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Contemporary Methods of insect-vector control


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

Published in: Health & Medicine

Contemporary Methods of insect-vector control

  3. 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. 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. 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. 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:
  7. 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: Table 1 contd.
  8. 8. Images of some insect vectors 8 (a) (b) Plate 1: a) Assassin bug, Triatoma infestans (b) Black fly, Simulium damnosum Source : (a) (b)
  9. 9. 9 (a) (b) Plate 2: (a) Eye gnat, Hippelates pallipes (b) Horsefly, Tabanus sulcifrons Source : (a) (b)
  10. 10. 10 (b) Plate 3: (a) Housefly, Musca domestica (b) Human louse, Pediculus humanus Source : (a) (b) (a)
  11. 11. 11 (a) (b) Plate 4: (a) Punkie, Culicoides sonorensis (b) Culex mosquito Source : (a) (b)
  12. 12. 12 (a) (b) Plate 5: (a) Aedes mosquito (b) female Anopheles mosquito Source : (a) (b)
  13. 13. 13 (a) (b) Plate 6: (a) Sand fly, Phlebotomus sergenti (b) Tsetse fly, Glossina longipalpis Source : (a) (b)
  14. 14. Some conditions transmitted by Arthropod (Insect) vectors 14 Plate 7: (a) Chikungunya rash (b) Leishmaniasis sore Source : (a) (b)
  15. 15. 15 (a) (b) Plate 8: (a) Chagas disease symptom (b) African Sleeping Sickness Source : (a) (b)
  16. 16. 16 (a) (b) Plate 9: (a) Lymphatic Filariasis (Elephantiasis) (b) Horsefly bite Source : (a) (b)
  17. 17. 17 (a) (b) Plate 10: (a) Sand fly bite (b) Eye lesions transmitted by Black fly Source : (a) (b)
  18. 18. 18 (a) (b) Plate 11: (a) Person suffering from malaria (b) Trypanosomes in Blood Source : (a) (b)
  19. 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. 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. 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. 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. 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. 24 Plate 12: Screw worm fly, Cochliomyia hominivorax source:
  25. 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. 26 Fig. 1: Diagram showing a cross between a male homozygous for the Lethal gene and a normal homozygous female source :
  27. 27. Plate 13: A genetically modified male insect carrying the female-specific “Dominant Lethal” gene with genetic markers that fluoresce its eyes Source: 27
  28. 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. 29 Plate: (a) (b) Plate 14: (a) Galleria mellonella densovirus (b) Wolbachia pipientis (red) in insect testes (green) Source : (a) (b)
  30. 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:
  31. 31. 31 Plate 16: Neem, Azadirachta indica Source: Fig. 2: Chemical structure of some neonicotinoids Source:
  32. 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. 33. Hormonal Control 33 Fig. 3: Hormonal Control of insects Source:
  34. 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. 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. 36 Fig. 4: Concept of Integrated Vector control Source:
  37. 37. 37 Plate 17: (a) LLIN (b) Mosquito fish (Gambusia affinis) Source : (a) (b) (a) (b)
  38. 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. 39 Fig. 5: Current Tsetse fly distribution in Africa Source:
  40. 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. 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:
  42. 42. 42 Plate 18: Spore of Entomopathogenic fungi, Beauvaria bassiana, encapsulating an insect. source:
  43. 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. 44