General
Information
There
are approximately 3,500 species of mosquitoes grouped into 41 genera.
Human malaria is transmitted only by females of the genus Anopheles.
Of the approximately 430 Anopheles species, only 30-40 transmit
malaria (i.e., are "vectors") in nature.
Geographic
Distribution
Anophelines
are found worldwide except Antarctica. Malaria is transmitted by different
Anopheles species, depending on the region and the environment.
Map of the world showing the distribution of predominant malaria vectors
Geographical
Distribution of Arthropod Borne Diseases and Their Principal Vectors.
Anophelines
that can transmit malaria are found not only in malaria-endemic areas,
but also in areas where malaria has been eliminated. The latter areas
are thus constantly at risk of re-introduction of the disease.
Images
In addition to the images further down this page, we have the following
images of an Anopheles mosquito having a blood meal.
Life
Stages
Like
all mosquitoes, anophelines go through four stages in their life cycle:
egg, larva, pupa, and adult. The first three stages are aquatic and last
5-14 days, depending on the species and the ambient temperature. The adult
stage is when the female Anopheles mosquito acts as malaria vector.
The adult females can live up to a month (or more in captivity) but most
probably do not live more than 1-2 weeks in nature.
Eggs
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Top: Anopheles Egg;
note the lateral floats. Bottom: Anopheles eggs are
laid singly |
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Anopheles
Larva. Note the position, parallel to the water surface
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Adult
females lay 50-200 eggs per oviposition. Eggs are laid singly directly
on water and are unique in having floats on either side. Eggs are not
resistant to drying and hatch within 2-3 days, although hatching may take
up to 2-3 weeks in colder climates.
Larvae
Mosquito
larvae have a well-developed head with mouth brushes used for feeding,
a large thorax and a segmented abdomen. They have no legs. In contrast
to other mosquitoes, Anopheles larvae lack a respiratory siphon
and for this reason position themselves so that their body is parallel
to the surface of the water.
Larvae
breathe through spiracles located on the 8th abdominal segment and therefore
must come to the surface frequently. The larvae spend most of their time
feeding on algae, bacteria, and other microorganisms in the surface microlayer.
They dive below the surface only when disturbed. Larvae swim either by
jerky movements of the entire body or through propulsion with the mouth
brushes.
Larvae
develop through 4 stages, or instars, after which they metamorphose into
pupae. At the end of each instar, the larvae molt, shedding their exoskeleton,
or skin, to allow for further growth.
The
larvae occur in a wide range of habitats but most species prefer clean,
unpolluted water. Larvae of Anopheles mosquitoes have been found
in fresh- or salt-water marshes, mangrove swamps, rice fields, grassy
ditches, the edges of streams and rivers, and small, temporary rain pools.
Many species prefer habitats with vegetation. Others prefer habitats that
have none. Some breed in open, sun-lit pools while others are found only
in shaded breeding sites in forests. A few species breed in tree holes
or the leaf axils of some plants.
Pupae
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Anopheles
Pupa
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The
pupa is comma shaped when viewed from the side. The head and thorax are
merged into a cephalothorax with the abdomen curving around underneath.
As with the larvae, pupae must come to the surface frequently to breathe,
which they do through a pair of respiratory trumpets on the cephalothorax.
After a few days as a pupa, the dorsal surface of the cephalothorax splits
and the adult mosquito emerges.
The
duration from egg to adult varies considerably among species and is strongly
influenced by ambient temperature. Mosquitoes can develop from egg to
adult in as little as 5 days but usually take 10-14 days in tropical conditions
Adults
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Anopheles
Adults. Note (bottom row) the typical resting position.
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Like
all mosquitoes, adult anophelines have slender bodies with 3 sections:
head, thorax and abdomen.
The
head is specialized for acquiring sensory information and for feeding.
The head contains the eyes and a pair of long, many-segmented antennae.
The antennae are important for detecting host odors as well as odors of
breeding sites where females lay eggs. The head also has an elongate,
forward-projecting proboscis used for feeding, and two sensory palps.
The
thorax is specialized for locomotion. Three pairs of legs and a pair of
wings are attached to the thorax.
The
abdomen is specialized for food digestion and egg development. This segmented
body part expands considerably when a female takes a blood meal. The blood
is digested over time serving as a source of protein for the production
of eggs, which gradually fill the abdomen.
Anopheles
mosquitoes can be distinguished from other mosquitoes by the palps, which
are as long as the proboscis, and by the presence of discrete blocks of
black and white scales on the wings. Adult Anopheles can also be
identified by their typical resting position: males and females rest with
their abdomens sticking up in the air rather than parallel to the surface
on which they are resting.
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Female
Anopheles dirus Feeding.
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Adult
mosquitoes usually mate within a few days after emerging from the pupal
stage. In most species, the males form large swarms, usually around dusk,
and the females fly into the swarms to mate.
Males
live for about a week, feeding on nectar and other sources of sugar. Females
will also feed on sugar sources for energy but usually require a blood
meal for the development of eggs. After obtaining a full blood meal, the
female will rest for a few days while the blood is digested and eggs are
developed. This process depends on the temperature but usually takes 2-3
days in tropical conditions. Once the eggs are fully developed, the female
lays them and resumes host seeking.
