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ARTHROPOD VECTORMosquitoes

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LECTURE OUTLINE

Mosquitoes as disease vectors-its role as important human disease vector for dengue,

microfilaria and malaria

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INTERESTING FACTS

Alexander the Great, conqueror of many nations, was vanquished by the bite of a tiny mosquito bearing malaria parasites

The Black Death, decimator of Europe, killer of tens of millions worldwide is the work of a tiny flea vectoring the bacilli that cause bubonic plague from rats to people.

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SOMETHING TO PONDER

What is the world’s deadliest animal?

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MOSQUITOES: INTRODUCTION

There are about 3000 species of mosquito, of which about 100 are vectors of human diseases

Mosquitoes and ticks account for the majority of transmissions of the most important vector-borne diseases, although some close relatives of mosquitoes also get involved, including sand flies and black flies

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MOSQUITOES: CLASSIFICATION

• There are 442 species of mosquitoes in Malaysia, compared to 553 species (Indonesia), 416 species (Thailand) and 310 species (Philippines)

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MOSQUITOES: BIOLOGY

Mosquitoes differ from the other biting Diptera in having a long slender body, long legs and long needle-shaped mouthparts

The wings sometimes have discernible patterns of scales

The adult insects measure between 2 mm and 12.5 mm in length

Some species bite in the morning or evening and at night; others feed during the day

Species may bite indoors or out of doors.

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MOSQUITOES: ANATOMY

Anatomy of a MosquitoThe Animal Files.com

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MOSQUITOES: MALE AND FEMALE

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MOSQUITOES: LARVA

Anatomy of a Mosquito LarvaThe Animal Files.com

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MOSQUITOES: LIFE-CYCLEMosquitoes undergo complete metamorphosis (egg larvapupa adult )

WHO, 2010

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Female mosquitoes usually lay about 30-300 eggs at one oviposition.

Larvae will emerge after 2-3 days in the tropics, and 7-14 days in cooler temperature weather

There are four active larval stages.

All larvae require water to develop

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Mosquito larvae feed on yeasts, bacteria, protozoa and numerous other plants, microorganisms found in the water.

Some of them are surface-feeders (e.g. Anopheles), while many others browse over the bottom.

Larval development ranged from 5-7 days to 7-14 days (in tropical areas)

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All mosquito pupae are aquatic and coma shaped

Pupae do not feed, but spend most of their time at the water surface taking in air through respiratory trumpets. When disturbed, they will swim up and down in a jerky fashion.

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MOSQUITOES: BEHAVIOUR

Female mosquitoes feed on animals and humans Attracted by the body odours, carbon dioxide and

heat emitted from the animal or person Some species prefer biting at certain hours, for

example at dusk and dawn or in the middle of the night

Feeding usually takes place during the night but daytime biting also occurs

Some species prefer to feed in forests, some outside of houses, others indoors.

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MOSQUITO-BORNE DISEASES

Mosquito-borne diseases are divided into three groups (according to the pathogens):

- Protozoa diseases (e.g. Malaria).- Viral diseases (e.g. dengue, Japanese encephalitis).- Nematode diseases (e.g. filariasis).

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Mosquito-borne diseases and their vectors (virus disease)

Disease Pathogen Vector species

Virus

JE JE virus Culex tritaeniorhynchus Culex fuscocephala Culex gelidus Culex vishui Culex pseudovishui

Dengue Dengue Aedes aegypti Dengue virus Aedes albopictus hemorrhagic

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Mosquito-borne diseases and their vectors (protozoa and nematode disease)

Disease Pathogen Vector species

Malaria Plasmodium vivax Anopheles sp. P. falciparum P.malariae P. ovale

Filariasis Wuchereria bancrofti Culex quinquefasciatus Anopheles sp.

