Vector biology

1. 1. Class Insecta:-
 Contains the majority of medically important species
 Mosquitoes, tsetse flies, sand flies, black flies, etc.
2. Class Arachnida:-
 ticks, mites, etc.
3. Class Crustaceans:
 copepods, shrimps, crabs, lobsters, water fleas, crayfish, etc.
PHYLUM ARTHROPODA
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Medically important Classes of Phylum Arthropoda:

2.  Some medically important orders of class insecta
include:
Order Diptera
Order Heteroptera
Order Phthiraptera (reading assignment)
Order Siphonaptera (reading assignment)
Class Insecta
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3.  Medically important families of order Diptera include:
 Family Culicidae,
 Family Simulidae,
 Family Psychodidae,
 Family Glossinidae ,
Class insecta
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Order Diptera

4. Some Medically important Genus of family Culicidae include:
 Aedes
 Aedes aegypti : vector for yellow fever & dengue viruses,
 Culex
 Culex quinquefasciatus: vector of filarial worms & arboviruses,
 Anopheles
 Anopheles mosquitoes: important vectors of malaria,
Class insecta . . . Diptera . . .
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Family Culicidae: The mosquitoes

5. Life cycle: Complete metamorphosis:
Egg --- larva ---- pupa ---- adult
Class insecta . . . Diptera . . .
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Family Culicidae: The mosquitoes

6. Class insecta . . . Diptera . . .
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Culicidae: The mosquitoes

7. Diptera . . . Culicidae . . .
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Morphology
Aedes aegypti

8. Aedes aegypti
 The main vector for yellow fever and Dengue virus,
 Maintains, across generations, & transmits yellow fever virus
 Vector of the four serotypes of Dengue virus (DENV1-4),
 There are ~50–100 million DENV infections each year,
 The mosquito populations exhibit a large amount of genetic
variation in their ability to become infected with, propagate,
and eventually transmit DENV1–4.
Diptera . . . Culicidae . . .
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9. Diptera . . . Culicidae . . .
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Culex quinquefasciatus

10. The main vector of filarial worms
Wuchereria bancrofti
 Endemic in 83 countries,
 1.2 billion people are at risk of infection,
 Around 120 million people are infected,
Transmission of filarial worms:
The bite of female mosquitoes,
L3 deposited on human skin during blood meal,
 L3 enter into human body via bite puncture wound,
Diptera . . . Culicidae . . .
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Culex quinquefasciatus

11.  Mosquitoes ingest L1 during the blood meal,
 L1 penetrates the mosquito’s stomach,
 Enter the body cavity (hemocoel),
 Migrate to flight muscles for growth,
 After 2 molts, L1 develops to L3,
 L 3 migrate to the mosquito’s head,
 L 3 (infective stage) reach the proboscis,
 L3 infects a man at the next blood meal,
Diptera . . . Culicidae . . .
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Culex quinquefasciatus
Transmission of filarial worm to a mosquito vector:

12. Diptera . . . Culicidae . . .
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13.  Are vectors of malaria
Malaria:
 Major health problem in 100 countries,
 Active transmission in 97 countries,
 1.2 billion are at high risk,
 ≈ 216 million global incidences,
 A remarkable decrease in cases and
deaths currently,
Diptera . . . Culicidae . . .
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Anopheles mosquitoes

14. Diptera . . . Culicidae . . .
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15. Diptera . . . Culicidae . . .
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Malaria vectorial system
Around 40 species of Anopheles
mosquitoes are vectors of malaria,
Malaria transmission depends on:
 The mosquitoes:-
 Occurrence,
 Feeding preference,
 Susceptibility to infection,
 Socioeconomic & environmental factors,
 Availability of matured gametocytes.

16. Diptera . . . Culicidae . . .
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Malaria Life cycle; vector part

17. Diptera . . . Culicidae . . .
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Pathogen development in Anopheles mosquitoes
Developmental sites include:
 mid-gut,
 Malpighian tubules,
 thoracic musculature, and
 salivary glands,
• These sites are appropriate
environments to initiate parasite
gene expression,
• Permit post-translational
modifications of previously
expressed gene products that
are required for development.

