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Dr. Nik Ahmad Irwan Izzauddin Nik Him
      Room 4o3, nikirwan@usm.my




                                        1
 Introduction and important aspects of vectors in
  disease transmission – important concepts, terms and
  definitions
 Arthropod vectors – Mosquitoes as disease vectors-its
  role as important human disease vector for dengue,
  microfilaria and malaria




                                                          2
Introduction and important aspects of vectors
      in disease transmission – important
        concepts, terms and definitions




                                                3
 The most successful animal
  group
 Arthropods have huge impact on
  health of humans & domestic
  animals
   Irritation & diseases
 Relatively few species involved
  but serious social & economic
  consequences                                   Leishmaniasis
     Transmit diseases (vectors)
     Inject venoms & transmit allergens
     Cause wounds                         Myiasis
     Create nuisance & phobias
 Other arthropod groups also
  very important
                                                                 4
 Major insect orders
   Diptera (flies)
   Hemiptera (true bugs)
   Phthiraptera (lice)
   Siphonaptera (fleas)
 Diseases & causative
  pathogens
 Other arthropod groups
   Acari (ticks & mites)
   Araneae (spiders)
                             Protozoan causing
   Scorpiones (scorpions)   sleeping sickness
                                                 5
 Nuisance mostly related to high
  densities & not real hazards
   Justified in case of biting, venomous &
     filth-frequenting species
 Major causes of nuisance &
  irritation
   Blood-feeding species
   Lachrymal-feeders
   Immunological reactions
   Phobic responses (delusory parasitosis)
 Large industries are focused on pest
  control
                                              Mosquito feeding frenzy
                                                                    6
 Vector
  - Definition: Organism that are capable of transmitting other
  organisms that cause disease in vertebrate host


 Carrier, bearer


 In parasitology  An organism or vehicle that
  transmits the causative agent or disease-causing
  organism from reservoir to the host


                                                                  7
 Vectors generally don’t become “ill” from carrying their
  various viral, protozoan and nematode infections.
 They might accrue some damage to their tissues, but in
  some cases this “damage” actually makes them more likely
  to transmit and infect.
 A mosquito with problems in its feeding apparatus will
  need to take additional bites to complete a blood meal.
 A flea with a gut clogged with plague bacteria will
  regurgitate more.



                                                             8
9
10
 Primary vector
  - If they are proved to be transmitting a pathogen to man or other
    animal
 Secondary vector
 - If they play a supplementary role in transmission, but would be
 unable to maintain a disease in the absence of primary vector




                                                                       11
Case sample:
In the transmission of dengue fever in Malaysia

                           Aedes egypti is the main
                           vector…




                           and Aedes albopictus is
                           the secondary vector



                                                      12
 In instances where a disease of unknown cause is
 occurring, certain general characteristics help to
 identify its vector

 Establishment of the relationship of arthropod
 populations to transmission of a particular disease
 agent is called vector incrimination

 The process of knowing which species of arthropod is
 serving as a vector of a particular disease
                                                         13
• There are 4 major criteria for incriminating
arthropods as vectors of human disease




Barnett, H. C. 1956. The transmission of western equine encephalitis virus
by the mosquito Culex tarsalis Coq. Am. J. Trop. Med. Hyg. 5: 86-98.         14
 Identifying contact between arthropod and host
  - suck blood for meal  YES

 Having a biological association in time
  - the rate of cases higher in correlation to the higher number of Aedes
   YES

 Repeated demonstration of disease between arthropod and host
  - The infective stage have been found consistently in Aedes and they
  transferred it during feeding  YES

 Replicable under experimental conditions
  - Can be rearing in the lab and the and can be replicated  YES

                                                                            15
 The time that elapses between the invasion of a susceptible host
  by an infectious agent and the onset of symptoms of the disease
  caused by that agent is called the incubation period

 The length of the incubation period varies greatly

 Extrinsic Incubation period
  – a period in a vector during which the disease-producing
  organism/parasite increases or transforms to a point where it can
  be transmitted.

