Seminar on Contemporary methods of insect vector control in the tropics presented at the University of Lagos in July 2012.

1. CONTEMPORARY METHODS OF
INSECT VECTOR CONTROL
IN THE TROPICS
BY
BELLO OLADOLAPO
080809028
1

2. OUTLINE
• INTRODUCTION
• MAJOR INSECT VECTORS OF TROPICAL DISEASES
AREAS FOUND
DISEASES TRANSMITTED
• INSECT VECTOR CONTROL APPROACHES
• MERITS AND DEMERITS OF SELECTED VECTOR CONTROL TACTIC
• CONTEMPORARY METHODS
• CASE STUDIES
• CONCLUSION
2

3. Introduction
• Tropical diseases are diseases that are prevalent in
or endemic to tropical and subtropical regions of the
world. A considerable number of these disease
agents are transmitted by insects.
• Insects especially haematophagous flies are by far
the most important tropical vectors, transmitting a
number of disease organisms.
• Most often, transmission mode is active.
3

4. Insect vector control
• Insect-vector control is defined as the application of
targeted site-specific activities that are cost-effective to
manage populations of insects which carry disease-causing
organisms
• Control of arthropod (insect) vectors is the primary
available intervention for some of the most devastating
tropical diseases.
4

5. MAJOR INSECT VECTORS OF TROPICAL
DISEASES
• As earlier stated, Insects are the most common
tropical disease vectors, transmitting a number of
disease organisms.
• Major insect vectors include:
Assassin bugs, Human Lice, Black flies, Sand flies,
Punkies, Mosquitoes, Horseflies, Eye-gnats,
Houseflies and Tsetse flies
5

6. Common
name
Order
(family)
Genera Pathogens carried Diseases
transmitted
Areas
Endemic
Assassin bug,
Kissing bug
Hemiptera
(Reduviidae)
Triatoma
Rhodnius
Trypanosoma cruzi Chagas Disease South &Central
America
Black fly Diptera
(Simuliidae)
Simulium Onchocerca volvulus Onchocerciasis
(River Bindness)
Africa, Mexico,
South, Central
America
Eye gnat Diptera
(Chloropidae)
Hippelates,
Siphunculina
Treponema pertenue Yaws,
Conjuctivitis
Asia, South
America, Africa.
Horse fly Diptera
(Tabanidae)
Tabanus Pasteurella tularensis,
Bacillus anthracis,
Loa loa
Tularemia,
Anthrax, Loiasis.
Tropical Africa
Housefly Diptera
(Muscidae)
Musca Shigella dysentariae,
Eberthella
typhosa, Vibrio comma,
Salmonella typhi
Dysentery,
Typhoid fever,
Cholera, Anthrax,
Tuberculosis,
Poliomyelitis
Found in
unsanitary
conditions
worldwide
6
Table 1: List of major insect vectors and endemicity of diseases transmitted
Adapted from: www.cals.ncsu.edu

7. Common
name
Order
(family)
Genera Pathogens
carried
Diseases
transmitted
Areas
Endemic
Human Lice Pthiraptera Pediculus Borellia Recurrentis,
Rickettsia spp.
Epidemic
Relapsing fever,
Epidemic typhus,
Trench fever
Asia, Africa
Mosquitoes Diptera
(Culicidae)
Aedes,
Anopheles,
Culex
Viruses,
Plasmodium spp.
Wuchereria
bancrofti,
Brugia malayi,
Malaria, Yellow
fever, Dengue
fever,
Encephalitis,
Filariasis,
Chikungunya
East, West Africa,
Asia.
Punkies,
Biting Midges
Diptera
(Ceratopogonidae)
Culicoides,
Forcipomyia,
Leptoconops
Acanthocheilonema,
Dipetalonema,
Mansonella,
Onchocerca
Itchy Red welts,
Allergic
Responses.
Any aquatic or
semi-aquatic
habitat
Sand fly Diptera
(Psychodidae)
Phlebotomus Bartonella
bacilliformis,
Leishmania
spp.
Carrion’s disease,
Sandfly fever,
Leishmaniasis
South America,
Asia, Africa.
Tsetse fly Diptera
(Glossidae)
Glossina Trypanosoma spp. Human African
Trypanosomiasis,
Nagana
Africa
7
Adapted from: www.cals.ncsu.edu
Table 1 contd.

