Exposing Anopheles mosquitoes to antimalarials blocks Plasmodium parasite transmission

Abstract

Bites of Anopheles mosquitoes transmit Plasmodium falciparum parasites that cause malaria, which kills hundreds of thousands of people every year. Since the turn of this century, efforts to prevent the transmission of these parasites via the mass distribution of insecticide-treated bed nets have been extremely successful, and have led to an unprecedented reduction in deaths from malaria1. However, resistance to insecticides has become widespread in Anopheles populations2,3,4, which has led to the threat of a global resurgence of malaria and makes the generation of effective tools for controlling this disease an urgent public health priority. Here we show that the development of P. falciparum can be rapidly and completely blocked when female Anopheles gambiae mosquitoes take up low concentrations of specific antimalarials from treated surfaces—conditions that simulate contact with a bed net. Mosquito exposure to atovaquone before, or shortly after, P. falciparum infection causes full parasite arrest in the midgut, and prevents transmission of infection. Similar transmission-blocking effects are achieved using other cytochrome b inhibitors, which demonstrates that parasite mitochondrial function is a suitable target for killing parasites. Incorporating these effects into a model of malaria transmission dynamics predicts that impregnating mosquito nets with Plasmodium inhibitors would substantially mitigate the global health effects of insecticide resistance. This study identifies a powerful strategy for blocking Plasmodium transmission by female Anopheles mosquitoes, which has promising implications for efforts to eradicate malaria.

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Fig. 1: A. gambiae exposure to ATQ aborts P. falciparum development.
Fig. 2: The transmission-blocking activity of ATQ is maintained at an exposure time of six minutes and at time points of exposure before and after infection.
Fig. 3: Malaria transmission model predicts that adding ATQ to insecticide-treated nets would increase bed-net effectiveness.
Fig. 4: Other cytochrome b inhibitors have P. falciparum transmission-blocking activity.

Data availability

Raw data for infection experiments are available as Supplementary Data. All further data are available upon request.

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Acknowledgements

We thank N. Singh, E. Lund and K. Thornburg for Plasmodium and Anopheles culture; M. Bernardi for help with graphics; D. Wirth, S. Bopp, H. Ranson, and the members of the Catteruccia laboratories for comments and suggestions on the manuscript. Malaria prevalence and LLIN coverage map data were retrieved from the Malaria Atlas Project (www.map.ox.ac.uk). Insecticide resistance data were retrieved from the IR Mapper database (www.irmapper.com). F.C. is funded by a Faculty Research Scholar Award by the Howard Hughes Medical Institute (HHMI) and the Bill & Melinda Gates Foundation (BMGF) (Grant ID: OPP1158190), and by the National Institutes of Health (NIH) (R01 AI124165, R01 AI104956). L.M.C. is supported by Simons Foundation Collaboration Grant 524390. C.O.B. is supported by NIGMS Maximizing Investigator's Research Award (MIRA) R35GM124715-02. The findings and conclusions within this publication are those of the authors and do not necessarily reflect positions or policies of the HHMI, the BMGF, Simons Foundation or the NIH.

Reviewer information

Nature thanks Jaline Gerardin, Janet Hemingway, Elizabeth Winzeler and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Contributions

D.G.P. carried out and analysed infection and fitness experiments. M.A.I. carried out immunofluorescent assays. L.M.C. and I.E.H. generated code and carried out mathematical modelling. C.O.B. and F.C. supervised the study.

Corresponding author

Correspondence to Flaminia Catteruccia.

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Competing interests

A patent application (US provisional application no. 62/726,757) covering the concept of the application of antimalarial compounds to mosquitoes has been filed on behalf of F.C. and D.G.P. by the President and Fellows of Harvard University. The authors state that they have no other competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Effects of ATQ exposure on survival and post-blood-feeding egg production in A. gambiae female mosquitoes.

a, ATQ exposure has no effect on the acute or long-term survival of A. gambiae female mosquitoes (two-sided log-rank Mantel–Cox, n = 189, df = 1, χ2 = 0.00, P = 0.9951). The sigmoidal fit used for subsequent modelling is shown. b, The production of eggs after an infectious blood meal is unaffected by ATQ exposure (two-sided, unpaired Student’s t-test, n = 75, df = 1, t = 0.826, P = 0.4115). Means and 95% confidence intervals of the mean are indicated. ns, not significant; n indicates the number of biologically independent mosquito samples. Source data

Extended Data Fig. 2 Model structure and population parameters.

a, Schematic of the mosquito life cycle model with the time step of one day. Mosquitoes spend three days as eggs (Ei) and ten days as larvae (Li) (which includes the pupal stage). Adult female mosquito compartments fall within the dashed box, and begin with a rest day (R0) followed by mating (M) or feeding (F). After feeding, female mosquitoes undergo two days of rest (Ri), followed by a day for egg laying (EL). Then, the cycle repeats. Shaded boxes denote when exposure to insecticide or ATQ could occur. These are the same compartments in which mosquitoes can become infected or transmit infections, assuming they have been infected for a period longer than the incubation time. b, Survival of the mosquito population as a function of age. The curve is a Gompertz distribution with scale parameter b = 0.1868 and shape parameter η = 0.0293. c, Functions relating infection levels of humans and mosquito to risk of infection. Subpanel (i) shows the risk of a human becoming infected, βH, as a function of the number of infectious feeders, f. Subpanel (ii) shows the risk of a mosquito becoming infected, βM, as a function of the fraction of the human population that is infected, IH. Source data

Extended Data Fig. 3 Sensitivity of model results to variation in prevalence, coverage and insecticide resistance.