 Intrinsic incubation period
  – a period in the vertebrate host before disease is expressed
  clinically

                                                                      16
Case sample: Malaria




Extrinsic incubation period   Intrinsic incubation period17
 Vectorial capacity describes the potential of a
group of arthropods to transmit a given pathogen
 Vector effectiveness
 There are 6 main determinants:
  – Abundance
  – Host preference and host-feeding patterns
  – Reproductive capacity
  – Longevity
  – Dispersal
  – Vector competence


                                                    18
 The more vectors there are, the higher the probability
  of disease transmission
 because it has a direct bearing on the probability of
  vector-host contact




                                                           19
 Knowing what the vectors feed upon allows for
  identification of disease transmission
 Patterns of host feeding that are determined by
  identification of blood meals in vectors are usually the
  end result of many factors, including host preference
  and host availability.




                                                             20
 A measure of the rate at which a population of vectors
  increases
 Fecundity is a related term which relates to the
  number of generations, broods, or litters produced per
  unit of time.
 The net reproductive rate of a population of vectors is
  a combination of fecundity and survival.
 Survival is influenced by various mortality factors,
  including predation and diseases, as well as accidents
  and natural aging
                                                            21
 Longevity is knowing how long the stages of the
  vector’s life cycle last

 A vector must feed more than once to transmit
  pathogens

 It is essential that a vector live a sufficient period of
  time

 Maximal longevity will permits vectors to serve as
  essential parts of the reservoir of infection.
                                                              22
 Dispersal is knowing how far a vector can fly or move about freely
         – The greater the movement, the greater chance for spread of disease

 Vectors that can move freely and for long distances will have greater
    chances for contact with humans, and will be more likely to move
    between infected and noninfected hosts.

 Superior mobility aids in the dissemination of pathogens over a
    wide area, so that their associated diseases are not limited or focal
    in nature.
   Flying vectors (e.g. mosquitoes, flies) generally make a good vector;
    however crawling vectors (e.g. fleas, lice, mites and ticks) are
    distributed by the relative mobility of their hosts.



                                                                                23
 The susceptibility of a group of arthropods to a given
 strain of pathogen and the ability of those arthropods
 to transmit the pathogen

 These traits are under genetic control, and although
 infection and transmission in vectors will vary with
 temperature, vector competence is considered to be an
 innate characteristic for a particular vector for a given
 microorganism


                                                             24
 Mechanical transmission
 Biological transmission
 - Cyclodevelopment transmission
 - Propagative transmission
 - Cyclopropagative transmission




                                   25
 This is when the pathogen adheres to body hairs,
 spines, sticky pads, or other structures of insects
 – In the case of certain insects, transmission may be by regurgitation or
 defecation
 - Biting flies may transmit pathogens by biting with contaminated
 mouthparts


 Nearly all mechanically transmitted diseases can also
  be transmitted in other ways (e.g., contaminated food
  and water)
 e.g. houseflies, cockroaches
                                                                             26
 The parasites undergo several molts in the body of the
 vector

 No multiplication takes place within the body of the
 vector

 The only pathogens that are transmitted this way are
 filarial nematodes
 - Start out as a microfilariae and develops into an infectious larvae


 Also known as Cyclical transmission

                                                                         27
28
 In this type of transmission, the pathogen multiplies
  within the body of the vector, but does not undergo
  any changes in form

 Most viral diseases fall into this category


 Plague (a bacterial disease) is also an example


 Any stage of these pathogens can infect a vertebrate
  host
                                                          29
 Both multiplication and changes in the life form of the
 pathogen occur within the vector

 Examples are malaria, leishmaniasis, both caused by
 protozoan parasites

 Also known as Propagative and Cyclical transmission




                                                            30
31
 Horizontal and vertical transmission describe the
  pathway a pathogen takes among vectors and hosts.
 Horizontal transmission
  - Horizontal transmission involves the pathogen being transmitted by
  a vector to a host in a cyclical pathway


 Ventricle transmission
  - Vertical transmission is more direct and does not involve a host, but
  occurs directly from infected mother/female to offspring
 – This is also called transovarial transmission


                                                                            32
33
 Dead end host
  – a vertebrate that harbors the pathogen and is severely
  affected by it, yet the level of pathogen in its body is too
  low for blood sucking vector to become infective after
  feeding on the host.

