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Asymptomatic malaria infections: detectability, transmissibility and public health relevance

Nature Reviews Microbiology volume 12, pages 833–840 (2014)Cite this article

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Abstract

Most Plasmodium falciparum infections that are detected in community surveys are characterized by low-density parasitaemia and the absence of clinical symptoms. Molecular diagnostics have shown that this asymptomatic parasitic reservoir is more widespread than previously thought, even in low-endemic areas. In this Opinion article, we describe the detectability of asymptomatic malaria infections and the relevance of submicroscopic infections for parasite transmission to mosquitoes and for community interventions that aim at reducing transmission. We argue that wider deployment of molecular diagnostic tools is needed to provide adequate insight into the epidemiology of malaria and infection dynamics to aid elimination efforts.

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Figure 1: Sensitivity of malaria diagnostic methods in relation to parasite density and transmission intensity.

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Acknowledgements

T.B. is supported by the Bill and Melinda Gates Foundation (grants OPP1024438 and OPP1034789). L.O. is supported by UK Medical Research Council Population Health Scientist Fellowship G1002387. I.F. is supported by the Swiss National Science Foundation (grant 3300C0-105994/1) and the Bill and Melinda Gates Foundation (grant 39777). C.D. is supported by the Bill and Melinda Gates Foundation (grant OPP1034789) and the Wellcome Trust (grant 091924). The authors thank R. Gosling, H. Sturrock, J. Mosha, G. Stresman, A. C. Eziefula and W. Moss for fruitful discussions. They also thank P. Bejon, A. van der Ven and G. Targett for critical reading of the manuscript.

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Authors and Affiliations

  1. Teun Bousema and Chris Drakeley are at the London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK.,

    Teun Bousema & Chris Drakeley

  2. Teun Bousema is also at the Radboud University Medical Centre, 6525 GA Nijmegen, The Netherlands.,

    Teun Bousema

  3. Imperial College, London, London, SW7 2AZ, UK

    Lucy Okell

  4. Swiss Tropical Institute, Basel, 4051, Switzerland

    Ingrid Felger

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PowerPoint slides

Glossary

Artemisinin combination therapy

(ACT). A treatment in which an artemisinin derivative is combined with another antimalarial drug, which is often a schizonticidal drug.

Asymptomatic malaria infections

Malaria infections that lack typical clinical symptoms but are detectable by microscopy, rapid diagnostic test or molecular methods.

Gametocytogenesis

The developmental process that leads to the formation of male and female sexual gametocytes, which later fertilize after ingestion by the mosquito vector.

LAMP

(Loop-mediated isothermal amplification). A method for DNA amplification in which the entire reaction is carried out in a single tube and at a constant temperature.

Malaria receptivity

A term used to describe the degree to which infection occurs. In areas of high receptivity, the presence of abundant anopheline vectors and the existence of other ecological and climatic factors favour parasite transmission.

Mosquito larviciding

The treatment of breeding sites with insecticides that specifically target the larval life stage of mosquitoes.

Osmiophilic body

A membrane-bound vesicle that is predominantly found in female gametocytes.

Schizont stage

An asexual stage in the Plasmodium spp. life cycle, in which the parasite divides several times to produce daughter cells that go on to invade new red blood cells.

Submicroscopic gametocyte carriage

Infections in which sexual gametocytes that are detectable by molecular methods, but not microscopy, are produced.

Submicroscopic malaria infections

Infections that do not necessarily produce gametocytes and are detectable by molecular methods but not microscopy. Submicroscopic infections are almost exclusively asymptomatic.

Superinfections

Infections in which an Individual is infected by more than one parasite clone.

Symptomatic malaria infections

Infections that are accompanied by fever and/or other symptoms that are indicative of malaria. These infections are, almost without exception, detectable by microscopy or rapid diagnostic test.

Transmission hot spots

Areas in which transmission intensity is higher than that in the surrounding area.

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Bousema, T., Okell, L., Felger, I. et al. Asymptomatic malaria infections: detectability, transmissibility and public health relevance. Nat Rev Microbiol 12, 833–840 (2014). https://doi.org/10.1038/nrmicro3364

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