Main Topics in Entomology: Insects as Disease Vectors

S. MANGUIN, J. MOUCHET AND P. CARNEVALE

1.1 Introduction

Insects of medical interest are numerous, and some have had a major impact on the course of human history due to epidemics of vector-borne diseases which have led to millions of deaths. Mosquitoes are at the origin of severe epidemics of malaria, dengue, yellow fever and chikungunya, all of which continue to kill millions of people throughout the world each year. Fleas are responsible for propagating the plague, which killed millions of people during three pandemics. Lice transmit typhus, which has been shown to weaken armies in periods of war. Among those insects of medical interest, the vast majority belongs to the insect order Diptera. This chapter deals with the most common insect vectors of diseases, whose classification is as follows.

1.2 Mosquitoes

There are more than 3400 species of mosquitoes, which belong to 37 genera joined together in only a single family: Culicidae, which is divided into three sub-families: (1) Toxorhynchitinae, (2) Anophelinae and (3) Culicinae. Mosquitoes have a worldwide distribution. They occur in tropical and temperate zones, even on the level of the Arctic Circle; there are, however, no mosquitoes in the Antarctic. Mosquitoes are also found in mountainous regions (at 5500 m altitude), as well as in caves and mines (at –1250 m altitude).

The mosquitoes that bite humans and which are the most important vectors of disease, belong to the genera Anopheles, Culex, Aedes, Mansonia, Haemagogus and Sabethes. Anopheles can transmit malaria parasites, but also lymphatic microfilariae (e.g. Wuchereria bancrofti, Brugia malayi and Brugia timori) and some arboviruses (e.g. O'nyong-nyong and equine encephalitis). Some Culex species also transmit the microfilariae responsible for Bancroftian lymphatic filariasis, as well as many arboviruses, such as the West Nile, Japanese encephalitis, Saint Louis encephalitis, Murray Valley encephalitis and Rift Valley arboviruses. The Aedes genus includes important vectors of the viruses responsible for yellow fever, dengue (and hemorrhagic dengue), chi- kungunya, eastern equine encephalitis (EEE), and many other arboviruses. Some Aedes species also transmit microfilarial parasites (Bancroftian lymphatic filariasis), such as Ae. polynesiensis in the Pacific islands.

1.2.1 General Morphology

The development cycle of mosquitoes includes two phases (see Figure 1.1): (1) an aquatic phase, with the succession of immature stages, such as eggs, larvae (four stages) and pupae, and (2) an air phase with the male and female adults.

The eggs are ovoid and measure approximately 0.5 mm. They are laid either on the surface of water (e.g. Anopheles and Culex) or near the surface of water (e.g. Aedes). The eggs may be laid separately (e.g. Anopheles and Aedes) or close together in the form of an "egg raft" at the time of oviposition (e.g. Culex and Coquillettidia). The eggs are able to float due to side floats (e.g. Anopheles) or apical floats (e.g. Culex). The variations in egg ornamentation have been used to dismember the complex Anopheles maculipennis in order to understand the phenomenon of "anophelism without malaria" in Western Europe.

This raises the concept of "species complex", in which sibling species are important disease vectors whilst others are not involved in pathogen transmission at all, despite the fact that these species cannot be differentiated morphologically. Identification of the individual species must be based on more sophisticated techniques, in particular molecular ones. Many of the main vectors of pathogenic diseases belong to a species complex, such as Anopheles gambiae, An. dirus, An. farauti, Culex pipiens, etc.

The larvae which emerge from the egg evolve in four stages (L1, L2, L3 and L4), intersected with three moults which allow the larvae to grow from 1 mm (L1) to 15 mm (L4) in a week (longer in temperate regions). The four larval stages have a comparable general morphology. The larva is composed of three parts: head, thorax and abdomen. The 8th abdominal segment is modified (the "respiratory segment") with two important structures: (1) on the lateral side, the "comb" (pecten) which is made up of spines and scales of different forms, sizes and numbers, according to the genus and species of the mosquito, and (2) on the dorsal face, the spiracular apparatus which is located either directly on the tegument for Anopheles, or at the end of a respiratory siphon for Culicinae. This is a useful characteristic with which to differentiate the position of the larvae, as Anopheles larvae (without siphon) stay parallel to the water surface, whilst Culex or Aedes larvae (with a siphon) have an oblique angle of suspension to the water surface. For Mansonia, the end of the siphon is modified in a hard organ used to bore plants. Mansonia larvae do not breathe like other mosquitoes but attach themselves to the roots, leaves and stems of aquatic plants in order to obtain their air supply.