 Amplifying host
  – a vertebrate that has high level of pathogen that a feeding
  vector will likely become infectious.

 Silent host
  – one that harbors the pathogen, but shows no obvious
  signs of disease.

                                                                  34
 Resistant host
 – one that is not naturally affected by a pathogen.

 Partially resistant host
 – one that harbors the pathogens for a long period
 before being overcome by it.

 Susceptible host
 – a victim of the pathogen
                                                       35
 Reservoir refer to any vector or host, capable of
 maintaining a pathogen for considerable
 periods of time.
 Usually, they show no evidence of serious disease,
 or a reservoir of infection may be maintained by
 continuous transmission among a group of
 severely affected animals.
 Most vectors are short-lived and would not serve
 as important components of reservoirs, except
 possibly through aestivation or hibernation.
                                                       36
 At the end of the nineteenth century, it was discovered that certain
  species of insects, other arthropods and freshwater snails were
  responsible for the transmission of some important diseases.

 The discovery of the insecticide dichlorodiphenyltrichloroethane
  (DDT) in the 1940s was a major breakthrough in the control of vector-
  borne diseases.

 In 1950s and early 1960s -->

  -To eradicate the diseases or to reduce transmission to such low level that control
  could be maintained through the general health care facilities without the need of
  control measures.

 Problem  resistance
               expensive
                                                                                        37
 Vector control methods suitable for community
 involvement should:
— be effective;
— be affordable;
— use equipment and materials that can be obtained
 locally;
— be simple to understand and apply;
— be acceptable and compatible with local customs,
 attitudes and beliefs;
— be safe to the user and the environment.
                                                     38
 Before starting any vector control activity, it is
  important to ask two questions:
 What result do you want to achieve: merely to protect
  yourself or your family from biting pests and the
  diseases they carry, or to reduce disease in the
  community?
 Are the health authorities already carrying out control
  measures and do you want to provide the community
  or your family with additional protection from disease?


                                                            39
 Important factors to take into consideration
 - Biological factors
 - Availability of technical tools
 - Socio-economic considerations




                                                 40
 Species affected
 Density of susceptible species
 Wildlife reservoir
 Vector transmission
 Transmissibility
 Current extent of disease
 Survival in the environment
 Carrier state
 Ease of clinical recognition



                                   41
 Diagnostic tests
 Vaccines
 Treatment
 Effectiveness of isolation/ quarantine
 Disinfection




                                           42
 Cost and benefits of intervention
 Ease of implementation
 Stake holder engagement
 Political will




                                      43
 2 control methods
1. Self-protection
2. Community control




                       44
 Self-protection measures are used to protect yourself, your
  family or a small group of people living or working together
  from insect pests or vectors of disease.
 These measures include personal protection,
1. the prevention of contact between the human body and
    the disease vector,
2. and environmental measures to prevent pests and vectors
    from entering, finding shelter in, or breeding in or
    around your house.
 These measures are usually simple and inexpensive, and
  can often be adopted without help from specialized health
  workers.

                                                                 45
   Example:
      Repellents
      Protective clothing
      Insecticide vaporizers
      Mosquito nets
      Insecticides
     - Insecticide spraying of walls
     - Space-spraying with insecticides
     - Treating fabrics with an insecticide

     Anti-mosquito screening
     Prevention of breeding
      - Source reduction
      - Biological control
      - Larvicides
      - Insect growth regulator


                                              46
 The type of control may be the same as for the
  protection of an individual or a family, but is, of
  course, larger.
 Before investing resources in community-wide control
  efforts, advice should be obtained from health workers
  on the type of measures most likely to be successful
  under local conditions.
 Before investing resources in community-wide control
  efforts, advice should be obtained from health workers
  on the type of measures most likely to be successful
  under local conditions.

