MEDICALLY IMPORTANT MOSQUITOES

INTRODUCTION

Mosquitoes are the most important group of insects. They are important as vectors of human diseases. In addition they cause a considerable biting nuisance to man. Mosquitoes are found world wide with over 3000 species included in 34 genera, distributed from the tropics to the arctic regions. However, the medically important species belong to only a few of these genera.

MORPHOLOGY

These are small slender flies with one pair of wings; forwardly projecting mouth parts; scales covered the body, legs and wings. The body is divisible to head, thorax and abdomen. The head bears the characteristic mouth pars suited for piercing and sucking. The mouth parts are collectively called the proboscis. It consists of a labium (lower lip), a labrum (upper lip) enclosing two pairs of cutting apparatus (mandibles and maxillae) and a stylet (hypopharynx) through which the salivary duct opens. The food channel is formed the labrum and the hypopharynx. The head bears a pair of palps and long segmented antennae. In the female the antennae have only few hairs, while in the males they are bushy and feathery. Internally trilobed salivary gland is situated ventrally in the thorax. Powerful flight muscles are also present in the thorax. The body cavity (hemocele) is bathed in fluid.

LIFE CYCLE

Only female mosquitoes suck blood, while the males thrive on plant and fruit juices. The blood meal is essential in providing nutrients for the maturation of eggs. The time taken from the blood meal to oviposition is referred to as the gonotrophic cycle (2-3 days in the tropic). The life cycle has a complete metamorphosis with egg, larva, pupa and adult stage.

I. EGG: A female mosquito carefully selects the breeding site, each species having its own preferred habit, 100-200 eggs are laid at a time on water or moist surface, either singly or in batches. Eggs hatch in around 2-5 days.

II. LARVA: Larvae are legless with prominent thorax. It is actively feeding with four larval stages (7-14 days). Larvae breathe air through spiracles found in the posterior ends. In general almost any collection of permanent or temporary water can be a larval habit, but larvae are not found in large expanses of water such as lakes and in fast flowing rivers (except at margins).

III. PUPA: They are ‘comma’ shaped with a large cephalothorax (fused head and thorax). Although non-feeding, they show active movements. Pupal stage lasts 2-3 days.

IV. ADULT: They emerge by splitting the pupal skin. In the tropics adults live for 2-3 weeks but in colder climates they live much longer.

ADULT BEHAVIOUR

Mosquitoes are rather selective in their choice of hosts for their blood meals. Some species prefer human blood to animal and are called anthropophilic while others feeding only on animals are termed zoophilic. The biting and resting behavior of adults are important in the epidemiology of disease transmission.

Medically important mosquitoes can be divided into two groups;

1. Anophelines
2. Culicines

ANOPHELINES

MORPHOLOGY

Length of the Palps of the females is equal to the length of the proboscis whereas palps of the male are clubbed in shape. Adults usually rest with body forming an angle to the resting surface. Larvae lie parallel to the surface of water with the help of palmate hairs on the abdomen. They do not have siphon tubes. Eggs are laid singly and usually have floats.

THE GENUS ANOPHELES

The genus Anopheles is the only mosquitoes that are medically important among Anophelines. Anopheles is the only known vectors of human malaria. In some countries they transmit Filariasis (Africa and SE Asia), and viruses as well (Africa).

Anopheles has world wide distribution. The genus contains about 400 species, but only about 30 species are efficient natural vectors of human malaria. Natural susceptibility of Anopheles to human malaria is largely unexplained. The frequency of feeding on man, mosquito density and longevity are some features of importance in determining the efficiency of a vector in nature. Further, the habits of a given species vary from country to country. Thus a particular species may be an important vector in one area and not in another.

Anopheles is crepuscular (around sunset) or nocturnal in habits such as mating, blood feeding and ovipositioning. They are generally not found at altitude above 2000-2500 m.

Anopheles culicifacies, Anopheles subpictus, Anopheles minimus, Anopheles vagus, Anopheles maculates and Anopheles annularis are some members of the genus Anopheles. They all are vectors of human malaria but distribution varies. Anopheles culicifacies is the only malarial vector in Sri Lanka whereas others are found as vectors in Africa.

Anopheles culicifacies

This is an important malarial vector in Sri Lanka, Pakistan, Bangladesh, India and Nepal. Adults are small or medium in size with characteristic light and dark spots on the wing margin; the palps are branded .Resting position is ‘culicine’.It breeds in small collections of clean, sunlit water: in slow running streams or rain water collections in clayey or sandy soil (eg. Sand pool/rock pools in riverbeds; hoof marks; cart tracks; gem pits).This is not usually found breeding with other species expect with An. Subpictus. Eggs are laid singly with floats.