The pupa's morphology is completely different from that of the larva, consisting of two parts: (1) a prominent cephalothorax equipped with two respiratory trumpets (the pupa does not have an oral apparatus as it breathes but does not feed), and (2) an abdomen made up of eight visible segments (the ninth segment is barely visible), the eighth segment carries a pair of swimming paddles. The pupa is quite mobile and dives when disturbed. Its lifespan is short (one to two days).

The adults. Male and female mosquitoes can be easily differentiated by observing the head and the end of the abdomen. The head comprises of two compound eyes made up of hundreds of ommatidia, and two antennae with 15 articles in the male and 16 articles in the female. In the male, there is a great number of large setae which allows for the easy recognition of the "plumose" antennae, whereas the female has "pilose" antennae. The oral apparatus is of the "sucker" type for the male and the "biter" type for the female which includes: (1) a labium folded up in a gutter and finished by two labellums; in this gutter, there are six piercing stylets which will penetrate the skin and search for a capillary for the intake of blood; (2) the labrum (or upper lip), which serves as the "roof" of the food channel; (3) the hypopharynx which is connected to salivary glands by the salivary channel and forms the floor of the food channel; and (4) two mandibles and two maxillae. On both sides of this female piercing apparatus, there is a pair of maxillary palps, which are as long as the proboscis for Anopheles but shorter than the proboscis for Culicinae. This difference allows for easily differentiation of Anopheles from other mosquitoes.

The thorax includes three segments: the prothorax, the mesothorax, and the metathorax. Each segment comprises a pair of legs made up of a hip (or "coxa"), a trochanter, a femur, a tibia and a tarsus with five articles; the last article carries at its end two claws which help the mosquito to hold on to the support. The legs carry more or less coloured scales which are used for mosquito identification. The second segment, or "wing segment", is the largest and carries a pair of wings (two wings = "Diptera") with veins and scales whose form and colour ("wing ornamentation") are also used for species identification. The third segment carries halters which are used for balance during flight. The morphology of the lateral parts of the thorax called "pleurites" is very much used in systematics. There are two respiratory spiracles (on the second and third segments). The dorsal part of the second segment is called the "scutum" which is prolonged by a "scutellum". This is simple and rounded for Anopheles or trilobed for Culicinae. The abdomen is composed of ten segments of which eight are quite visible. Each segment comprises a dorsal chitinized part (tergite) and a ventral chitinized part (sternite), connected by a very extensible pleural membrane which allows for the swelling of the abdomen of the female after a blood meal or the maturation of the ovaries. Segments nine (genital segment) and ten (anal segment) are quite modified. The genital apparatus on the male is very complicated and its morphology is used in systematics (especially for Culex). Between 12 and 24 hours after the emergence of the adult, the male genital apparatus undergoes a 180° rotation and becomes ready for mating. The terminalia surround a complex penis (the "phallosome") which is located on the tenth segment. The abdomen of the female ends with two cercus.

1.2.2 Internal Anatomy

The internal mosquito anatomy is composed of: a digestive tract with the pharynx and its pump which aspirate the blood; the oesophagus; the stomach (midgut); and the posterior intestine (hindgut) which ends in the rectum and the anus. The salivary duct arrives at the lower face of the pharynx and the salivary pump allows for excretion of saliva during the bite. The salivary channel is connected to a pair of trilobed salivary glands. If parasites (Plasmodium) are in the mosquito's salivary glands, they are inoculated during the bite. In the female, the genital apparatus is composed of two ovaries with many ovarioles, and their oviducts meet to form an odd oviduct which arrives at the vagina. During maturation, the ovarioles evolve in five "Christopher's" stages. The spermatheca, which is a duct that opens in the vagina, is an organ where the spermatozoa, inoculated by the male during fecundation, are stored. There is one spermatheca in the Anopheles female and two in the Culicinae.

1.2.3 Biology

According to the species, mosquito larvae can develop in practically all possible types of habitats: freshwater to brackish water; clean water to heavy polluted one; stagnant water to running water; natural habitats to man-made breeding sites; small habitats (puddles, footprints, artificial containers, etc.) to large ones (rice plantations, lakes, etc.). Information on larval ecology is crucial in order to carry out appropriate vector control programs targeting specific mosquitoes. Three keys elements concern the larvae: (1) they feed, therefore it is possible to use insecticides of ingestion like Bacillus thuringiensis or Bacillus sphaericus (useless against pupae which do not feed); (2) they moult, therefore growth regulators like juvenoids or ecdysteroids can be used; and (3) they breathe at the water surface, therefore it is possible to use methods aiming at asphyxiating them, like monolayers, oils or polystyrene chips for example.