18. Diptera . . . Culicidae . . .
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Migratory routes & developmental sites within the
mosquito for malaria parasites and filarial worms

19. Diptera . . . Culicidae . . .
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Developmental sites and migratory routes
Malaria parasites:
 Infected blood meal (A),
 Enter into the mid-gut (B),
 Escape the peritrophic matrix (C),
 Replicate in the mid-gut epithelial cells (D),
 Travel through the haemocoel (E),
 Replicate & reside in the salivary glands (H),

20. Diptera . . . Culicidae . . .
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Developmental sites and migratory routes
The filarial worms:
 Infected blood meal (A),
 Enter into the mid-gut (B),
 Escape the peritrophic matrix (C),
 Penetrate the mid-gut epithelium (D),
 Develop in the thoracic musculature (G),
 L3 break out & enter the haemocoel (E),
 L3 migrate to the head region,
 L3 actively emerge from the head region,

21. Diptera . . . Culicidae . . .
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Migratory steps pose potential barriers:
 Blood coagulation inhibit pathogens movement,
 The proteolytic enzymes, secreted into the lumen
for blood digestion, kill parasites,
 The pharyngeal armature can cause physical damage
to microfilariae (~250-300mm),
 Blood feeding initiates the formation of peritrophic matrix,
which eventually surrounds the blood meal and physically
separates it from the mid gut epithelium.
 The mosquitoes innate immune system kills the parasites,

22. Diptera . . . Culicidae . . .
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In Americas:
 An. albimanus,
 An. darlingi,
In South America:
 An. darlingi,
 An. aquasalis,
 An. albitarsis,
 An. bellator,
 An. cruzii,
In Asia:
An. sacharovi,
An. superpictus,
An. stephensi,
An. arabiensis,
An. culicifacies
Malaria vectorial system . . .

23. Diptera . . . Culicidae . . .
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Malaria vectorial system . . .

24. Diptera . . . Culicidae . . .
 Sub Sahara
 Principal vectors:-
 An. gambiae s.s.,
 An. arabiensis,
 Secondary vectors:-
 An. pharoensis,
 An. funestus,
 An. rivurolum,
 An. nili
 In Ethiopia
 Principal vector:-
 An. arabiensis,
 Secondary vector:-
 An. pharoensis,
 An. funestus,
 An. nili,
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Malaria vectorial system . . .

25. VECTOR CONTROL
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Malaria control would be achieved by:-
 Preventing humans from the bit of vector,
 Selectively destroying vector species,
 Clearing gametocytes in all carriers,
 Previous control efforts led to the rapid
selection of insecticide-resistant strains
of mosquitoes,

26. VECTOR CONTROL . . .
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 Measures directed against vectors designed to:
 Limit reproduction of vectors,
 Reduce the longevity of vectors,
 Minimize vector-human contact,
 Wider mechanisms have been used,
 Biological agents,
 Bacterial toxins,
 Insect growth regulators,
 Botanical repellents,
 Immunological,
 Chemical insecticides,

27. VECTOR CONTROL . . .
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 Four groups have been used extensively:-
 Organochlorine (DDT),
 Organophosphates,
 Carbamates,
 Pyrethroids,
 Organochlorine (DDT) for agriculture:
 Introduced in 1934,
 Banned in 1983,
 DDT in malaria control:
 Launched in 1949 as IRS,
 Completely removed in 2000,
CHEMICAL INSECTICIDES

28. VECTOR CONTROL . . .
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Organophosphates as IRS:
 Malathion,
 Fenitrothion,
 Pirimiphos-methyl,
Pyrethroids as LLINs,
 Most popular in malaria
control due to:
 Low cost
 Low mammalian toxicity,
 Higher efficiency to
invertebrates,
 Permethrin,
 Deltamethrin,
 Cypermethrin
 Cyfluthrin,
 Etofenprox,
 Lambda-cyhalothrin
Synthetic Pyrethroids:
CHEMICAL INSECTICIDES . . .