Biting habits: feeds readily on man and cattle; indoor and night biter; enters houses at dusk and rests indoors after a blood meal. Thus principal control measure is the spraying of residual insecticide on walls.

CULICINE MOSQUITOS

MORPHOLOGY

Length of the palps of the females are much shorter than that of proboscis and palps of male are pointed ( not clubbed).Adults usually rests with body parallel to resting surface. Larvae hang down from water surface and have siphon tubes. Eggs may be in clusters or singly but do not have floats.

Culicine mosquitoes include around 30 genera of which the medically important ones are Aedes, Culex, Mansonia and Armigeres.

AEDES

Aedes are world wide distribution extending up to the Arctic Circle where they are a serious biting nuisance to man during summer months. However their major medical importance is as vector of arboviruses. Aedes aegipti and Aedes albopictus are members of this family.

Aedes aegypti & Aedes albopictus

The adults are small, dark, delicate mosquitoes. They have patterns on their thorax due to silvery/white scales. Legs are banded, but wings are clear. Eggs are laid singly on damp surfaces just above water level and can withstand desiccation for long periods.

Aedes aegypti is recognized by the characteristic markings on the dorsum of the thorax (two curved lines enclosing two straight lines), while Aedes albopictus has a single line.

Aedes aegypti is a domestic mosquito common with urbanization. It is a container breeder laying eggs in rain water collections in discarded tin cans, coconut shells, tires and also in vegetation such as leaf axils, bamboo stumps, etc. it even breeds indoors: in flower vases and ants traps.

Aedes albopictus is a less urban species and they breed outdoors. Adults are day bitters, both indoors and outdoors. Larvae have short barrel shaped dark siphon tubes.

These two species are medically importance as they are the vectors of many diseases. Dengue, Chikungunya, Filariasis (Dirofilaria ripens) and yellow fever are the some importance diseases transmitted by these mosquitoes.

CULEX

These mosquitoes are importance as vectors of Filariasis and arboviruses. Culex quinquefasciatus, Culex fatigans, Culex tritaeniorhynchus, Culex gelidus, Culex fuscocephala and Culex pseudovishnui are some importance members of this group.

Culex quinquefasciatus

It is a very common urban mosquito throughout the tropics. It is the most important vector of Bancroftian Filariasis. Adults are small brown mosquitoes without any prominent markings. Adult female is a night biter, both indoors and outdoors and it is highly anthropophilic. Following the blood meal, rests on the clothes and dark places, under furniture or if outdoors in the shade.

It breeds in polluted, stagnant water with high organic content (eg. Catch pits of bucket latrines, cess pools, blocked drains, gutters and septic tanks). Eggs are long, cylindrical, laid upright on water surface and place together (300) to form rafts. Larval siphon tube is usually long.

Other Culex species

Culex tritaeniorhynchus, Culex gelidus, Culex fuscocephala and Culex pseudovishnui are important vectors of Japanese encephalitis.

Culex gelidus breeds in water containing much organic matter such as husk pits and muddy ground pools.

Culex tritaeniorhynchus breeds in clear water habits specially paddy fields and marshes. These mosquitoes are late evening outdoor biters and are principally bird and animal feeders. However, they readily bite man on contact.

MANSONIA

They are principally found in wet tropical areas. They are important as vectors of Brugian Filariasis in South and South East Asia. Mansonia uniformis and Mansonia annulifera are some important members of this group.

Body, legs and wings of adults are covered with light and dark, broad asymmetrical scales. This gives characteristic speckled appearance. Adults are night biters mainly outdoors but they are known to enter houses. Eggs are dark, cylindrical and laid in a mass on under surface of floating vegetation (eg. Pistia, Salvinia). Eggs cannot withstand desiccation. Larvae are characteristic, found attached to aquatic plants with their siphon tubes to obtain air. The siphon tubes are conical, dark and possess serrations. Pupa is also attached to roots of plants with the respiratory trumpets.

ARMIGERES

This is a very common group of mosquitoes and is found in everywhere. It brings considerable biting nuisance.

Armigeres subabatus

This is a member of Armigeres group and they are found in urban areas. It is not known to transmit major pathogens in nature. However, evidence suggests that it transmits Dirofilaria ripens. Adults are dark, medium sized with prominent banded abdomen and a bent proboscis. It breeds in polluted collections of water in urban situations.