ALL
ABOUT MOSQUITOES
Mosquitoes are a family
of small, midge-like flies: the Culicidae. Although a few species are harmless
or even useful to humanity, the females of most species are ectoparasites whose
tube-like mouthparts (called a proboscis) pierce the hosts' skin to suck the
blood. The word "mosquito" (formed by mosca and diminutive ito) is
from the Spanish or Portuguese for "little fly".Thousands of species
feed on the blood of various kinds of hosts, mainly vertebrates, including
mammals, birds, reptiles, amphibians, and even some kinds of fish. Some
mosquitoes also attack invertebrates, mainly arthropods. Though the loss of
blood is seldom of any importance to the victim, the saliva of the mosquito
often causes an irritating rash that is a serious nuisance. Much more serious
though, are the roles of many species of mosquitoes as vectors of diseases. In
passing from host to host, some transmit extremely harmful infections such as
malaria, yellow fever, west nile virus and filariasis.
The biting Diptera are
two-winged flying insects that suck blood from humans and animals. In many
parts of the world their biting is a considerable nuisance. More importantly,
they are carriers of a number of diseases, mostly in the tropics, causing
illness and death on a large scale.
The most important
group of biting Diptera is the mosquitos, which have a long, slender body and
long, needle-shaped, piercing mouthparts. Others include the blackflies,
phlebotomine sandflies, tsetse flies, biting midges, horseflies (tabanids) and
stable flies, which generally have shorter biting mouthparts and more robust
bodies. The last three groups are of limited importance as vectors of human
disease.
Mosquitos
Mosquitos differ from
the other biting Diptera in having a long slender body, long legs and long
needle-shaped mouthparts (Fig. 1.1a). 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.
Table shows Diseases transmitted by mosquitos and
VECTORS
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DISEASES
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Mosquitos
(Culicidae)
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Anopheles
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Malaria, lymphatic filariasis
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Culex
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Lymphatic filariasis, Japanese encephalitis, other
viral diseases
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Aedes
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Yellow fever, dengue, dengue haemorrhagic fever,
other viral diseases, lymphatic filariasis
|
Mansonia
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Lymphatic filariasis
|
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The
biting Diptera – mosquitos
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Mosquitos
Mosquitos
are important vectors of several tropical diseases, including malaria,
filariases, and numerous viral diseases, such as dengue, Japanese encephalitis
and yellow fever. In countries with a temperate climate they are more important
as nuisance pests than as vectors.
There
are about 3000 species of mosquito, of which about 100 are vectors of human
diseases. Control measures are generally directed against only one or a few of
the most important species and can be aimed at the adults or the larvae.
Life
cycle
Mosquitos
have four distinct stages in their life cycle: egg, larva, pupa and adult . The
females usually mate only once but produce eggs at intervals throughout their
life. In order to be able to do so most female mosquitos require a blood-meal .
Males do not suck blood but feed on plant juices. The digestion of a blood-meal
and the simultaneous development of eggs takes 2 - 3 days in the tropics but
longer in temperate zones. The gravid females search for suitable places to
deposit their eggs, after which another blood-meal is taken and another batch
of eggs is laid. This process is repeated until the mosquito dies.
The life cycle of the mosquito
Depending
on the species, a female lays between 30 and 300 eggs at a time. Many species
lay their eggs directly on the surface of water, either singly (Anopheles)
or stuck together in floating rafts (e.g. Culex). In the tropics,
the eggs usually hatch within 2 - 3 days. Some species (e.g. Aedes) lay
their eggs just above the water line or on wet mud; these eggs hatch only when
flooded with water. If left dry they can remain viable for many weeks.
Once
hatched, the larvae do not grow continuously but in four different stages
(instars). The first instar measures about 1.5 mm in length, the fourth about 8
- 10 mm. Although they have no legs, they have a well developed head and body
covered with hairs, and swim with sweeping movements of the body. They feed on
yeasts, bacteria and small aquatic organisms. Most mosquito larvae have a
siphon located at the tip of the abdomen through which air is taken in and come
to the water surface to breathe; they dive to the bottom for short periods in
order to feed or escape danger. Anopheles larvae, which feed
and breathe horizontally at the surface, have a rudimentary siphon. Larvae of Mansonia do
not need to come to the surface to breathe, since they can obtain air by
inserting the siphon into a water plant, to which they remain attached for most
of the time.
In
warm climates, the larval period lasts about 4 - 7 days, or longer if there is
a shortage of food. The fully grown larva then changes into a comma-shaped
pupa, which does not feed and spends most of its time at the water surface. If
disturbed it dives swiftly to the bottom. When mature, the pupal skin splits at
one end and a fully developed adult mosquito emerges. In the tropics the pupal
period lasts 1 - 3 days. The entire period from egg to adult takes about 7 - 13
days under good conditions.
Biting
behaviour
Female
mosquitos feed on animals and humans. Most species show a preference for certain
animals or for humans. They are 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.
Because
digestion of the blood-meal and development of the eggs takes several days, a
blood-fed mosquito looks for a safe resting place that is shaded and offers
protection from desiccation. Some species prefer to rest in houses or cattle
sheds, while others prefer to rest outdoors, on vegetation or at other natural
sites. Mosquitos do not usually bite while eggs are developing.
