Class Insecta. The role of insects as vectors and mechanical carriers

At a glance

1. Insects (class Insecta) are arthropods with a body in three parts: a head with one pair of antennae, mouthparts and, in most, compound eyes; a thorax with three pairs of legs and usually two pairs of wings; and an abdomen.
2. They grow either by complete metamorphosis, egg, larva, pupa and adult, as in flies, mosquitoes and fleas, or by incomplete metamorphosis, egg, nymphs and adult, as in lice and bugs.
3. A biological vector is one in which the pathogen multiplies, develops or both before it is passed on: Anopheles and Plasmodium, the tsetse fly and trypanosomes, the sand fly and Leishmania.
4. A mechanical carrier only transports the pathogen, unchanged, on its body or through its gut: the housefly carries the germs of diarrhoea and typhoid, and the cysts and eggs of intestinal parasites, from faeces to food, and its relative the eye-seeking fly carries trachoma from eye to eye.
5. Vector-borne diseases make up more than 17% of all infectious diseases and kill more than 700 000 people a year, most from malaria. Many can be prevented by controlling the vector and protecting people from bites.

01The class Insecta

Insects are the largest class of the phylum Arthropoda. Like all arthropods they have a hard exoskeleton of chitin, which they must moult to grow, a segmented body and jointed legs. What sets them apart is their body plan, in three parts.

• The head carries one pair of antennae, in most a pair of compound eyes (fleas have only simple eyespots or none), and the mouthparts, which take a different form for each way of feeding: piercing and sucking in mosquitoes, bugs, lice and fleas; sponging in the housefly; chewing in cockroaches.
• The thorax, of three segments, carries three pairs of legs, one on each, and in most adult insects two pairs of wings. Flies (order Diptera, two-winged) keep only the front pair; the hind pair is reduced to halteres, small knobs that steady them in flight. Lice and fleas have lost their wings altogether.
• The abdomen holds most of the gut and the reproductive organs.

Inside, they breathe through tracheae, fine air tubes that open through spiracles along the sides of the body; their blood, the haemolymph, flows freely round the organs in an open circulation; and they excrete through Malpighian tubules that drain into the gut.

02How insects grow

Insects grow only when they moult, so each passes through a series of stages. They do it in one of two ways.

• Complete metamorphosis (holometabolous development): egg, larva, pupa and adult. The larva looks nothing like the adult, often lives somewhere else and eats something else; inside the pupa, which does not feed, its body is broken down and rebuilt as the adult. Flies, mosquitoes, sand flies and fleas grow this way.
• Incomplete metamorphosis (hemimetabolous development): egg, nymphs and adult. The nymphs are small copies of the adult, without working wings, live in the same place and eat the same food, and reach the adult form through a series of moults, with no pupa. Lice, bed bugs, triatomine bugs and cockroaches grow this way.

In the blood-sucking insects, which stages feed on blood depends on the way they grow. In complete metamorphosis only the adult does, and in mosquitoes, sand flies and black flies only the female, which needs blood to make her eggs; the larvae live in water, soil or the host's nest. In incomplete metamorphosis every nymph and both sexes of the adult feed on blood, as in lice, bed bugs and triatomine bugs.

03Insects as vectors

A vector is a living organism that carries a pathogen from one person to another, or from an animal to a person. Many vectors are blood-sucking insects: they take up the pathogen with a blood meal from an infected host and pass it on at a later meal, often for the rest of their lives. There are two kinds.

• A biological vector is one in which the pathogen multiplies, develops, or both, before it can be passed on, so a vector is not infective at once: there is an extrinsic incubation period of days to weeks. The pathogen is passed on with the bite, in the saliva (malaria, dengue) or brought up from the front of the gut (leishmaniasis; plague, from a flea whose gut is blocked by the bacteria); from the mouthparts as the vector feeds (filarial larvae); in the faeces, rubbed into the bite (Chagas disease, epidemic and murine typhus); or when a louse is crushed on the skin (louse-borne relapsing fever).
• A mechanical carrier, or mechanical vector, only transports the pathogen, which does not change or multiply in it, on its body, its mouthparts or through its gut.

Biological transmission is of three kinds:

• Propagative: the pathogen only multiplies, as the dengue and yellow fever viruses do in Aedes, and Yersinia pestis in the rat flea.
• Cyclo-developmental: it only develops from one stage to the next, without multiplying, as filarial larvae do in mosquitoes and black flies. One larva taken up can become at most one infective larva.
• Cyclo-propagative: it both develops and multiplies, as Plasmodium does in Anopheles and the trypanosomes in the tsetse fly.

