The Effects of Age, Exposure History and Malaria Infection on the Susceptibility of Anopheles Mosquitoes to Low Concentrations of Pyrethroid

Abstract

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.

Delayed mortality effects cut the malaria transmission potential of insecticide-resistant mosquitoes

“…A previous study tested for a cumulative impact of low-dose insecticide exposure in Anopheles but found no evidence of higher mosquito mortality following repeated exposures (33). Similarly, our results show no association between the immediate mortality of mosquitoes following exposure, and the number of times they had been previously exposed.…”

“…However, we also show that mosquitoes' natural mortality varies with age. Older mosquitoes have been previously shown to be more susceptible to pyrethroids than their younger counterparts (33,38). Our findings suggest this result may have been driven by changes in the natural mortality of mosquitoes over time (i.e., senescence) rather than increases in susceptibility to insecticide exposure.…”

“…Alternatively, contact with LLINs could prompt behavioral changes that increase the transmission potential of IR mosquitoes, by, for example, changing the time and location of their biting to avoid nets [e.g., "behavioral resistance" (32)]. Furthermore, previous studies have suggested that resistance is associated with changes in the susceptibility of mosquitoes to infection [ranging from an enhancement, reduction, or no change (33)(34)(35)]. IR also drives various physiological modifications that may ultimately impact survival and parasite competence (28).…”

The threat (or not) of insecticide resistance for malaria control

“…The standard WHO assays prescribe the use of 3-to 5-d-old mosquitoes, yet several studies observe that adult female Anopheles become more susceptible to insecticides with increasing age (13,14). This rising susceptibility is important, as it is only the older mosquitoes that are responsible for transmitting malaria parasites, so if they remain susceptible, transmission can still be reduced (15).…”

Behavioral Responses of Two Dengue Virus Vectors,Aedes aegyptiandAedes albopictus(Diptera: Culicidae), to DUET and its Components

“…Populations of mosquitoes consist of individuals in a variety of different ages and physiological states (i.e., teneral, unfed, bloodfed, and gravid), with differences in insecticide susceptibility reported among different ages (Davidson 1958, Rowland and Hemingway 1987, Hodjati and Curtis 1999, Glunt et al 2011. Effect of pesticide treatments in a wind tunnel on percentage of time in the postspray period that female Ae.…”