Only females bite to take a blood meal for egg maturation, but males and females feed on flower nectar from which they get their energy necessary for flight. Fecundation occurs two to three days after adult emergence, with generally only one fecundation, although several can take place. The female's life is conditioned by the succession of blood meals and the development of the ovaries, this is the gonotrophic cycle, which starts with the unfed female, then after blood-feeding, it becomes half-gravid, and gravid. This cycle must be known for each species or each situation considered in vector control programs, as its duration conditions, the frequency of the contacts host/mosquito, and the ingestion (from man to mosquito) or the transmission (from mosquito to man) of pathogens responsible of the disease considered. After egg laying the female seeks another blood meal, and the "gonotrophic cycle" repeats itself every two to three days. In tropical regions, the blood meal is accompanied by a maturation of the ovaries; this is the "trophogonic concordance". On the other hand, in temperate regions during a cold period there can be a "trophogonic dissociation" for which the blood meal is not followed by the development of the ovaries; the females can even enter into complete diapause, allowing hibernation. Mosquitoes can take their blood meal from humans (anthropophilic) or animals (zoophilic), or both. The trophic preferences of species are very important to know, the more anthropophilic a mosquito, the higher its vectorial role. The blood meal can be taken indoors (endophagic) or outside (exophagic). Some species bite essentially during the night (nocturnal) like Anopheles, others during the day (diurnal) like Aedes, and others during the morning or at sundown. After the blood meal, the mosquitoes have a phase of digestion which lasts approximately 48 hours, during which they rest either indoors (endophilic) or outdoors (exophilic). All these behaviours are very important to know in the definition of vector control strategies. It is clear that indoor spraying with remanent insecticides will be particularly efficient against anthropophilic, endophagic and endophilic mosquitoes, but effects against the exophagic and exophilic mosquitoes will be quite reduced, and the addition of insecticide could even increase exophilic behaviour and greatly reduce the impact. Some products have effects known as "deterrent" (the mosquito avoids entering the treated house), "excito-repulsive" or "irritant", where the mosquito avoids contact with a treated surface or remains in contact with the product for only a short period of time and so the insecticide does not have a lethal effect. The mosquitoes can thus survive outdoors and continue to bite the human population in spite of indoor treatment. Mosquitoes generally have a lifespan of about one month in tropical areas, although in temperate areas, mosquitoes can survive during the winter in diapause or semi-diapause. Their range of active flight is generally rather weak (active dispersion), three kilometres for Anopheles and Aedes hardly move away from their larval habitat, therefore vector control can target areas near breeding sites. Mosquitoes can, however, be transported by the wind (passive dispersion) and by modern means of transportation (e.g. "airport malaria" cases can occur as a result of Ano- pheles vectors travelling by plane from an endemic zone into a malaria-free area and inoculating Plasmodium parasites).

1.2.3.1 Culicinae

The subfamily of Culicinae includes 33 genera; the most important ones in medical entomology are the Culex, Aedes, Mansonia, Sabethes and Haemagogus. Aedes, Culex and Mansonia are found in the temperate and tropical regions; the genera Sabethes and Haemagogus are found only in Central and South America. Culicinae are easily distinguishable from Anopheles at the larval and adult stages (see Table 1.1).

1.2.3.1.1 Culex. Culex species are widespread in the whole World, except the most northern zones of temperate regions and the poles. There are thought to be some 800 species divided into 21 sub-genera.

The eggs, brown, long and cylindrical, are deposited on the surface of water and bound to form a "raft" composed of some 300 eggs which are laid in a large variety of aquatic habitats: small puddles, pools, permanent or temporary ponds, flooded marshes, borrow pits, ditches, rice plantations, as well as anthropogenic sites such as cans, cisterns, and even sewage drains with polluted water. The most important species, Culex quinquefasciatus, is strongly associated with anarchistic urbanization, with poor hygiene conditions and worn-out water drainage systems containing organic matter where the larvae can develop (e.g. polluted stagnant water, gutters, septic tanks, sewage drains, etc.). The density of mosquito populations can be very high under such conditions and constitutes a major cause of nuisance for the people affected.