Brugia malayi Mansonia sp. Aedes togoi Anopheles sinensis

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Anopheline eggs are laid singly on the water surface, possess floats

All Aedes lay their eggs singly, on the ground, at or above the waterline, never possess floats

Culex eggs are deposited in rafts of 100 or more

Anopheline – larvae never have a siphon. Lie parallel to water surface

Culicinae – all larvae have a short or long siphon. Subtend an angle from the water surface

Anophelines rest in a position where their head, thorax, and abdomen are in a straight line, usually at an angle of 40 to 90°, whereas the culicines rest in a position almost parallel to the surface.

Characteristics of anophelines and culicines. (From Pictorial Keys to Some Arthropods and Mammals of Public Health Importance, U.S. Department of Health, Education, and Welfare, Public Health Services, Washington, D.C., 1964.)

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ANOPHELES: BIOLOGY

About 380 species of Anopheles occur around the world

Some 60 species are sufficiently attracted to humans to act as vectors of malaria

A number of Anopheles species are also vectors of filariasis and viral diseases.

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ANOPHELES: LIFE-CYCLE Larval habitats vary from species to species, but are frequently

exposed to sunlight and commonly found in association with emergent vegetation, such as grass or mats of floating vegetation or algae.

The most preferred breeding sites are pools, seepages, quiet places in slow-running streams, rice fields, leaf axils of certain epiphytic plants and puddles of rainwater.

Artificial containers, such as pots, tubs, cisterns and overhead tanks are not usually suitable, except in the case of Anopheles stephensi in south-west Asia

The eggs, laid singly on the water surface where they float until hatching, are elongated, have a pair of lateral floats, and are about 1 mm in length.

Hatching occurs in 2–3 days. The larvae float in a horizontal position at the surface, where they feed

on small organic particles. In the tropics the duration of development from egg to adult is 11–13

days.

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ANOPHELES: BEHAVIOUR

Anopheles mosquitos are active between sunset and sunrise.

Each species has specific peak biting hours, and there are also variations in their preference for biting indoors or outdoors.

The anophelines that enter houses to feed often rest indoors for a few hours after feeding.

They may then leave for outdoor sheltered resting sites, among them vegetation, rodent burrows, cracks and crevices in trees or in the ground, caves and the undersides of bridges.

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Alternatively, they may stay indoors for the whole period needed to digest the blood-meal and produce eggs.

Indoor resting is most common in dry or windy areas where safe outdoor resting sites are scarce.

Once the eggs are fully developed the gravid mosquitoes leave their resting sites and try to find a suitable breeding habitat.

Many Anopheles species feed on both humans and animals.

They differ, however, in the degree to which they prefer one over the other.

Some species feed mostly on animals while others feed almost entirely on humans.

The latter species are the more dangerous as vectors of malaria.

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ANOPHELES: HEALTH IMPORTANT

Important malaria vectors in MalaysiaAn. aconitus

An. balabacensis

An. dirus

An. donaldi

An. flavirostris

An. letifer

An. leucosphyrus

An. maculatus

An. minimus

An. nigerrimus

An. subpictus

An. sundaicus

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AEDES: BIOLOGY

Aedes mosquitos occur around the world and there are over 950 species

They can cause a serious biting nuisance to people and animals, both in the tropics and in cooler climates

In tropical countries Aedes aegypti is an important vector of dengue, dengue haemorrhagic fever, yellow fever and other viral diseases

A. aegypti is easily recognized by the contrasting black and white rings on its legs and the lyre-shaped pattern of silver markings on the upper surface of the thorax.

A closely related species, Aedes albopictus, can also transmit dengue

In some areas Aedes species transmit filariasis

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AEDES: LIFE-CYCLE

The eggs are laid singly on damp surfaces just above or near the water line in temporary pools and other habitats where the water level rises and falls.

They can withstand desiccation for many months and hatch only when flooded with water.

All species of Aedes which occur in regions with cold winters survive these periods in the egg stage.

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Some species breed in coastal salt marshes and swamps that are flooded at intervals by unusually high tides or heavy rains, while others have adapted to agricultural irrigation practices.