8. Images of some insect vectors
8
(a) (b)
Plate 1: a) Assassin bug, Triatoma infestans (b) Black fly, Simulium damnosum
Source : (a) http://www.healthline.com
(b)www.epa.gov

9. 9
(a) (b)
Plate 2: (a) Eye gnat, Hippelates pallipes (b) Horsefly, Tabanus sulcifrons
Source : (a) cals.ncsu.edu
(b) c-r-alpacas.com

10. 10
(b)
Plate 3: (a) Housefly, Musca domestica (b) Human louse, Pediculus humanus
Source : (a) cals.ncsu.edu
(b) http://www.nlm.nih.gov
(a)

11. 11
(a) (b)
Plate 4: (a) Punkie, Culicoides sonorensis (b) Culex mosquito
Source : (a) cals.ncsu.edu
(b) gizmag.com

12. 12
(a) (b)
Plate 5: (a) Aedes mosquito (b) female Anopheles mosquito
Source : (a) health.ezinemark.com
(b) medimanage.com

13. 13
(a) (b)
Plate 6: (a) Sand fly, Phlebotomus sergenti (b) Tsetse fly, Glossina longipalpis
Source : (a) cals.ncsu.edu
(b) cals.ncsu.edu

14. Some conditions transmitted by Arthropod
(Insect) vectors
14
Plate 7: (a) Chikungunya rash (b) Leishmaniasis sore
Source : (a)mosquitozone.com
(b)mosquitozone.com

15. 15
(a) (b)
Plate 8: (a) Chagas disease symptom (b) African Sleeping Sickness
Source : (a)mosquitozone.com
(b)mosquitozone.com

16. 16
(a) (b)
Plate 9: (a) Lymphatic Filariasis (Elephantiasis) (b) Horsefly bite
Source : (a)mosquitozone.com
(b)mosquitozone.com

17. 17
(a)
(b)
Plate 10: (a) Sand fly bite (b) Eye lesions transmitted by Black fly
Source : (a)mosquitozone.com
(b)mosquitozone.com

18. 18
(a) (b)
Plate 11: (a) Person suffering from malaria (b) Trypanosomes in Blood
Source : (a)mosquitozone.com
(b)mosquitozone.com

19. INSECT VECTOR CONTROL APPROACHES
The most common insect vector control measure adopted over the
years has been the use of chemicals in various forms (Chemical
control).
CHEMICAL CONTROL:
• This involves the application of chemical compounds (mostly
synthetic) as repellents or killing agents for insect vectors.
• Insecticides such as larvicides, adulticides and repellents have been
in use to control vectors. For example, larvicides can be used in
breeding zones of many insect vectors with aquatic immature
stages.
• Insecticides can be applied to house walls or bed nets, and use of
personal repellents can reduce incidence of insect bites and thus
infection. The use of pesticides for vector control is promoted by
the World Health Organization (WHO) and has proven to be highly
effective.
19

20. Scorecard for Chemical control
Merits
• Wide spectrum of activity
• Ease of application
• Rapid action on target species
• Availability & affordability
• Residual activity
Demerits
• Over-reliance/abuse of
chemicals
• Insecticide resistance
development
• Vector population resurgence
• Adverse effects on non-target
species
• Ecosystem pollution
• Public health issues
20