29. VECTOR CONTROL . . .
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 Have their own mode and site of action,
 Active molecules:-
 Quickly infiltrates through the integument,
 Reach the site of action,
 Bind with the action sites such as:-
 vital enzyme,
 nerve tissue,
 receptor proteins,
 kill insects at the threshold concentration,
MODE OF ACTION OF INSECTICIDES

30. VECTOR CONTROL . . .
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 Properties of chemical insecticides:-
 Lethal,
 Sub-lethal,
 Excitation/repellent,
 Insecticides efficiency is affected by:-
 Insecticide resistance,
 Inadequate resources,
 Poor quality,
 Operational failure:
• overuse or misuse,
MODE OF ACTION OF INSECTICIDES . . .

31. INSECTICIDE RESISTANCE
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 It is a decrease in sensitivity to a given chemical:-
The resistance should be
 Statistically defined,
 Heritable to the progeny,
 Evaluated relative to the susceptible population,
 Insecticide resistance in insects occur due to
one or the combined effect of the following:-
1. The physiological resistance,
2. Behavioral resistance,
3. Reduced toxicant uptake:-
 Due to altered cuticle,
 Selection of resistant forms,
 Evolution of resistant populations,
For a given insect population, insecticide resistance means:

32. INSECTICIDE RESISTANCE . . .
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I. Metabolic resistance:
 Modification of the three enzyme groups:
i. Cytochrome P450 Mono-oxygenases (CYP450s),
ii. Carboxyl/cholinesterases,
iii. Glutathione-S-transferases (GSTs),
• Results in over production of detoxifying enzymes,
II. Target site modification:
 Mutational changes in neuronal VGSC,
(e.g., kdr mutation),
 In these case insecticides no longer bind to target sits,
1. THE PHYSIOLOGICAL RESISTANCE

33. INSECTICIDE RESISTANCE . . .
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Example: Pyrethroid resistance have been developed
in An. gambiae population b/s of two mechanisms
1. Metabolic resistance:
 Due to elevated level of detoxifying enzymes,
2. Altered target site (kdr):
 Two point mutations at VGSC,
∆ leucine to phenylalanine (L1014F), W. African type,
∆ leucine to serine (L1014S), East African type,
PHYSIOLOGICAL RESISTANCE . . .

34. INSECTICIDE RESISTANCE . . .
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2. THE BEHAVIORAL RESISTANCE
 Insects move away from insecticide-treated areas,
 Often without lethal consequences,
 Changes the insects customary behavior,
 Movement of insects is of two types:-
 Direct contact or excitation (irritancy),
 after making physical (tarsal) contact,
 Non-contact or spatial repellency,
 without making physical contact,
 An. gambiae s.s. ,for example, is changing its behavior in Africa:
 Its endophilic nature is changing to exophilic,
 shifting from human to animal feeding pattern,

35. INSECTICIDE RESISTANCE . . .
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3. Reduced toxicant uptake

36. INSECTICIDE RESISTANCE . . .
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I ) Biotic factors:-
 Rapid reproduction of insects,
 The inherent nature of insect species,
 Abundant progeny,
 Migration of insects & wide host range,
II ) Abiotic factors:-
 Dosage of insecticides,
 Period and frequency of applications,
 Insecticides with similar mode of action,
Factors for resistance development (IRD)

37. INSECTICIDE RESISTANCE . . .
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 Makes insecticide use ineffective,
 Limits the available vector control options,
 In most cases, resistance confers cross-resistance,
 i.e. Resistance to one insecticide confers resistance to
another insecticide,
 Previous selection with an insecticide can confer
resistance to new materials, even without the pre-
exposure to the new one,
IMPACT OF INSECTICIDE RESISTANCE

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