Aedes aegypti mainly breeds in the domestic environment: its preferred habitats are water storage tanks and jars inside and outside houses, and roof gutters, leaf axils, bamboo stumps and temporary containers such as jars, drums, used car tyres, tin cans, bottles and plant pots.

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All these habitats typically contain relatively clean water.

Aedes albopictus originally occurred only in Asia and Madagascar but recently invaded North and South America, as well as West Africa, where it may become important in the transmission of dengue and other viral diseases.

Like Aedes aegypti, it breeds in temporary containers but prefers natural ones in forests, such as tree holes, leaf axils, ground pools and coconut shells, and breeds more often outdoors in gardens and less frequently indoors in artificial containers.

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AEDES: BEHAVIOUR

Aedes mosquitos bite mainly in the morning or evening (crepuscular bitters)

Most species bite and rest outdoors but in tropical towns Aedes aegypti breeds, feeds and rests in and around houses.

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AEDES: HEALTH IMPORTANT

Two important species of Aedes in Malaysia are Ae. aegypti and Ae. albopictus.

Both are vectors of dengue/dengue hemorrhagic fever in Southeast Asia

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CULEX: BIOLOGY About 550 species of Culex have been described, most of

them from tropical and subtropical regions

Some species are important as vectors of bancroftian filariasis and arboviral diseases, such as Japanese encephalitis • The most common species,

Culex quinquefasciatus, a major nuisance and vector of bancroftian filariasis, breeds especially in water polluted with organic material, such as refuse and excreta or rotting plants

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CULEX: LIFE-CYCLE Rafts of 100 or more eggs are laid on the water surface. The rafts remain afloat until hatching occurs 2–3 days

later. Culex species breed in a large variety of still waters,

ranging from artificial containers and catchment basins of drainage systems to large bodies of permanent water.

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Examples of such breeding sites are soakaway pits, septic tanks, pit latrines, blocked drains, canals and abandoned wells.

In many developing countries Culex quinquefasciatus is common in rapidly expanding urban areas where drainage and sanitation are inadequate.

Culex tritaeniorhynchus, the vector of Japanese encephalitis in Asia, prefers cleaner water.

It is most commonly found in irrigated rice fields and in ditches.

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CULEX: BEHAVIOUR

Culex sp is a markedly domestic species The adult females bite people and animals

throughout the night, indoors and outdoors During the day they are inactive and are often

found resting in dark corners of rooms, shelters and culverts

They also rest outdoors on vegetation and in holes in trees in forested areas

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CULEX: HEALTH IMPORTANT

Culex quinquefasciatus (vector of Brancroftian filariasis

Culex tritaeniorhynchus (vector of Japanese encephalitis-B)

Some important Culex sp. as a vector for malariaCulex tritaeniorhynchus

Culex fuscocephala

Culex gelidus

Culex vishui

Culex pseudovish

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MANSONIA: BIOLOGY

Mansonia mosquitos are mostly found in marshy areas in tropical countries.

Some species are important as vectors of brugian filariasis in south India, Indonesia and Malaysia.

The body, including the legs and wings, is covered with dark-brown and pale scales, giving it a rather dusty appearance, as if sprinkled with salt and pepper.

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MANSONIA

The same mosquito, almost fully blood engorged

Adult taking a meal

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MANSONIA: LIFE-CYCLE

The species that transmit filariasis normally lay their eggs in masses that are glued to the lower sides of plants hanging or floating near the water surface

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Because the larvae and pupae attach themselves to aquatic plants for the purpose of breathing they occur only in water bodies containing permanent vegetation, such as swamps, ponds, grassy ditches and irrigation canals, and may be difficult to find.

They can also occur in deeper water where there is floating vegetation, and are very often attached to the underwater parts of floating aquatic weeds

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Larvae of Mansonia uniformis attached to plant roots

The pupa of Mansonia uniformis. Like the larvae, the pupa have modified trumpets for piercing plant materials to gain their oxygen. They normally only rest on the water surface like this just before emerging.