The
cycle repeats itself until the female dies. Females can survive up to
a month (or longer in captivity) but most probably do not live longer
than 1-2 weeks in nature. Their chances of survival depend on temperature
and humidity, but also their ability to successfully obtain a blood meal
while avoiding host defenses.
Factors
Involved in Malaria Transmission and Malaria Control
Understanding
the biology and behavior of Anopheles mosquitoes can help understand
how malaria is transmitted and can aid in designing appropriate control
strategies. Factors that affect a mosquito's ability to transmit malaria
include its innate susceptibility to Plasmodium, its host choice
and its longevity. Factors that should be taken into consideration when
designing a control program include the susceptibility of malaria vectors
to insecticides and the preferred feeding and resting location of adult
mosquitoes.
See
also: Vector Control
Preferred
Sources For Blood Meals
One
important behavioral factor is the degree to which an Anopheles
species prefers to feed on humans (anthropophily) or animals such as cattle
(zoophily). Anthrophilic Anopheles are more likely to transmit
the malaria parasites from one person to another. Most Anopheles
mosquitoes are not exclusively anthropophilic or zoophilic. However, the
primary malaria vectors in Africa, An. gambiae and An. funestus,
are strongly anthropophilic and, consequently, are two of the most efficient
malaria vectors in the world.
Life
Span
Once
ingested by a mosquito, malaria parasites must undergo development within
the mosquito before they are infectious to humans. The time required for
development in the mosquito (the extrinsic incubation period) ranges from
10-21 days, depending on the parasite species and the temperature. If
a mosquito does not survive longer than the extrinsic incubation period,
then she will not be able to transmit any malaria parasites.
It
is not possible to measure directly the life span of mosquitoes in natures.
But indirect estimates of daily survivorship have been made for several
Anopheles species. Estimates of daily survivorship of An. gambiae
in Tanzania ranged from 0.77 to 0.84 meaning that at the end of one day
between 77% and 84% will have survived. (Charlwood et al., 1997, Survival
And Infection Probabilities of Anthropophagic Anophelines From An Area
of High Prevalence of Plasmodium falciparum in Humans, Bulletin
of Entomological Research, 87, 445-453).
Assuming
this is constant through the adult life of a mosquito, less than 10% of
female An. gambiae would survive longer than a 14-day extrinsic
incubation period. If daily survivorship increased to 0.9, over 20% of
mosquitoes would survive longer than a 14-day extrinsic incubation period.
Control measures that rely on insecticides (e.g. indoor residual spraying)
may actually impact malaria transmission more through their effect on
adult longevity than through their effect on the population of adult mosquitoes.
Patterns
of Feeding And Resting
Most
Anopheles mosquitoes are crepuscular (active at dusk or dawn) or
nocturnal (active at night). Some Anopheles mosquitoes feed indoors
(endophagic) while others feed outdoors (exophagic). After blood feeding,
some Anopheles mosquitoes prefer to rest indoors (endophilic) while
others prefer to rest outdoors (exophilic). Biting by nocturnal, endophagic
Anopheles mosquitoes can be markedly reduced through the use of
insecticide-treated bed nets (ITNs) or through improved housing construction
to prevent mosquito entry (e.g. window screens). Endophilic mosquitoes
are readily controlled by indoor spraying of residual insecticides. In
contrast, exophagic/exophilic vectors are best controlled through source
reduction (destruction of the breeding sites)
Insecticide
Resistance
Insecticide-based
control measures (e.g. indoor spraying with insecticides, ITNs) are the
principal way to kill mosquitoes that bite indoors. However, after prolonged
exposure to an insecticide over several generations, mosquitoes, like
other insects, may develop resistance, a capacity to survive contact with
an insecticide. Since mosquitoes can have many generations per year, high
levels of resistance can arise very quickly. Resistance of mosquitoes
to some insecticides has been documented with just within a few years
after the insecticides were introduced. There are over 125 mosquito species
with documented resistance to one or more insecticides. The development
of resistance to insecticides used for indoor residual spraying was a
major impediment during the Global Malaria Eradication Campaign. Judicious
use of insecticides for mosquito control can limit the development and
spread of resistance. However, use of insecticides in agriculture has
often been implicated as contributing to resistance in mosquito populations.
It is possible to detect developing resistance in mosquitoes and control
programs are well advised to conduct surveillance for this potential problem.
Related Sources:
Susceptibility/Refractoriness
Some
Anopheles species are poor vectors of malaria, as the parasites
do not develop well (or at all) within them. There is also variation within
species. In the laboratory, it has been possible to select for strains
of An. gambiae that are refractory to infection by malaria parasites.
These refractory strains have an immune response that encapsulates and
kills the parasites after they have invaded the mosquito's stomach wall.
Scientists are studying the genetic mechanism for this response. It is
hoped that some day, genetically modified mosquitoes that are refractory
to malaria can replace wild mosquitoes, thereby limiting or eliminating
malaria transmission.
Other
Sources of Information outside CDC
Page last modified : September
7, 2004
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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