field control of insect vectors

Question 1

aims of athropod control strategies

Answer 1

• reduce disease-causing vector population
• repel the vector
• form a barrier between vector and potential host
• reduce lifespan of vector and potentially infected vector
• with the public healht goal of:
  
  • reduced outbreaks, deaths, illness
  • protection of population subgroups with reduced immunity

Question 2

factors to consider in vector control

Answer 2

• transmission chain
• local vector habits and habitat
• vector number
• existing measures
• target epidemiological endpoint
  
  • eradicate vector/disease? reduced mortality/morbidity?

Question 3

malaria in the UK

Answer 3

• putatively indigenous
• no more transmission
• swamp draining → reduced A. atroparvus density
• better ventilation and accommodation
  
  • change in biting preference
• quinine use in 19thC reducing human reservoirs
• fall in chinchon aprice
  
  • source of quinine

Question 4

choice of control method

Answer 4

• consider local behavioural traits
  
  • time of feeding
  • location of feeding
  • adult resting location
  • heat preference in hosts
  • breeding site location

Question 5

A. gambiae ss

Answer 5

• larval habitat
  
  • clean shallow freshwater
  • temporary pools
• blood feeding
  
  • indoors at night
  • highly anthropophagic
  • P. falciparum vector

Question 6

A. arabiensis

Answer 6

• larval habitat
  
  • sunny clean pools, little vegetation
  • arid savannas
• blood feeding
  
  • prefers mammals
  • exophilic/phagic
  • falciparum and vivax

Question 7

A. stephensi

Answer 7

• larval habitat
  
  • urban breeding in man-made vectors
    
    • water tanks
  • fresh/brackish/polluted water
• blood feeding
  
  • highly anthropophagic
  • endophagic and exophagic
  • endophilic

Question 8

studying feeding and resting

Answer 8

• human bait traps
  
  • aspirator when sat on human
  • no use anymore
• UV light traps now

Question 9

studying host preference

Answer 9

• light traps
• analysis of bloodmeal to identify blood source from antigens

Question 10

identifying breeding sites

Answer 10

• dipping
  
  • someone stands in water to identify larvae presence

Question 11

aspects of traditional mosquito control

Answer 11

• insecticides
  
  • chemical control
• biological control
  
  • predators/competitors introduced
  • can be slow
• eliminate breeding sites

Question 12

insecticide treatment

Answer 12

• IRS
• ITNs
• space spraying
  
  • large otudoor environment
    
    • insecticide baits and traps

Question 13

IRS

Answer 13

• prevent disease transmission
• kill or repel
• porous walls
• 80% of homes sprayed
• anopheles, aedes, triatomines

Question 14

ITNs

Answer 14

• prevent entry and biting
• also protects others in the room
• anopheles and tsetse

Question 15

IRS vs ITN

Answer 15

• IRS
  
  • unstable/epidemic malaria
  • rapid response
  • demanding on logistics/planning/infrastructure
  • difficult to scale up
• ITNs
  
  • target of at risk individuals
  • less demanding
  • community protection if scaled up

Question 16

space spraying

Answer 16

• target urban or epidemic areas
• expensive and damaging
• precisely defined target area
• organophosphates/pyrethrids
• anopheles, aedes, culex, sandflies

Question 17

insecticide baits/traps

Answer 17

• attract and killa dults
• tsetse, musca domestica, other flies
• 99.9% reduction inivory coast/uganda villages with permethrin

Question 18

larval control

Answer 18

• identify breeding sites in adequate numbers
• chemical, biological, environmental control

Question 19

chemical control of larvae

Answer 19

• larvicides are low toxicity and safe in drinking water
• resistance, washing away
• anopheles, aedes, culex, blakc fly, musca

Question 20

biological larval control

Answer 20

• predation, parasitism, infection of target species
• reduces population
• active human management
• fish, toxins, wolbachia

Question 21

biological control with fish

Answer 21

• contorl how far they move
• rice paddies
• mosquito fish, wild guppies, green carp
• consume vegetation to reduce larval sites
• attract predatory birds
• locals use as food source
• reduction in other aquatic species

Question 22

control with bacterial spores/toxins

Answer 22

• BT toxin
  
  • selectively lethal in mosquito and black fly larvae
  • directly breaches cuticle to get to haemocoel

Question 23

wolbachia control

Answer 23

• wolbachia = intracellular bacterium transmitted via insect eggs
• natural occurence in 60% of insect species
• engineer into dengue/malaria mosquitoes
• cytoplasmic incompatibility allows spread
• some wolbachia strains provide pathogen resistance e.g. plamsodium resistance

Question 24

cytoplasmic incompatibility

Answer 24

• wolbachia infected insects can only mate with toher ifnected offspring
  
  • produce infected offspring
  • also infected if uninfected male mates with infected female
• sterile offspring if uninfected female mates with infected male
• removal of uninfected females from the population
• spread of infection

Question 25

environmental control of larvae

Answer 25

• longer lasting
• easier in urban areas
  
  • depends on breeding site identification
• results in source reduction
  
  • best strategy
• can exploit mate seeking behaviour if breeding sites are inaccessible
  
  • genetic control

Question 26

genetic control

Answer 26

• aim to suppress or replace infected population
• suppression
  
  • reduced ability to reproduce (SIT)
• replacement
  
  • spread of refractory elements into the wild
  • e.g. gene drive

Question 27

phased testing of genetic control

Answer 27

• first contained use
  
  • stable inheritance?
  • relationship between organism and environment?
  • fitness?
  • test for insecticide resistance (can be removed)
• semi-field
  
  • mimic daylight times with cages facing environment
  • temperature/humidity fluctuations
• release in field

Question 28

first release of genetically engineered mosquitoes for vector control

Answer 28

• RIDL technique in aedes aegypti
• Cayman islands - isolated
• release at end of rainy season
  
  • low populations
• no transmission here to not affect spread of disease
• test mating competitiveness and efficacy of suppression

Question 29

considerations in selection of field site

Answer 29

• contained isolated release zones
• contingency plan in case of breakdown
  
  • other control measures
• assessment of single vectors vs other vectors
• population size/density/distribution
• transport and release logistics
• incidence origin and cost of malaria
• unpredictable events

Question 30

result of first transgenic release

Answer 30

• manual separation of males from females was challenging
• ovitrap capture and then lab monitoring
• ovitrap index reduced to o in a few months
  
  • = percentage of traps positive for aedes eggs