The graphs show the enhanced effectiveness of insecticide combined with ATQ (relative to insecticide alone) in reducing human prevalence under varying levels of coverage (across panels), prevalence (along x axis), and insecticide resistance (IR) (bar colour). The enhanced effectiveness of the interventions is defined as human prevalence with only insecticide − human prevalence with insecticide and ATQ, divided by human prevalence with insecticide alone, and is represented by positive values when the addition of ATQ is beneficial. Prevalence is quantified after ten years of simulation. The coverage is varied from 20–80% (top left panel 20%; top right panel 40%; bottom left panel 60%; and bottom right panel 80%). In each panel, the position of the bars determines the malaria prevalence under no intervention, from 20–80%. In the complete absence of insecticide resistance, all mosquitoes that contact insecticide are killed; all dark-green bars equal zero. Source data

Extended Data Fig. 4 Malaria transmission model predicting the effects of adding ATQ to insecticide-treated nets in additional malaria prevalence settings.

a, b, The heat maps show changes in malaria transmission for bed net-like interventions using insecticide alone or insecticide plus an ATQ-like compound, relative to no intervention at varying coverage and varying insecticide resistance levels. The model considers both 20% (a) and 70% (b) prevalence of malaria. The effectiveness of the interventions is defined as (1 − proportion reduction in malaria transmission relative to no intervention), and is represented as colours ranging from yellow (no change in malaria transmission) to dark blue (elimination of malaria transmission) at varying levels of coverage (axis) and insecticide resistance (y axis). Insecticide resistance is the percentage of mosquitoes that are impervious to insecticide. Coverage is the probability of a mosquito encountering an intervention during a single feeding episode. The model output demonstrates that addition of ATQ significantly increases the ability of an LLIN-like intervention to reduce and even eliminate malaria transmission. Source data

Extended Data Fig. 5 Testing additional compounds for fitness costs and transmission-blocking activity through tarsal contact.

a, Mosquito survival relative to an untreated control 48 h after exposure to ATQ, DEC, PYR, HYD, ACE and PER. The proportion of female A. gambiae surviving exposure to each compound (1 mmol per m2, 60 min) relative to the proportion of individuals surviving exposure to an untreated control is shown. PER exposure causes almost complete mortality (proportionate survival relative to controls = 0.055, pairwise two-sided chi-squared test with Bonferroni correction, n = 80, df = 1, χ2 = 76.10, P < 0.0001), whereas all other compounds behave comparably to controls. b, Neither PYR nor DEC (1 mmol per m2, 6 min) are capable of reducing the prevalence of P. falciparum through tarsal contact, relative to controls (pairwise two-sided chi-squared test with Bonferroni correction: DEC, n = 93, df = 1, χ2 = 2.42, P = 0.12; PYR, n = 92, df = 1, χ2 = 0.55, P = 0.46). Similarly, DEC and PYR had no effect on the intensity of infection, compared to a mock-treated control (Wilcoxon with Dunn’s post hoc test, n = 183, df = 3, P = 0.31 (DEC) and P = 0.99 (PYR)). Letters indicate groups that are statistically different from one another. ****P < 0.0001. Medians are indicated; n denotes the number of biologically independent mosquito samples. Source data

Extended Data Fig. 6 ATQ exposure via a netting substrate completely inhibits P. falciparum development.

A. gambiae female mosquitoes were allowed to rest for 60 min on 100-denier polyester netting that had been treated with either a 0.5 mg ml−1 (0.05% w/v) solution of ATQ in acetone, or acetone alone. Females exposed to ATQ in this way failed to become infected after an infectious P. falciparum blood meal, demonstrating that a netting substrate is also capable of delivering sufficiently high doses of ATQ to inhibit infection (two-sided chi-squared test, n = 98, df = 1, χ2 = 75.55, P < 0.0001). ****P < 0.0001. Medians are indicated; n denotes the number of biologically independent mosquito samples. Source data

Extended Data Table 1 Chemical properties and structures of study compounds and bed-net-approved chemicals

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Paton, D.G., Childs, L.M., Itoe, M.A. et al. Exposing Anopheles mosquitoes to antimalarials blocks Plasmodium parasite transmission. Nature 567, 239–243 (2019). https://doi.org/10.1038/s41586-019-0973-1

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