Anopheles mosquitoes are the only vectors that can transmit human malaria. There are approximately 30-40 of the over 400 Anopheles species that are capable of transmitting malaria. They are found worldwide except in Antarctica. The life cycle includes egg, larva, pupa and adult stages, with the aquatic stages lasting 5-14 days depending on temperature. Only female Anopheles mosquitoes transmit malaria by taking a blood meal from an infected human host and passing the Plasmodium parasites to a new human host during subsequent blood feeding.

1. Anopheles

2. 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
430Anopheles 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.
• 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.

3. 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 : 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.

4. Top: Anopheles Egg;
note the lateral floats.
Bottom: Anopheles egg
s are laid singly.
Anopheles Larva. Note the
position, parallel to the
water surface.
Larvae of Anopheles gambiae,
the major malaria vector in Africa,
can breed in diverse habitats.
Three habitats are shown from
left to right: tire tracks, rice fields,
and irrigation water.

5. • 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 : 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.

7. Adults : 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.
• 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.

9. • 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.

11. 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.
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 Anophelesmosquitoes 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).

12. Anopheles freeborni mosquito pumping blood
Sequential images of the mosquito
taking its blood meal

13. 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 to 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 nature.
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.

14. Disease
Malaria : Anopheles are vectors of malaria, Bancroftian and Brugian filariasis and of
multiple arboviruses (dengue fever; yellow fever; encephalitides and
haemorrhagic fevers). Malaria is caused by Plasmodium falciparum, P. vivax, P.
malariae and P. ovale. Transmission of the disease occurs in virtually all of tropical
Africa, Central and South America, and the Middle and Far East. South East Asia is
a particular problem due to multiple drug resistance. P. falciparum is found in
Africa and other tropical countries as well as in subtropics. P. malariae has a low
prevalence in both tropics and subtropics. P .vivax is the most widespread in
temperate regions and subtropics but may also be found in the tropics. P. ovale
has a low prevalence in West Africa. In Africa alone, 370 million people live in
endemic areas.
• P .vivax causes benign tertian malaria (43% of cases) and P. falciparum results in
malignant tertian or sub-tertian malaria and pernicious malaria (50% of cases). P
.ovale (mild tertian malaria, 1% of cases) and P. malariae (quartan malaria, 7% of
cases) contribute a small percentage of malarial cases.
• The global malaria situation is serious and becoming worse: 300-500 million
clinical cases occur annually. 1.5 - 2.7 million people die of malaria each year with
approximately one million deaths among children under five years of age are
attributed to malaria alone or in combination with other diseases. Countries in
tropical Africa account for more than 90% of the total malaria incidence and the
great majority of malaria deaths (WHO data).

15. Filariasis : Anopheline mosquitoes also transmit
the filarial worms Wuchereria bancrofti,
Brugia malayi and Brugia timori. Wuchereria
bancrofti is the main cause of "elephantiasis"
(Bancroftian filariasis) and the most widely
distributed filarial parasite of man.
• Brugian (Malayan) filariasis is less widespread,
less common and less serious than its
Bancroftian counterpart. The life cycle is
identical to that of Wuchereria bancrofti with
Brugia malayi limited to Asia and B. timori
restricted to Indonesia.

16. Treatments and Control
Malaria : If the infective species is not known, or the infection is known to be
mixed, initial treatment should be with quinine, mefloquine or rarely
halofantrine. Falciparum (malignant) malaria is often resistant to
chloroquine and should be treated with quinine, mefloquine,
halofantrine, quinidine or pyrimethamine-sulphadoxine. Benign malaria
(P. vivax) should be treated with chloroquine.
• Prevention is most dependent upon coverage of exposed skin and the
use of insect repellent, mosquito nets impregnated with permethrin and
correct prophylaxis. The vector may be controlled by water clearance
programs, house spraying (DDT) and destruction of breeding areas. Drug
resistance to DDT and ethical resistance to its use have limited its
effectiveness.
Filariasis : Diethylcarbamazine (DEC) kills microfilaria. Ivermectin suppresses
microfilaria production but its overall effectiveness remains untried and
elephantiasis can be treated surgically. Control measures comprise
draining of mosquito breeding sites and killing larvae. Many mosquitoes
are resistant to insecticides but mosquito repellents and nets are
effective.
• Anopheline larvae may be suffocated in their breeding sites but culicine
larvae (Mansonia sp.) derive oxygen from plants and are not amenable to
such measures. Control depends upon the use of mosquito nets and
periodic mass treatment.