21. CONTEMPORARY METHODS OF VECTOR
CONTROL
• The myriads of challenges associated with chemical
control of disease vectors in the tropics have
necessitated a call for viable alternative(s) to combat
their contributions to disease burden in the regions.
• In this regard, a number of hitherto old methods of
vector control are being reviewed, while entirely new
approaches are currently under study.
21

22. NOVEL APPROACHES TO VECTOR CONTROL
Some novel approaches to vector control include:
• Sterile Insect Technique (SIT)
• Genetic Modification (GM)
• Modification of age structure
• “New Generation” chemicals
• Hormonal control
• Integrated Vector Management (IVM)
22

23. Sterile Insect Technique (SIT)
• Sterile Insect Technique (SIT) is an applied form of biological
control, whereby the natural reproductive fitness of specific insects
is interrupted.
• The process involves the exposure of male individuals (mostly pupal
stage) of insects to specified doses of gamma radiation to achieve
sterility.
• These are then released into the environment to compete with wild
fertile males for mature female insects.
• The ultimate aim is the gradual reduction in number of eggs laid,
with resultant reduction in insect population.
23

24. 24
Plate 12: Screw worm fly, Cochliomyia hominivorax
source: www-naweb.iaea.org

25. Genetic modification
• Genetic modification is the alteration and recombination of genetic
material by technological means, resulting in transgenic
organisms(insects)
• The latest method works by introducing a repressible "Dominant Lethal"
gene into male insects. The insects can also be given genetic markers, such
as fluorescence that make monitoring the progress of eradication easier.
• Most advanced forms of this technique have a female-specific dominant
lethal gene.
• These males are then released in large numbers into the affected region
and after mating, any female offspring produced will die
25

26. 26
Fig. 1: Diagram showing a cross between a male homozygous for the
Lethal gene and a normal homozygous female
source : bioone.org

27. Plate 13: A genetically modified male insect carrying the female-specific “Dominant
Lethal” gene with genetic markers that fluoresce its eyes
Source: mi2g.com
27

28. MODIFICATION OF AGE STRUCTURE
• Most pathogens vectored by arthropods, (insects)undergo an Extrinsic Incubation
Period (EIP) in the vector(during which they replicate and infect the salivary
glands), before they can be transmitted to a new host.
• The duration of EIP consumes a significant proportion of the vector’s lifespan.
Thus, only the most mature vectors are of epidemiological importance.
• Some biological agents induce mortality effects late in adult life and skew vector
population towards younger individuals. These include
 Densonucleosis viruses (densoviruses).
 The use of virulent strains of the common bacterial endosymbiont, Wolbachia,
 Entomopathogenic fungi.
28

29. 29
Plate:
(a) (b)
Plate 14: (a) Galleria mellonella densovirus (b) Wolbachia pipientis (red)
in insect testes (green)
Source : (a) virology.wisc.edu/virusworld
(b)serc.carleton.edu

30. Innovative chemicals (Insecticidal agents)
Natural compounds
• Natural insecticides, such as
nicotine, pyrethrum, neem
and ryanodine extracts are
made by plants as defences
against insects
Synthetic Compounds
Neonicotinoids
• Neonicotinoids are synthetic
analogues of the natural
insecticide nicotine, with a
much lower acute mammalian
toxicity and greater field
persistence.
Ryanoids
• Ryanoids are synthetic
chemicals with the same mode
of action as ryanodine, a
natural insecticide extracted
from Ryania speciosa .
30
Plate 15: Ryana speciosa
source: naturekind.org

31. 31
Plate 16: Neem, Azadirachta indica
Source: nipahutgardens.com
Fig. 2: Chemical structure of some
neonicotinoids
Source: uni-ulm.de

