2. LECTURE OUTLINE
Mosquitoes as disease vectors-its role as important
human disease vector for dengue, microfilaria and
malaria
2
3. 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.
3
5. 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
5
6. MOSQUITOES: CLASSIFICATION
• There are 442 species of mosquitoes in Malaysia, compared6
to 553 species (Indonesia), 416 species (Thailand) and 310
species (Philippines)
7. 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.
7
12. 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
12
13. 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)
13
14. 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.
14
16. 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.
16
17. 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).
17
18. 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
18
19. 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
19
20. 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.
20
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.)
21. 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.
21
22. 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. 22
23. 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.
23
24. 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. 24
25. ANOPHELES: HEALTH IMPORTANT
Important malaria vectors in Malaysia
An. aconitus
An. balabacensis
An. dirus
An. donaldi
An. flavirostris
An. letifer
An. leucosphyrus
An. maculatus
An. minimus
An. nigerrimus
An. subpictus
An. sundaicus
25
26. 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
26
In some areas Aedes species transmit filariasis
27. 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.
27
28. 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.
28
29. 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.
29
30. 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.
30
31. 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
31
32. 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
32
33. 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.
33
34. 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.
34
35. 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
35
36. CULEX: HEALTH IMPORTANT
Culex quinquefasciatus (vector of Brancroftian filariasis
Culex tritaeniorhynchus (vector of Japanese encephalitis-
B)
Some important Culex sp. as a vector for malaria
Culex tritaeniorhynchus
Culex fuscocephala
Culex gelidus
Culex vishui
Culex pseudovish
36
37. 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.
37
39. 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
39
40. 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
40
41. Larvae of Mansonia The pupa of Mansonia uniformis.
uniformis attached to plant roots 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. 41
42. 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.
42
43. 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
43
44. 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
44
45. 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
45
46. 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
46
47. 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
47
48. 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
48
49. 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
49
50. 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 50