                                                           47
 Many factors need to be taken into account:
1. the vector species and its behaviour,
2. the compatibility of control methods with the local
   culture,
3. affordability in the long term, the need for expert
   advice




                                                         48
49
50

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8 Vectors intro

  • 1. Dr. Nik Ahmad Irwan Izzauddin Nik Him Room 4o3, nikirwan@usm.my 1
  • 2.  Introduction and important aspects of vectors in disease transmission – important concepts, terms and definitions  Arthropod vectors – Mosquitoes as disease vectors-its role as important human disease vector for dengue, microfilaria and malaria 2
  • 3. Introduction and important aspects of vectors in disease transmission – important concepts, terms and definitions 3
  • 4.  The most successful animal group  Arthropods have huge impact on health of humans & domestic animals  Irritation & diseases  Relatively few species involved but serious social & economic consequences Leishmaniasis  Transmit diseases (vectors)  Inject venoms & transmit allergens  Cause wounds Myiasis  Create nuisance & phobias  Other arthropod groups also very important 4
  • 5.  Major insect orders  Diptera (flies)  Hemiptera (true bugs)  Phthiraptera (lice)  Siphonaptera (fleas)  Diseases & causative pathogens  Other arthropod groups  Acari (ticks & mites)  Araneae (spiders) Protozoan causing  Scorpiones (scorpions) sleeping sickness 5
  • 6.  Nuisance mostly related to high densities & not real hazards  Justified in case of biting, venomous & filth-frequenting species  Major causes of nuisance & irritation  Blood-feeding species  Lachrymal-feeders  Immunological reactions  Phobic responses (delusory parasitosis)  Large industries are focused on pest control Mosquito feeding frenzy 6
  • 7.  Vector - Definition: Organism that are capable of transmitting other organisms that cause disease in vertebrate host  Carrier, bearer  In parasitology  An organism or vehicle that transmits the causative agent or disease-causing organism from reservoir to the host 7
  • 8.  Vectors generally don’t become “ill” from carrying their various viral, protozoan and nematode infections.  They might accrue some damage to their tissues, but in some cases this “damage” actually makes them more likely to transmit and infect.  A mosquito with problems in its feeding apparatus will need to take additional bites to complete a blood meal.  A flea with a gut clogged with plague bacteria will regurgitate more. 8
  • 9. 9
  • 10. 10
  • 11.  Primary vector - If they are proved to be transmitting a pathogen to man or other animal  Secondary vector - If they play a supplementary role in transmission, but would be unable to maintain a disease in the absence of primary vector 11
  • 12. Case sample: In the transmission of dengue fever in Malaysia Aedes egypti is the main vector… and Aedes albopictus is the secondary vector 12
  • 13.  In instances where a disease of unknown cause is occurring, certain general characteristics help to identify its vector  Establishment of the relationship of arthropod populations to transmission of a particular disease agent is called vector incrimination  The process of knowing which species of arthropod is serving as a vector of a particular disease 13
  • 14. • There are 4 major criteria for incriminating arthropods as vectors of human disease Barnett, H. C. 1956. The transmission of western equine encephalitis virus by the mosquito Culex tarsalis Coq. Am. J. Trop. Med. Hyg. 5: 86-98. 14
  • 15.  Identifying contact between arthropod and host - suck blood for meal  YES  Having a biological association in time - the rate of cases higher in correlation to the higher number of Aedes  YES  Repeated demonstration of disease between arthropod and host - The infective stage have been found consistently in Aedes and they transferred it during feeding  YES  Replicable under experimental conditions - Can be rearing in the lab and the and can be replicated  YES 15
  • 16.  The time that elapses between the invasion of a susceptible host by an infectious agent and the onset of symptoms of the disease caused by that agent is called the incubation period  The length of the incubation period varies greatly  Extrinsic Incubation period – a period in a vector during which the disease-producing organism/parasite increases or transforms to a point where it can be transmitted.  Intrinsic incubation period – a period in the vertebrate host before disease is expressed clinically 16