Each has its own topic: mosquitoes and sand flies and lice, bed bugs and fleas, and the parasites they carry each have theirs.

Ticks and mites are vectors too, though not insects.

04Insects as mechanical carriers

Musca domestica, the housefly, is the classic mechanical carrier. It breeds in manure and rubbish, feeds on faeces and on our food alike, and flies several kilometres. It cannot bite: it pours saliva and vomit onto solid food to dissolve it and sucks it up through the sponging pads of its proboscis, and it defecates where it feeds.

• It carries germs on its hairs and the sticky pads of its feet, on its mouthparts, and in its vomit and faeces; few survive long on the outside of the fly, so most reach food when it vomits or defecates.
• It is a mechanical vector of more than 100 pathogens, among them those of typhoid, cholera, salmonellosis, bacillary dysentery, tuberculosis and eye infections, and it carries the cysts of Entamoeba histolytica and Giardia and the eggs of Ascaris, Trichuris, Enterobius and Hymenolepis nana.
• Flies drawn to children's eyes and noses, above all Musca sorbens, the eye-seeking or bazaar fly, a close relative of the housefly that breeds in human faeces, carry Chlamydia trachomatis, the cause of trachoma, from one child to another, where there is little water and no latrines.

Cockroaches carry germs the same way, from drains and rubbish to food.

The answer is hygiene more than insecticide: clean water, latrines, covered food and waste, and screens.

05Fly larvae in living tissue

Myiasis is infestation of a living person or animal by the larvae of flies, which feed on its living or dead tissue for a time.

• Furuncular myiasis: a single larva grows in the skin under a boil-like swelling with a small breathing hole. In Africa the tumbu fly, Cordylobia anthropophaga, lays its eggs on clothes dried outdoors, and the larvae burrow into the skin when they are worn, which is why clothes there are ironed. In Central and South America the human botfly, Dermatobia hominis, sticks its eggs to a mosquito or another fly, which carries them to the skin. Covering the hole thickly with petroleum jelly makes the larva come up for air, and it is then removed.
• Wound myiasis: flies lay their eggs in wounds, ulcers or the nose, most often in people who are old, neglected, homeless or diabetic; screwworm larvae can eat deep into living tissue, and in southern Europe Wohlfahrtia magnifica does the same.

The larvae of the green bottle fly, Lucilia sericata, disinfected and laid in a non-healing wound, eat only the dead tissue: maggot debridement therapy.

06Controlling vectors

Many vector-borne diseases can be prevented. The tools act either on the insect or on the bite.

• Removing where vectors breed: emptying and covering water containers for Aedes, draining standing water, clearing rubbish, latrines and sanitation.
• Insecticides: insecticide-treated nets, the main tool against malaria, which also protect against other night-biting mosquitoes; spraying the inside walls of houses (indoor residual spraying); larvicides in water.
• Protecting people: screens on doors and windows, repellents, long clothes, and nets for those who sleep by day.
• Insecticide resistance is spreading, so control is planned as integrated vector management, combining several methods, with the community's part in it and surveillance of the vectors.

Climate change, travel and unplanned cities are spreading vectors into new places and lengthening the seasons when they bite. In Europe the tiger mosquito, Aedes albopictus, now lives across the south and has spread dengue and chikungunya locally in France, Italy and Croatia; Culex carries West Nile virus, and sand flies leishmaniasis, around the Mediterranean.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1Which feature tells an adult insect apart from a tick?

Jointed legsThree pairs of legs and a body in three parts, head, thorax and abdomenA hard exoskeletonFeeding on blood

2Which of these insects goes through complete metamorphosis, with a pupa?

The head louseThe bed bugThe fleaThe triatomine bug

3How does the housefly pass on the bacteria of typhoid and dysentery?

The bacteria multiply in its salivary glands and are injected when it bitesIt carries them unchanged on its hairs and feet, and in what it vomits and passes onto foodThrough its eggs, to the next generation of fliesIt cannot pass them on

4In Anopheles, Plasmodium both develops from one stage to another and multiplies. What kind of biological transmission is this?

MechanicalPropagativeCyclo-developmentalCyclo-propagative

5Which insect carries Trypanosoma cruzi, the cause of Chagas disease?

The tsetse flyThe triatomine (kissing) bugThe sand flyThe black fly