32. Hormonal control
• A hormone is a chemical secreted by an endocrine gland or some
nerve cells that regulates various aspects of growth and
development such as the change from larva to adult.
• Ecdysone, a hormone in insects that promote metamorphosis and
ecdysis (moulting), triggers larva-larva moults as long as another
hormone, called Juvenile Hormone (JH) is present. When JH is low
or negligible, ecdysone promotes the pupa-to-adult moult. Thus
normal metamorphosis seems to occur when the output of JH
diminishes spontaneously
• When solutions act on such insects that undergo normal
metamorphosis (of the latter kind), their normal development is
upset. This raises the possibility of using JH as an insecticide.
Unfortunately, JH is too unstable to be practical, but some synthetic
JH-mimics, e.g. Methoprene, Pyriproxyfen and Diofenolan are now
being used.
32

33. Hormonal Control
33
Fig. 3: Hormonal Control of insects
Source: biology-forums.com

34. Integrated Vector Management (IVM)
• A new WHO Global Strategic Framework for Integrated Vector
Management defines IVM as a strategy to
. . . improve the efficacy, cost-effectiveness, ecological soundness and
sustainability of disease vector control. IVM encourages a multi-disease
control approach, integration with other disease control measures and the
considered and systematic application of a range of interventions, often in
combination and synergistically.
• The general objective of Integrated Vector Management is the reduction
of vector-borne diseases, particularly through the prevention, reduction
and or interruption of disease transmission, via the utilization of multiple
control measures in a compatible manner.
34

35. Components of I.V.M.
Components of IVM include:
• The use of personal protective measures such as:
 Wearing of protective clothings
 Environmental Control Measures
• Biological Control Measures
Biological control or “biocontrol” is the use of natural enemies to manage
mosquito populations.
• Chemical control such as:
 Larviciding
 Long Lasting Insecticidal nets (LLINs)
 Direct bodily use of repellents which appear in various forms.
 Indoor Residual Spraying
 Outdoor spraying
35

36. 36
Fig. 4: Concept of Integrated Vector control
Source: helid.digicollection.org

37. 37
Plate 17: (a) LLIN (b) Mosquito fish (Gambusia affinis)
Source : (a) vectorcontrol.bayer.com
(b) tropicalfishtanksonline.com
(a)
(b)

38. Case report
Becker 2011
• Tsetse fly (Glossina spp.)
• 43 species implicated as disease vectors in Africa
• Vertebrate blood feeder (both sexes)
• Disease agent is a protozoan of the Genus Trypanosoma
– Human sleeping sickness
– Nagana in cattle
• Eradicated via SIT in Zanzibar (East Africa)
38

39. 39
Fig. 5: Current Tsetse fly distribution in Africa
Source: www.indiana.edu

40. Case study
• In a laboratory-based setting, Blanford et al (2005) examined the
survival and sporozoite burden of Anopheles stephensi exposed to
an isolate of the fungus Beauvaria bassiana.
• Results indicated that short periods of exposure of mosquitoes to
cage mesh sprayed with oil-based formulations of Beauvaria were
sufficient to cause > 90% mortality by day 14 after contact (the
approximate EIP for malaria).
• Importantly, exposure of mosquitoes infected with the rodent
malaria, Plasmodium chabaudi, to surfaces sprayed with Beauvaria
spores reduced the transmission risk by a factor of 80.
• At day 14 post-exposure, 31% of malaria-infected control
mosquitoes were alive and able to transmit, compared with only
0.4% of mosquitoes in the Beauvaria and malaria treatment
40

41. 41
Fig. 6 :
Cumulative proportional survival rates of adult A. stephensi after exposure to oil-based
spray residues containing the fungal pathogen B. bassiana
Source: www.sciencemag.org/cgi/content/full/308/5728/1638/DC1

42. 42
Plate 18: Spore of Entomopathogenic fungi, Beauvaria bassiana, encapsulating an insect.
source: microbeworld.org

43. CONCLUSION
Vector control remains a viable approach to the
management of many diseases in the tropics and
subtropics. The careful consideration, selection
and adoption of locally relevant tactics is
imperative. This will ultimately enhance disease
management in the tropical regions of the world.
43

44. 44