Abstract:
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BACKGROUND: It is anticipated that malaria elimination efforts
in Africa will be hampered by increasing resistance to the
limited arsenal of insecticides approved for use in public
health. However, insecticide susceptibility status of vector
populations evaluated under standard insectary test conditions
can give a false picture of the threat, as the thermal
environment in which the insect and insecticide interact plays a
significant role in insecticide toxicity. METHODS: The effect of
temperature on the expression of the standard WHO insecticide
resistance phenotype was examined using Anopheles arabiensis and
Anopheles funestus strains: a susceptible strain and the derived
resistant strain, selected in the laboratory for resistance to
DDT or pyrethroids. The susceptibility of mosquitoes to the
pyrethroid deltamethrin or the carbamate bendiocarb was assessed
at 18, 25 or 30 degrees C. The ability of the pyrethroid
synergist piperonyl-butoxide (PBO) to restore pyrethroid
susceptibility was also assessed at these temperatures. RESULTS:
Temperature impacted the toxicity of deltamethrin and
bendiocarb. Although the resistant An. funestus strain was
uniformly resistant to deltamethrin across temperatures,
increasing temperature increased the resistance of the
susceptible An. arabiensis strain. Against susceptible An.
funestus and resistant An. arabiensis females, deltamethrin
exposure at temperatures both lower and higher than standard
insectary conditions increased mortality. PBO exposure
completely restored deltamethrin susceptibility at all
temperatures. Bendiocarb displayed a consistently positive
temperature coefficient against both susceptible and resistant
An. funestus strains, with survival increasing as temperature
increased. CONCLUSIONS: Environmental temperature has a marked
effect on the efficacy of insecticides used in public health
against important African malaria vectors. Caution must be
exercised when drawing conclusions about a chemical's efficacy
from laboratory assays performed at only one temperature, as
phenotypic resistance can vary significantly even over a
temperature range that could be experienced by mosquitoes in the
field during a single day. Similarly, it might be inappropriate
to assume equal efficacy of a control tool over a geographic
area where local conditions vary drastically. Additional studies
into the effects of temperature on the efficacy of
insecticide-based interventions under field conditions are
warranted. |