management of insecticide resistance

Question 1

resistance

Answer 1

• inherited abiity of a strain of an organism to survive doses of a toxicant that would kill the majority of individuals in a normal population of the same species
• WHO 1957

Question 2

assessment of resistance

Answer 2

• expose adult field samples to discriminating dose
• 1 hour
• count how many dead
• fewer than 80% dead → resistance

Question 3

discriminating dose

Answer 3

• predetermined value defined as the minimal dose that kills all susceptible individuals of a reference strain

Question 4

insecticide resistance

Answer 4

• widespread and increasing
• has arisen against all four classes
• particularly from ITN use
  
  • massive selection pressure
• no new classes for public health since 1970s

Question 5

susceptible insects

Answer 5

• inseciticde penetrates body
• some insecticide is:
  
  • excreted
  • degraded
  • reaches and binds target
• each provides potential for resistant mechanisms

Question 6

resistance mechanisms

Answer 6

• avoid places where there is insecticide
  
  • behavioural resistance
• reduce penetration
• increased excretion/degradation
  
  • metabolic resistance
• mutated target prevetning high affinity binding
• usually combinations of these

Question 7

behavioural resistance

Answer 7

• altered biting behaviour
  
  • human landing catch experiments (humans = bait)
  • observe locaiton of biting
  • before ITN → indoor biting predominates
  • 1 and 3 years after ITN → outdoor biting

Question 8

reduced penetration

Answer 8

• A. funestus
• thickness of leg cuticle measured under microscope
• thicker cuticle in resistant strain

Question 9

increased excretion

Answer 9

• insecticides = hydrophobic
• excretion deals with hydrophilic ocmpounds
• need modification of compounds → hydrophilic
  
  • GST gene products
  • catalyse addition of glutathione
• gene duplication or upregulation of GST → more product → higher turnover of insecticide
• also direct dechlorination by GSTs → less toxic, easier excretion

Question 10

increased detoxification

Answer 10

• cytochrome P450-dependent monooxygenases
• bind O2
  
  • add one O atom to substrate as OH
  • less toxic, more hydrophilic
  • gene duplication or transposon insertion → higher expression
• esterases
  
  • break ester bonds
  • reduced toxicity and increased excretion

Question 11

identifying responsible resistance variations

Answer 11

• plenty of GSTs/CYPs in mosquitoes
  
  • which is responsible?
• microarrays/RNAseq
  
  • gene expression profiling
• staistical analysis of expression
  
  • identify lead candidates
  • further analysis to confirm

Question 12

gene downregulation

Answer 12

• occurs in genes invovled in bioactivation of compounds
  
  • makes them more toxic if upregulated
• shifting of metabolic pathways involved
  
  • not just upregulation

Question 13

target site modification

Answer 13

• AChe
• voltage gated sodium channels

Question 14

voltage gated sodium channels

Answer 14

• single aa substitution → kdr mutation
• kdrs usually map to leucine in centre of channel
  
  • probably binding residue
  • reduced insecticide affinity
• independent emergence in A. gambiae:
• leu → phe west africa
• leu → ser kenya

Question 15

AChE

Answer 15

• modified AChE phenotypes = MACE
• side chains not involved in reaction
  
  • replaced by bulkier side chains
  • e.g. gly → ser
• smalle rbinding pocket
  
  • ACh binds but not insecticide (too bulky)
• not cost free
  
  • 25% of WT activity remains

Question 16

pyrethroid resistance

Answer 16

• esterases, CYPs, kdr

Question 17

DDT resistance

Answer 17

• CYPs, GSTs, kdr

Question 18

cross-resistance

Answer 18

• some classes have same target
• resistance to one provides resistance to another
  
  • e.g. kdr mutants resistant to DDT and pyrethroids

Question 19

steps of managing resistance

Answer 19

• where is it/what type of resistance
• reliable disease and resistance surveillance
• prevent/delay resistance development
• maintain effect disease control even in presence of resistance

Question 20

changes in allele frequency

Answer 20

• remove insecticide → remove selection pressure
  
  • no more advantage
  • frequency of resistant allele decreases due to reduced fitness
  • WT allele frequency increases
• field data supports this
  
  • Bangkok, Mexico

Question 21

theory of rotations

Answer 21

• assumes fitness cost of resistance
• apply insecticide → increased frequency of resistant allele
  
  • reaches threshold frequency
  • switch to insecticide with different mode of action
  • switch again at threshold
• or mix insecticides

Question 22

mosaic formation

Answer 22

• spatial distribution of insecticides
• insects move between regions and encounter different selection pressure
• Mexico field trials
  
  • rotation and mosaic → low levels of resistance
  • no carbamate resistance

Question 23

problems with rotation/mosaic

Answer 23

• logistics of implementation
  
  • especially mosaic
• cross-resistance
  
  • some insecticides should be considered same class
• some insecticides very stable/persistent
• agricultural/domestic use can interfere
• pollutants
  
  • heavy metals select for same mutations
• ancient resistance
  
  • other genetic changes counterbalance fitness reduction → resistant allele frequency doesn’t decrease

Question 24

points to consider in resistance management

Answer 24

• use when and where needed
• WHO-recommended concentrations
• avoid using same class in adults and larvae
• buys time only
  
  • still need other technologies and developments