CONTEMPORARY METHODS OF INSECT VECTOR CONTROL
IN THE TROPICS
BY
BELLO OLADOLAPO
080809028
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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
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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.
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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.
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MAJOR INSECT VECTORS OF TROPICAL DISEASES
• As earlier stated, Insects are the most commontropical disease vectors, transmitting a number ofdisease organisms.
• Major insect vectors include:
Assassin bugs, Human Lice, Black flies, Sand flies,Punkies, Mosquitoes, Horseflies, Eye-gnats,Houseflies and Tsetse flies
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Common name
Order(family)
Genera Pathogens carried Diseases transmitted
AreasEndemic
Assassin bug,Kissing bug
Hemiptera(Reduviidae)
TriatomaRhodnius
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, Eberthellatyphosa, Vibrio comma, Salmonella typhi
Dysentery, Typhoid fever, Cholera, Anthrax, Tuberculosis, Poliomyelitis
Found in unsanitaryconditions worldwide
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Table 1: List of major insect vectors and endemicity of diseases transmitted
Adapted from: www.cals.ncsu.edu
Common name
Order(family)
Genera Pathogens carried
Diseases transmitted
AreasEndemic
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.Wuchereriabancrofti,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,AllergicResponses.
Any aquatic or semi-aquatic habitat
Sand fly Diptera(Psychodidae)
Phlebotomus Bartonellabacilliformis,Leishmaniaspp.
Carrion’s disease,Sandfly fever, Leishmaniasis
South America, Asia, Africa.
Tsetse fly Diptera(Glossidae)
Glossina Trypanosoma spp. Human African Trypanosomiasis,Nagana
Africa
7Adapted from: www.cals.ncsu.edu
Table 1 contd.
Images of some insect vectors
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Plate 1: a) Assassin bug, Triatoma infestans (b) Black fly, Simulium damnosum
Source : (a) http://www.healthline.com(b)www.epa.gov
(a) (b)
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Plate 2: (a) Eye gnat, Hippelates pallipes (b) Horsefly, Tabanus sulcifrons
Source : (a) cals.ncsu.edu(b) c-r-alpacas.com
(a) (b)
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(b)
Plate 3: (a) Housefly, Musca domestica (b) Human louse, Pediculus humanus
Source : (a) cals.ncsu.edu (b) http://www.nlm.nih.gov
(a)
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Plate 4: (a) Punkie, Culicoides sonorensis (b) Culex mosquito
Source : (a) cals.ncsu.edu(b) gizmag.com
(a) (b)
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Plate 5: (a) Aedes mosquito (b) female Anopheles mosquito
Source : (a) health.ezinemark.com(b) medimanage.com
(b) (a)
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Plate 6: (a) Sand fly, Phlebotomus sergenti (b) Tsetse fly, Glossina longipalpis
Source : (a) cals.ncsu.edu (b) cals.ncsu.edu
(b) (a)
Some conditions transmitted by Arthropod (Insect) vectors
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Plate 7: (a) Chikungunya rash (b) Leishmaniasis sore
Source : (a)mosquitozone.com(b)mosquitozone.com
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Plate 8: (a) Chagas disease symptom (b) African Sleeping Sickness
Source : (a)mosquitozone.com(b)mosquitozone.com
(b) (a)
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Plate 9: (a) Lymphatic Filariasis (Elephantiasis) (b) Horsefly bite
Source : (a)mosquitozone.com(b)mosquitozone.com
(b) (a)
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(a)
Plate 10: (a) Sand fly bite (b) Eye lesions transmitted by Black fly
Source : (a)mosquitozone.com(b)mosquitozone.com
(b)
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Plate 11: (a) Person suffering from malaria (b) Trypanosomes in Blood
Source : (a)mosquitozone.com(b)mosquitozone.com
(a) (b)
INSECT VECTOR CONTROL APPROACHESThe 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.
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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
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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.
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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
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 pupalstage) 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.
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Plate 12: Screw worm fly, Cochliomyia hominivorax
source: www-naweb.iaea.org
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
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Fig. 1: Diagram showing a cross between a male homozygous for the Lethal gene and a normal homozygous female
source : bioone.org
Plate 13: A genetically modified male insect carrying the female-specific “Dominant Lethal” gene with genetic markers that fluoresce its eyes
Source: mi2g.com27
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.
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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
Innovative chemicals (Insecticidal agents)
Natural compounds
• Natural insecticides, such as nicotine, pyrethrum, neemand 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 .
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Plate 15: Ryana speciosasource: naturekind.org
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Plate 16: Neem, Azadirachta indica
Source: nipahutgardens.com
Fig. 2: Chemical structure of some neonicotinoids
Source: uni-ulm.de
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.
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Hormonal Control
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Fig. 3: Hormonal Control of insects
Source: biology-forums.com
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.
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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 MeasuresBiological 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
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Fig. 4: Concept of Integrated Vector control
Source: helid.digicollection.org
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Plate 17: (a) LLIN (b) Mosquito fish (Gambusia affinis)
Source : (a) vectorcontrol.bayer.com(b) tropicalfishtanksonline.com
(a) (b)
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)
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Fig. 5: Current Tsetse fly distribution in Africa
Source: www.indiana.edu
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 Beauvariaspores 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
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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
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Plate 18: Spore of Entomopathogenic fungi, Beauvaria bassiana, encapsulating an insect.
source: microbeworld.org
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.
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