Insecticides and insecticide resistance Flashcards
What is larviciding
key to vector control before DDT
using insecticide to target immature/aquatic stage
effective if breeding sites are readily identified and or small in number
generally more suitable for urban rather than rural disease (dengue and urban malaria vectors)
often expensive and logistically challenging
Larviciding for Anopheles mosquitos
WHO recommends when sites are fixed, few and findable
unrealistic when very large numbers of breeding sites as sites may require frequent insecticide treatment
more options for larvicides than adulticides
applications of larvicides likely to increase in African malaria control programmes as part of integrated programmes
Aedes aegypti breeding habitats
remove, cover and treat
ultraslow volume/ space spraying/ aerial spraying
expensive, often ineffective and may be environmentally damaging
used mainly in urban areas or epidemic situations e.g. dengue epidemics in Brazil
has played an important role in the control of HAT
Importance of insecticide-based malaria vector control
reduction in malaria cases between 2000 and 2015 attributed to anti-malaria chemotherapy, ITNs and IRS
mostly due to vector control
IRS
most dangerous malaria vectors bite indoors after feeding (endophilic and anthropophilic)
an insecticidal residue on the walls will kill them when they land
prior to the use of INS, this was the mainstay of malaria control worldwide
also used for control of chugs disease (triatomic rest in wall cracks) and leishmaniasis (sandflies rest indoors)
still widely practiced and can be extremely effective if done properly
pyrethroids cheap but also need to increase efficiency and quality of application technology and techniques
not always ideal due to smell and staining but can reduce non targets e.g. cockroaches
pyrethroids, carbamates and organophosphates most used
important to monitor spraying efficacy
ITNs
untreated nets reduce biting by nocturnal mosquitos but some biting occurs through the net
ITNs reduce biting and act as baited trap - community protection - irritation of mosquito may prevent completion of feeding and insecticide will kill
nets impregnated with pyrethroids
lifespan assumed to be 3 years but variable in practice
coverage hugely increased
provide both personal and community protection
effective against many species of anopheles and culex
Targets of insecticides
DDT and pyrethroids - sodium channel on axons - delay channel closing
OPs and carbamates target acetylcholinesterase at synapses- phosphorylate or carbylate the active site serine of AChE prevents breakdown of acetylcholine leading to repetitive nerve firing
organochlorines except DDT target GABA (gamma-aminobutyric acid) receptors
insecticide resistance issues
increased cost of control more insecticides used with poses an increased risk to the environment increased numbers of insect vectors loss of efficacy of control programmes loss of motivation to engage in control increased disease
definitions of insecticide resistance
selection of heritable characteristics in an insect population that results in the repeated failure of an insecticide product to provide the intended level of control when used as recommended
the development of an ability to tolerate doses that would prove lethal to a majority of individuals in a normal population of the same species
important distinction between operational resistance in def 1 and loss of susceptibility def 2
in vectors, def 1 involves the failure to control disease and can be difficult to demonstrate but evidence is increasing
insecticide resistance different species
pyrethroid resistance to African malaria vectors widespread and increasing
pyrethroid resistance to a. aegypti also widespread and may be increasing
resistance to chemical insecticides widespread in insect disease vectors but not tsetse
other classes of insecticide
insect growth regulators which mimic juvenile hormone and block development - methoprene, Pyriproxyfen
microbial insecticides e.g. bacillus thuringinesis isralensis
chitin synthesis inhibitors which interfere with cuticular formation - benzoylphenylureas
phenylpyrazoles and cyclodienes which bind to GABA receptors e.g fipronil and dieldrin
neonicotinoids which bind to nicotinic acetylcholine receptors e.g, clothinidin
halogenated pyrrole - disrupt mitochondrial ATP production e.g chlorfenapyr
general methods to diagnose insecticide resistance and product efficacy
discriminating dose bioassays - adults
dose response determination bioassays - adults r larvae
substrate testing adults - adults
discriminating dose assays
detect prevalence of resistance in a population
fixed time of exposure at a fixed dose
dose time for diagnosis predetermined from demonstration of reliable 100% susceptible lab strains
99 or above is susceptible
90-98 is suspected resistance
below 90 is resistant (below 80 for CDC)
dose response assays for adults or larvae
use a range of doses (concentrations with fixed time or exposure times with fixed dose including 0)
usually transform count and concentration data to linearise
compute and compare slopes and intercepts
res ratio = LDa test strain/LDn ref
