellaris are important vectors of diurnal and subperiodic Bancroftian filariasis, as well as Ae. togoi, which is also a vector of Brugia filariasis.

Aedes aegypti has a worldwide distribution and has colonized the majority of the tropical countries. A species complex has been recognized with two subspecies (or forms), Aedes aegypti aegypti and Aedes aegypti formosus, which differ in their biology, behaviour, susceptibility to dengue viruses and present genetic variations.6'7 The pale, anthropophilic, domestic form (Ae. ae. aegypti) develops mainly in anthropogenic habitats and is involved worldwide in dengue epidemics. The dark, non-anthropophilic, peri-domestic form (Ae. ae. formosus) occurs primarily in Africa, it preferentially colonizes natural sites and has only been reported in a forest cycle of dengue in Western Africa.8

Aedes albopictus is described as being the principal vector of dengue only when Ae. aegypti is absent or present at low density, generally in continental regions and suburban or rural zones. Its receptivity to the dengue virus is less than Ae. ae. aegypti (pale domestic form), but better than Ae. ae. formosus (dark peri-domestic form). Aedes albopictus is also thought to be responsible for the maintenance of infection because its rate of sexual and transovarian transmission is higher than for Ae. aegypti.9

1.2.3.1.3 Mansonia. Very aggressive during the day, Mansonia mosquitoes (25 species) occur in wet tropical regions, but some species have also been found in Sweden and Tasmania. Some Mansonia species are of medical importance. Mansonia uniformis is a vector of lymphatic filariasis agents,3 such as Brugia malayi in India and Southeast Asia and Wuchereria bancrofti in Asia and New Guinea. Mansonia dives and Ma. titillans (see Figure 1.4)

Figure 1.4 Adult Mansonia titillans (Photo © Sean McCann).

are also vectors of W. bancrofti in Asia and the tropical Americas, respectively; Ma. annulifera, Ma. annulata and Ma. indiana are vectors of B. malayi (nocturnal subperiodic) in Southeast Asia and sometimes also in India. In Africa, Mansonia (Ma. africana and Ma. uniformis) transmit arboviruses, such as "Spondweni" (Flavivirus) and Rift Valley fever (Phlebovirus). Its control is very difficult due to its peculiar larval ecology.

1.2.3.1.4 Haemagogus. Haemagogus mosquitoes (24 species) are only found in Central and South America (Neotropical region). They are first and foremost forest mosquitoes. The adults bite during the day, primarily in the canopy feeding on monkeys. Under certain environmental conditions, in particular at the edge of forests, during tree cutting or in the dry season, they can leave the canopy and bite humans. Several species of Haemagogus are involved in the transmission of the selvatic yellow fever (e.g. Hg. spegazzinii, Hg. leuco-celaenus, Hg. capricornii, Hg. janthinomys, etc.).10

1.2.3.1.5 Sabethes. Sabethes mosquitoes, which include 39 species classified into five subgenera, are distributed in the Neotropical region. They are diurnal biters and forest (canopy) mosquitoes but, as with Haemagogus, they can bite humans when flying near the ground. Sabethes chloropterus has been involved in the transmission of selvatic yellow fever in monkeys, accidentally in man, as well as arboviruses.10

1.2.3.2 Anophelinae

The 484 described species of Anopheles11 are distributed all around the world, except the polar zones and most of the Pacific islands. Anopheles species are distributed by geographic zone,12 whereas some species of Aedes and Culex have a panmictic distribution. The adults are night biters (from sundown to sunrise), although some specimens can bite during the daytime in forests or when cloudy.13 Their behaviour can be anthropophilic/zoophilic, endophagic/ exophagic or endophilic/exophilic. The biting activity is important to know in order to establish appropriate vector control methods. Insecticide treated nets (ITN) are obviously particularly efficient against endophagic species biting especially during the second part of the night. Biting behaviour is more commonly not restricted but depends on the accessibility of hosts and resting places. Some species are more opportunistic than others, but even strongly anthropophilic species can blood feed on animals if their preferential hosts are unavailable. Some authors have proposed a vector control method called "zooprophylaxy" by deviating the anthropophilic Anopheles species towards animals.14

Anopheles species are principally known for their involvement as vectors of malaria pathogens (see Figure 1.5). The efficiency of vector control programs requires a precise identification of the species concerned, especially important and arduous for species complexes, and a thorough knowledge of the biology of the actual vector in order to set up and evaluate strategies adapted to the local

Figure 1.5 Anopheles mosquitoes: Anopheles gambiae (A) (Photo © IRD/Jean-Pierre Hervy) and Anopheles albimanus (B) (Photo courtesy CDC/James Gathany).

eco-epidemiological context and the behaviour of the targeted species. Vector control encounters many technical constraints, such as vector resistance to insecticides, or socio-cultural problems, for example the acceptability of control methods such as IRS or the regular use of ITN. However, vector control remains the best strategy for malaria prevention and control, while vaccines are unavailable and drug resistance in P. falciparum strains continues to develop worldwide.

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