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MANSONIA: BEHAVIOUR

Mansonia species usually bite at night, mostly out of doors, but some species enter houses.

Resting after a blood-meal normally takes place out of doors.

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MANSONIA: HEALTH IMPORTANT

Important as vectors of brugian filariasis in south India, Indonesia and Malaysia.

Serves as a biological vector – it is required for the developmental cycle of the parasite

This also known as Elephantiasis

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PREVENTION AND CONTROL

Personal protection- Personal protection methods, used by individuals or small groups of people to protect themselves from biting insects and the diseases they may carry, act by preventing contact between the human body and the insects- Repellents - Protective clothing - Insecticide vaporizers- Mosquito nets

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Insecticides- The insecticide should be selected for its effectiveness against the target mosquito species- Insecticidal aerosols are sometimes used for the killing of flying and resting insects in situations where immediate results are needed- Because the insecticidal action does not last long it is usually necessary to repeat the procedure several times. - Space sprays are usually applied in and around houses in cities or villages and sometimes on outdoor resting places in dense vegetation or salt marshes- e.g Malathion, Permethrin, Methoprene, Resmethrin

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Prevention of breeding- Source reduction

- Such measures include covering or screening water containers, draining ponds and marshes, and filling in ditches, pools, etc- Larvicides

- killing immature mosquitoes by applying agents

- Insect growth regulators (IGRs) - Microbial larvicides - Organophosphates (OPs) - Surface oils and films

- Habitats in and around houses- Habitats in the field

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Biological control- The biological control of mosquitoes and other pests involves introducing into the environment their natural enemies, such as parasites, disease organisms and predatory animals

- Fish that eat mosquito larvae can be released into breeding sites

- Predatory mosquitoes of the genus Toxorhynchites, the larvae of which feed on other mosquito larvae

- Dragonflies, the larvae of which feed on mosquito larvae

- Cyclopoid copepods, small crustaceans that attack first- and second-instar larvae of mosquitoes;

- Nematode worms that are parasites of mosquito larvae

- Fungi that grow in the bodies of mosquito larvae

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MOSQUITOES BEHAVIOR AND ITS IMPLICATION ON CONTROL STRATEGIES Adult stage - Resting place after taking blood meals (outdoor or indoor)

Indoor e.g Anopheles- Insecticide spraying of walls- Mosquitoes resting on sprayed walls come into contact with insecticide through their feet and are killed.- Some insecticides irritate mosquitoes and cause them to leave houses- Hungry mosquitoes entering a house may bite first and then be killed when resting on a treated wall Outdoor e.g Mansonia, Culex- Resting after a blood-meal normally takes place out of doors.- Space-spraying- It has an immediate effect on adult populations of insects and is therefore suitable for the control of disease outbreaks- It kills mosquitoes that do not rest in houses

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Some species breed in clean water containers in and near houses, whereas others prefer polluted water in sanitation systems, or man-made and natural habitats in rural areas eliminating or changing the breeding place to make it unsuitable for development of larvae; making the breeding place inaccessible to adult mosquitos; - e.g Aedes - Change water in flower vases weekly and scrub to remove adhering mosquito eggs before refilling with fresh water

Egg Larvae Pupae = aquatic stages releasing fish or other predators that feed on larvae;- mosquito fish (Gambusia affinis) and the guppy (Poecilia reticulata) applying larvicides Insect growth regulator

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The larvae and pupae of Mansonia attach themselves to the submerged parts of water plants on which they depend for breathing removing or destroying the vegetation

Many mosquitoes attack people at night inside houses include the use of aerosols, mosquito coils, vaporizing mats and repellent smoke Screening of doors, windows and other openings in houses prevents insects from entering, while maintaining some ventilation Insecticide-treated screening and curtains- Treated screening or curtains provide a toxic barrier to mosquitoes that try to enter houses

Mosquitoes net