  • 17. Case sample: Malaria Extrinsic incubation period Intrinsic incubation period17
  • 18.  Vectorial capacity describes the potential of a group of arthropods to transmit a given pathogen  Vector effectiveness  There are 6 main determinants: – Abundance – Host preference and host-feeding patterns – Reproductive capacity – Longevity – Dispersal – Vector competence 18
  • 19.  The more vectors there are, the higher the probability of disease transmission  because it has a direct bearing on the probability of vector-host contact 19
  • 20.  Knowing what the vectors feed upon allows for identification of disease transmission  Patterns of host feeding that are determined by identification of blood meals in vectors are usually the end result of many factors, including host preference and host availability. 20
  • 21.  A measure of the rate at which a population of vectors increases  Fecundity is a related term which relates to the number of generations, broods, or litters produced per unit of time.  The net reproductive rate of a population of vectors is a combination of fecundity and survival.  Survival is influenced by various mortality factors, including predation and diseases, as well as accidents and natural aging 21
  • 22.  Longevity is knowing how long the stages of the vector’s life cycle last  A vector must feed more than once to transmit pathogens  It is essential that a vector live a sufficient period of time  Maximal longevity will permits vectors to serve as essential parts of the reservoir of infection. 22
  • 23.  Dispersal is knowing how far a vector can fly or move about freely – The greater the movement, the greater chance for spread of disease  Vectors that can move freely and for long distances will have greater chances for contact with humans, and will be more likely to move between infected and noninfected hosts.  Superior mobility aids in the dissemination of pathogens over a wide area, so that their associated diseases are not limited or focal in nature.  Flying vectors (e.g. mosquitoes, flies) generally make a good vector; however crawling vectors (e.g. fleas, lice, mites and ticks) are distributed by the relative mobility of their hosts. 23
  • 24.  The susceptibility of a group of arthropods to a given strain of pathogen and the ability of those arthropods to transmit the pathogen  These traits are under genetic control, and although infection and transmission in vectors will vary with temperature, vector competence is considered to be an innate characteristic for a particular vector for a given microorganism 24
  • 25.  Mechanical transmission  Biological transmission - Cyclodevelopment transmission - Propagative transmission - Cyclopropagative transmission 25
  • 26.  This is when the pathogen adheres to body hairs, spines, sticky pads, or other structures of insects – In the case of certain insects, transmission may be by regurgitation or defecation - Biting flies may transmit pathogens by biting with contaminated mouthparts  Nearly all mechanically transmitted diseases can also be transmitted in other ways (e.g., contaminated food and water)  e.g. houseflies, cockroaches 26
  • 27.  The parasites undergo several molts in the body of the vector  No multiplication takes place within the body of the vector  The only pathogens that are transmitted this way are filarial nematodes - Start out as a microfilariae and develops into an infectious larvae  Also known as Cyclical transmission 27
  • 28. 28
  • 29.  In this type of transmission, the pathogen multiplies within the body of the vector, but does not undergo any changes in form  Most viral diseases fall into this category  Plague (a bacterial disease) is also an example  Any stage of these pathogens can infect a vertebrate host 29
  • 30.  Both multiplication and changes in the life form of the pathogen occur within the vector  Examples are malaria, leishmaniasis, both caused by protozoan parasites  Also known as Propagative and Cyclical transmission 30
  • 31. 31
  • 32.  Horizontal and vertical transmission describe the pathway a pathogen takes among vectors and hosts.  Horizontal transmission - Horizontal transmission involves the pathogen being transmitted by a vector to a host in a cyclical pathway  Ventricle transmission - Vertical transmission is more direct and does not involve a host, but occurs directly from infected mother/female to offspring  – This is also called transovarial transmission 32
  • 33. 33
