22 research outputs found
The impact of temperature on insecticide toxicity against the malaria vectors Anopheles arabiensis and Anopheles funestus
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
Empirical and theoretical investigation into the potential impacts of insecticide resistance on the effectiveness of insecticide-treated bed nets.
In spite of widespread insecticide resistance in vector mosquitoes throughout Africa, there is limited evidence that long-lasting insecticidal bed nets (LLINs) are failing to protect against malaria. Here, we showed that LLIN contact in the course of host-seeking resulted in higher mortality of resistant Anopheles spp. mosquitoes than predicted from standard laboratory exposures with the same net. We also found that sublethal contact with an LLIN caused a reduction in blood feeding and subsequent host-seeking success in multiple lines of resistant mosquitoes from the laboratory and the field. Using a transmission model, we showed that when these LLIN-related lethal and sublethal effects were accrued over mosquito lifetimes, they greatly reduced the impact of resistance on malaria transmission potential under conditions of high net coverage. If coverage falls, the epidemiological impact is far more pronounced. Similarly, if the intensity of resistance intensifies, the loss of malaria control increases nonlinearly. Our findings help explain why insecticide resistance has not yet led to wide-scale failure of LLINs, but reinforce the call for alternative control tools and informed resistance management strategies
Long-lasting insecticidal nets no longer effectively kill the highly resistant Anopheles funestus of southern Mozambique
BACKGROUND: Chemical insecticides are crucial to malaria control
and elimination programmes. The frontline vector control
interventions depend mainly on pyrethroids; all long-lasting
insecticidal nets (LLINs) and more than 80% of indoor residual
spraying (IRS) campaigns use chemicals from this class. This
extensive use of pyrethroids imposes a strong selection pressure
for resistance in mosquito populations, and so continuous
resistance monitoring and evaluation are important. As
pyrethroids have also been used for many years in the Manhica
District, an area in southern Mozambique with perennial malaria
transmission, an assessment of their efficacy against the local
malaria vectors was conducted. METHODS: Female offspring of
wild-caught Anopheles funestus s.s. females were exposed to
deltamethrin, lambda-cyhalothrin and permethrin using the World
Health Organization (WHO) insecticide-resistance monitoring
protocols. The 3-min WHO cone bioassay was used to evaluate the
effectiveness of the bed nets distributed or available for
purchase in the area (Olyset, permethrin LLIN; PermaNet 2.0,
deltamethrin LLIN) against An. funestus. Mosquitoes were also
exposed to PermaNet 2.0 for up to 8 h in time-exposure assays.
RESULTS: Resistance to pyrethroids in An. funestus s.s. was
extremely high, much higher than reported in 2002 and 2009. No
exposure killed more than 25.8% of the mosquitoes tested
(average mortality, deltamethrin: 6.4%; lambda-cyhalothrin:
5.1%; permethrin: 19.1%). There was no significant difference in
the mortality generated by 3-min exposure to any net (Olyset:
9.3% mortality, PermaNet 2.0: 6.0%, untreated: 2.0%; p = 0.2).
Six hours of exposure were required to kill 50% of the An.
funestus s.s. on PermaNet 2.0. CONCLUSIONS: Anopheles funestus
s.s. in Manhica is extremely resistant to pyrethroids, and this
area is clearly a pyrethroid-resistance hotspot. This could
severely undermine vector control in this district if no
appropriate countermeasures are undertaken. The National Malaria
Control Programme (NMCP) of Mozambique is currently improving
its resistance monitoring programme, to design and scale up new
management strategies. These actions are urgently needed, as the
goal of the NMCP and its partners is to reach elimination in
southern Mozambique by 2020
The Effects of Age, Exposure History and Malaria Infection on the Susceptibility of Anopheles Mosquitoes to Low Concentrations of Pyrethroid
Chemical insecticides are critical components of malaria control programs. Their ability to eliminate huge numbers of mosquitoes allows them to swiftly interrupt disease transmission, but that lethality also imposes immense selection for insecticide resistance. Targeting control at the small portion of the mosquito population actually responsible for transmitting malaria parasites to humans would reduce selection for resistance, yet maintain effective malaria control. Here, we ask whether simply lowering the concentration of the active ingredient in insecticide formulations could preferentially kill mosquitoes infected with malaria and/or those that are potentially infectious, namely, old mosquitoes. Using modified WHO resistance-monitoring assays, we exposed uninfected Anopheles stephensi females to low concentrations of the pyrethroid permethrin at days 4, 8, 12, and 16 days post-emergence and monitored survival for at least 30 days to evaluate the immediate and long-term effects of repeated exposure as mosquitoes aged. We also exposed Plasmodium chabaudi- and P. yoelii-infected An. stephensi females. Permethrin exposure did not consistently increase mosquito susceptibility to subsequent insecticide exposure, though older mosquitoes were more susceptible. A blood meal slightly improved survival after insecticide exposure; malaria infection did not detectably increase insecticide susceptibility. Exposure to low concentrations over successive feeding cycles substantially altered cohort age-structure. Our data suggest the possibility that, where high insecticide coverage can be achieved, low concentration formulations have the capacity to reduce disease transmission without the massive selection for resistance imposed by current practice
The impact of temperature on insecticide toxicity against the malaria vectors Anopheles arabiensis and Anopheles funestus
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
Schedules for malaria experiments.
<p>For each experiment, female mosquitoes from a single cohort were divided into treatment groups. (A) In both <i>P. chabaudi</i> experiments, females were bloodfed at 3–5 days post-emergence from either uninfected or infected mice and then exposed to permethrin 14 days later. (B) In the <i>P. yoelii</i> experiment, females were fed uninfected or infected blood five days after emergence, or were only given sugar. These groups were exposed to permethrin twice, as indicated by the red stars. Knockdown was assessed at the end of each 1 h exposure and survival assessed 24 h later.</p
The effects of exposure history on survival after 24 hours.
<p>The 4 panels show the results of the permethrin exposures on days 4, 8, 12, or 16. In each panel, the cluster of bars on the left represent the proportion of insecticide-naïve females surviving 24 h after exposure. The bars on the right half show the corresponding results for previously-exposed females. In the day 12 panel, for example, multiple exposures had occurred at days 4, 8 and 12, whereas the single exposures occurred on day 12 only. All mosquitoes on day 4 were naïve (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024968#pone-0024968-g001" target="_blank">Fig. 1</a>).</p
Kaplan-Meier survival curves of mosquitoes exposed to permethrin in exposure-history experiment 1.
<p>Panels reflect different permethrin concentrations, as denoted. Red vertical lines show timing of exposures. Plotted data were from the total number of mosquitoes in the 3 replicate cups in each treatment group (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024968#s2" target="_blank">methods</a>), so that n = 60 at the start of each curve. Colors correspond to the timelines in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024968#pone-0024968-g001" target="_blank">Fig. 1</a>.</p