The
behaviour of mosquitos determines whether they are important as nuisance
insects or vectors of disease, and governs the selection of control methods.
Species that prefer to feed on animals are usually not very effective in
transmitting diseases from person to person. Those that bite in the early
evening may be more difficult to avoid than species that feed at night.
Mosquitos that rest indoors are the easiest to control.
Distinguishing
features of vector mosquitos
Among
the mosquitos there are two groups that suck human blood and may transmit
disease.
a)
The anophelines; the genus Anopheles is best known for its
role in transmitting malaria, but in some areas it can also transmit
filariasis.
b)
The culicines, which include the following genera:
v Culex:
vectors of filariasis and some viral diseases
v Aedes: vectors of dengue, yellow fever
and other viral diseases, and sometimes of filariasis
v Mansonia: vectors of brugian filariasis
v Haemagogus and Sabethes:
vectors of yellow fever in the forests of South and Central America.
Main characteristics for differentiating
Anopheles, Aedes and Culex mosquitos
Anopheles, Culex and Aedes mosquitos
can be distinguished from each other as shown in.
The most useful characteristics for
distinguishing anophelines from other mosquitos are:
v the length of the palps is equal to that of the
proboscis;
v while at rest they usually keep their mouthparts and
abdomen in a straight line at an angle to the resting surface; the angle varies
with the species and in some cases is almost perpendicular to the surface. Anopheles
culicifacies, a malaria vector in south Asia, is an exception, keeping its
body almost parallel to the surface. As its name suggests, it looks
superficially like a Culex mosquito.
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| Anopheles mosquito |
Anopheles mosquitos
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.
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.
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. 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 mosquitos leave their
resting sites and try to find a suitable breeding habitat.
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An Anopheles mosquito in flight
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An Anopheles mosquito at rest
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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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| Culex Mosquito |
Culex mosquitos
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.
In some areas they are a considerable nuisance
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. 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. 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.
Behaviour
Culex
quinquefasciatus 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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Aedes
aegypti at rest. 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.
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Aedes mosquitos
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 (Fig. 1.7) is an important vector of dengue, dengue
haemorrhagic fever, yellow fever and other viral diseases. A closely related
species, Aedes albopictus, can also transmit dengue. In some areas Aedes species
transmit filariasis.
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Aedes aegypti in flight
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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. 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. 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.
Behaviour
Aedes mosquitos bite mainly in the morning or
evening. Most species bite and rest outdoors but in tropical towns Aedes
aegypti breeds, feeds and rests in and around houses.
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Mansonia mosquitos
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.
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. 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 (Eichhornia,
Pistia, Salvinia) and grasses (Fig. 1.8).
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.
Spread of disease and prevention
Aedes
aegypti is a vector for transmitting several tropical fevers. Only the female
bites for blood, which she needs to mature her eggs. To find a host, these
mosquitoes are attracted to chemical compounds emitted by mammals. These
compounds include ammonia, carbon dioxide, lactic acid, and octenol. Scientists
at the Agricultural Research Service have studied the specific chemical structure
of octenol to better understand why this chemical attracts the mosquito to its host.They
found the mosquito has a preference for "right-handed"
(dextrorotatory) octenol molecules.
The
yellow fever mosquito can also contribute to the spread of reticulum cell sarcoma
among Syrian hamsters.
The
CDC traveler's page on preventing dengue fever suggests using mosquito
repellents that contain DEET (N, N-diethylmetatoluamide, 20% to 30%
concentration, but not more).
It also suggests:
v Although Aedes aegypti mosquitoes most commonly bite
at dusk and dawn, indoors, in shady areas, or when the weather is cloudy,
"they can bite and spread infection all year long and at any time of
day."
v The mosquitoes prefer to breed in areas of stagnant
water, such as flower vases, uncovered barrels, buckets, and discarded tires,
but the most dangerous areas are wet shower floors and toilet tanks, as they
allow the mosquitos to breed in the residence. Research has shown that certain
chemicals emanating from bacteria in water containers stimulate the female
mosquitoes to lay their eggs. They are particularly motivated to lay eggs in
water containers that have the correct amounts of specific fatty acids
associated with bacteria involved in the degradation of leaves and other
organic matter in water. The chemicals associated with the microbial stew are
far more stimulating to discerning female mosquitoes than plain or filtered
water in which the bacteria once lived.
v Wear long-sleeved clothing and long trousers when
outdoors during the day and evening
v Spray permethrin or DEET repellents on clothing, as
mosquitos may bite through thin clothing
v Use mosquito netting over the bed if the bedroom is
not air conditioned or screened, and for additional protection, treat the
mosquito netting with the insecticide permethrin
v Spray permethrin or a similar insecticide in the
bedroom before retiring
Mosquito
control is currently the best method for disease prevention. This primarily
includes source reduction, pesticide spraying or "fogging", or the
use of mosquito traps like the lethal ovitrap.
Although
the lifespan of an adult A. aegypti is two to four weeks depending on
conditions,[9] the eggs can be viable for over a year in a dry state, which
allows the mosquito to re-emerge after a cold winter or dry spell.[10]
The
preference for biting humans is dependent on expression of the odorant receptor
AeegOr4.
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| Prevention of mosquito |
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