  • 34.  Dead end host – a vertebrate that harbors the pathogen and is severely affected by it, yet the level of pathogen in its body is too low for blood sucking vector to become infective after feeding on the host.  Amplifying host – a vertebrate that has high level of pathogen that a feeding vector will likely become infectious.  Silent host – one that harbors the pathogen, but shows no obvious signs of disease. 34
  • 35.  Resistant host – one that is not naturally affected by a pathogen.  Partially resistant host – one that harbors the pathogens for a long period before being overcome by it.  Susceptible host – a victim of the pathogen 35
  • 36.  Reservoir refer to any vector or host, capable of maintaining a pathogen for considerable periods of time.  Usually, they show no evidence of serious disease, or a reservoir of infection may be maintained by continuous transmission among a group of severely affected animals.  Most vectors are short-lived and would not serve as important components of reservoirs, except possibly through aestivation or hibernation. 36
  • 37.  At the end of the nineteenth century, it was discovered that certain species of insects, other arthropods and freshwater snails were responsible for the transmission of some important diseases.  The discovery of the insecticide dichlorodiphenyltrichloroethane (DDT) in the 1940s was a major breakthrough in the control of vector- borne diseases.  In 1950s and early 1960s --> -To eradicate the diseases or to reduce transmission to such low level that control could be maintained through the general health care facilities without the need of control measures.  Problem  resistance  expensive 37
  • 38.  Vector control methods suitable for community involvement should: — be effective; — be affordable; — use equipment and materials that can be obtained locally; — be simple to understand and apply; — be acceptable and compatible with local customs, attitudes and beliefs; — be safe to the user and the environment. 38
  • 39.  Before starting any vector control activity, it is important to ask two questions:  What result do you want to achieve: merely to protect yourself or your family from biting pests and the diseases they carry, or to reduce disease in the community?  Are the health authorities already carrying out control measures and do you want to provide the community or your family with additional protection from disease? 39
  • 40.  Important factors to take into consideration - Biological factors - Availability of technical tools - Socio-economic considerations 40
  • 41.  Species affected  Density of susceptible species  Wildlife reservoir  Vector transmission  Transmissibility  Current extent of disease  Survival in the environment  Carrier state  Ease of clinical recognition 41
  • 42.  Diagnostic tests  Vaccines  Treatment  Effectiveness of isolation/ quarantine  Disinfection 42
  • 43.  Cost and benefits of intervention  Ease of implementation  Stake holder engagement  Political will 43
  • 44.  2 control methods 1. Self-protection 2. Community control 44
  • 45.  Self-protection measures are used to protect yourself, your family or a small group of people living or working together from insect pests or vectors of disease.  These measures include personal protection, 1. the prevention of contact between the human body and the disease vector, 2. and environmental measures to prevent pests and vectors from entering, finding shelter in, or breeding in or around your house.  These measures are usually simple and inexpensive, and can often be adopted without help from specialized health workers. 45
  • 46. Example:  Repellents  Protective clothing  Insecticide vaporizers  Mosquito nets  Insecticides - Insecticide spraying of walls - Space-spraying with insecticides - Treating fabrics with an insecticide  Anti-mosquito screening  Prevention of breeding - Source reduction - Biological control - Larvicides - Insect growth regulator 46
  • 47.  The type of control may be the same as for the protection of an individual or a family, but is, of course, larger.  Before investing resources in community-wide control efforts, advice should be obtained from health workers on the type of measures most likely to be successful under local conditions.  Before investing resources in community-wide control efforts, advice should be obtained from health workers on the type of measures most likely to be successful under local conditions. 47
  • 48.  Many factors need to be taken into account: 1. the vector species and its behaviour, 2. the compatibility of control methods with the local culture, 3. affordability in the long term, the need for expert advice 48
  • 49. 49
  • 50. 50