• MAJOR INSECT VECTORS OF TROPICAL DISEASES
• INSECT VECTOR CONTROL APPROACHES
• MERITS AND DEMERITS OF SELECTED VECTOR CONTROL TACTIC
• CONTEMPORARY METHODS
• CASE STUDIES
• 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.
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
• Control of arthropod (insect) vectors is the primary
available intervention for some of the most devastating
5. MAJOR INSECT VECTORS OF TROPICAL
• As earlier stated, Insects are the most common
tropical disease vectors, transmitting a number of
• Major insect vectors include:
Assassin bugs, Human Lice, Black flies, Sand flies,
Punkies, Mosquitoes, Horseflies, Eye-gnats,
Houseflies and Tsetse flies
Genera Pathogens carried Diseases
Trypanosoma cruzi Chagas Disease South &Central
Black fly Diptera
Simulium Onchocerca volvulus Onchocerciasis
Eye gnat Diptera
Treponema pertenue Yaws,
Horse fly Diptera
Tabanus Pasteurella tularensis,
Musca Shigella dysentariae,
typhosa, Vibrio comma,
Table 1: List of major insect vectors and endemicity of diseases transmitted
Adapted from: www.cals.ncsu.edu
Human Lice Pthiraptera Pediculus Borellia Recurrentis,
East, West Africa,
Itchy Red welts,
Any aquatic or
Sand fly Diptera
Tsetse fly Diptera
Glossina Trypanosoma spp. Human African
Adapted from: www.cals.ncsu.edu
Table 1 contd.
8. Images of some insect vectors
Plate 1: a) Assassin bug, Triatoma infestans (b) Black fly, Simulium damnosum
Source : (a) http://www.healthline.com
Plate 11: (a) Person suffering from malaria (b) Trypanosomes in Blood
Source : (a)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
• 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
• 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
20. Scorecard for Chemical control
• Wide spectrum of activity
• Ease of application
• Rapid action on target species
• Availability & affordability
• Residual activity
• Over-reliance/abuse of
• Insecticide resistance
• Vector population resurgence
• Adverse effects on non-target
• Ecosystem pollution
• Public health issues
21. CONTEMPORARY METHODS OF VECTOR
• 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.
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)
23. Sterile Insect Technique (SIT)
• Sterile Insect Technique (SIT) is an applied form of biological
control, whereby the natural reproductive fitness of specific insects
• The process involves the exposure of male individuals (mostly pupal
stage) of insects to specified doses of gamma radiation to achieve
• 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.
25. Genetic modification
• Genetic modification is the alteration and recombination of genetic
material by technological means, resulting in transgenic
• 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
• These males are then released in large numbers into the affected region
and after mating, any female offspring produced will die
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
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,
30. Innovative chemicals (Insecticidal agents)
• Natural insecticides, such as
nicotine, pyrethrum, neem
and ryanodine extracts are
made by plants as defences
• Neonicotinoids are synthetic
analogues of the natural
insecticide nicotine, with a
much lower acute mammalian
toxicity and greater field
• Ryanoids are synthetic
chemicals with the same mode
of action as ryanodine, a
natural insecticide extracted
from Ryania speciosa .
Plate 15: Ryana speciosa
Plate 16: Neem, Azadirachta indica
Fig. 2: Chemical structure of some
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
• 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
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.
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
• Chemical control such as:
Long Lasting Insecticidal nets (LLINs)
Direct bodily use of repellents which appear in various forms.
Indoor Residual Spraying
Fig. 4: Concept of Integrated Vector control
38. Case report
• 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)
Fig. 5: Current Tsetse fly distribution in Africa
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
Fig. 6 :
Cumulative proportional survival rates of adult A. stephensi after exposure to oil-based
spray residues containing the fungal pathogen B. bassiana
Plate 18: Spore of Entomopathogenic fungi, Beauvaria bassiana, encapsulating an insect.
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.