Identifying suitable methods for evaluating the sterilizing effects of pyriproxyfen on adult malaria vectors: a comparison of the oviposition and ovary dissection methods.

BACKGROUND: Nets containing pyriproxyfen, an insect growth regulator that sterilizes adult mosquitoes, have become available for malaria control. Suitable methods for investigating vector susceptibility to pyriproxyfen and evaluating its efficacy on nets need to be identified. The sterilizing effects of pyriproxyfen on adult malaria vectors can be assessed by measuring oviposition or by dissecting mosquito ovaries to determine damage by pyriproxyfen (ovary dissection). METHOD: Laboratory bioassays were performed to compare the oviposition and ovary dissection methods for monitoring susceptibility to pyriproxyfen in wild malaria vectors using WHO bottle bioassays and for evaluating its efficacy on nets in cone bioassays. Blood-fed mosquitoes of susceptible and pyrethroid-resistant strains of Anopheles gambiae sensu lato were exposed to pyriproxyfen-treated bottles (100 µg and 200 µg) and to unwashed and washed pieces of a pyriproxyfen long-lasting net in cone bioassays. Survivors were assessed for the sterilizing effects of pyriproxyfen using both methods. The methods were compared in terms of their reliability, sensitivity, specificity, resources (cost and time) required and perceived difficulties by trained laboratory technicians. RESULTS: The total number of An. gambiae s.l. mosquitoes assessed for the sterilizing effects of pyriproxyfen were 1745 for the oviposition method and 1698 for the ovary dissection method. Fertility rates of control unexposed mosquitoes were significantly higher with ovary dissection compared to oviposition in both bottle bioassays (99-100% vs. 34-59%, P < 0.05) and cone bioassays (99-100% vs. 18-33%, P < 0.001). Oviposition rates of control unexposed mosquitoes were lower with wild pyrethroid-resistant An. gambiae s.l. Cové, compared to the laboratory-maintained reference susceptible An gambiae sensu stricto Kisumu (18-34% vs. 58-76%, P < 0.05). Sterilization rates of the Kisumu strain in bottle bioassays with the pyriproxyfen diagnostic dose (100 µg) were suboptimal with the oviposition method (90%) but showed full susceptibility with ovary dissection (99%). Wild pyrethroid-resistant Cové mosquitoes were fully susceptible to pyriproxyfen in bottle bioassays using ovary dissection (> 99%), but not with the oviposition method (69%). Both methods showed similar levels of sensitivity (89-98% vs. 89-100%). Specificity was substantially higher with ovary dissection compared to the oviposition method in both bottle bioassays (99-100% vs. 34-48%) and cone tests (100% vs.18-76%). Ovary dissection was also more sensitive for detecting the residual activity of pyriproxyfen in a washed net compared to oviposition. The oviposition method though cheaper, was less reliable and more time-consuming. Laboratory technicians preferred ovary dissection mostly due to its reliability. CONCLUSION: The ovary dissection method was more accurate, more reliable and more efficient compared to the oviposition method for evaluating the sterilizing effects of pyriproxyfen on adult malaria vectors in susceptibility bioassays and for evaluating the efficacy of pyriproxyfen-treated nets.

Mosquito Shield™, a transfluthrin passive emanator, protects against pyrethroid-resistant Anopheles gambiae sensu lato in central Benin.

BACKGROUND: Spatial repellents can provide personal and household protection against biting vector mosquitoes by volatizing repellents into the air within a given area. Mosquito Shield™ is a transfluthrin passive emanator undergoing evaluation for malaria control. Studies evaluating its entomological impact against different local malaria vector populations would help guide its deployment in endemic countries. METHODS: A two-arm single-blinded small-scale household randomised entomological trial was conducted to assess the impact of Mosquito Shield™ on the human landing rate of wild pyrethroid-resistant Anopheles gambiae sensu lato (s.l.) vector mosquitoes in houses in the Ganhoua village of the Zakpota District of central Benin. From a total of 30 houses, 15 were randomly allocated to receive Mosquito Shield™, while the remainder received a placebo product. The trial lasted through the life of the Mosquito Shield™ product (32 days). Mosquito sampling was performed by human landing catches at baseline and at 6 timepoints post-intervention (days 0-1, 7-8, 14-15, 21-22, 28-29 and 31-32). Collections were performed for 2 nights at each sampling time point. WHO cylinder bioassays were conducted during the trial with F1 An. gambiae s.l. mosquitoes that emerged from larvae from the study area to assess the intensity of resistance to pyrethroids in the wild vector population. RESULTS: The vector population in the study area showed a high intensity of resistance to pyrethroids. Baseline An. gambiae s.l. human landing rates were similar in houses in both study arms before product application (11.53/person/night vs 11.67/person/night, p > 0.05). A total of 5736 mosquitoes were collected in the placebo control arm and 3862 in the Mosquito Shield™ arm post-intervention. Overall An. gambiae s.l. post-intervention human landing rates were significantly lower in houses in the Mosquito Shield™ arm (18.13/person/night) compared to the houses in the placebo control arm (26.84/person/night, IRR = 0.658, p < 0.001). Over the lifespan of the product, Mosquito Shield™ provided a significant protective efficacy of 34.2% (22.1-44.4%, p < 0.001) against wild pyrethroid-resistant An. gambiae s.l. vectors compared to the placebo. Human landing rates of other nuisance vector mosquito species (Culex and Mansonia) were also reduced in houses treated with Mosquito Shield™ compared to the placebo. CONCLUSION: Mosquito Shield™, a transfluthrin passive emanator, provided significant protection against pyrethroid-resistant malaria vectors to households in Benin. The spatial repellent shows potential to reduce malaria transmission by pyrethroid-resistant An. gambiae s.l. vector mosquitoes and cover gaps in malaria control when deployed to complement existing vector control interventions.