A team of international researchers, including Dr Mario Recker from the University of Exeter, has studied how the parasite, Plasmodium falciparum, can lie seemingly dormant during unfavourable conditions.

The study explains how the parasite can remain in a person without showing observable symptoms, before allowing malaria to re-emerge when mosquito populations resurge during the rainy season.

The study is published in leading journal Nature Medicine. Malaria is a still a major cause of death in Africa, killing hundreds of thousand people each year, most of whom are children under the age of 5 years.

Although the majority of cases occur during the rainy season, due to the abundance of mosquitoes that spread P. falciparum, the ability of the parasite to persist in the human host for long periods of time allows it to bridge the dry period when mosquitoes become rare and the opportunity for transmission very low.

However, how the parasite is able to remain in the human host under continuous attack by the immune system is poorly understood.

In the new study the team led by Silvia Portugal from the University of Heidelberg, Germany, followed individuals in Mali over consecutive dry and rainy seasons and found that by the end of the dry season the parasites had a distinct pattern of gene transcription.

This pattern was associated with reduced adherence of infected red blood cells to blood vessels, which enabled the infected blood cells to be cleared to low levels by the spleen.

Dr Recker, from the University of Exeter and who led the mathematical modelling part of this study said: "We knew that the parasite can prolong infections by continuously altering its appearance to the immune system.

"What this study shows is that the parasite also adopts another strategy that effectively allows it to hide in plain view by using the spleen to keep its numbers below the immune radar."

The authors conclude that these characteristics contribute to the maintenance of a low reservoir of P. falciparum in the body that evades detection and elimination by the immune system and can fuel the malaria transmission cycle in the subsequent rainy season.

Further research is needed to elucidate how environmental changes affect the transcriptional profiles of P. falciparum, which allow it to survive under specific conditions.

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Materials provided by University of Exeter. Note: Content may be edited for style and length.

Journal Reference:

1. Carolina M. Andrade, Hannah Fleckenstein, Richard Thomson-Luque, Safiatou Doumbo, Nathalia F. Lima, Carrie Anderson, Julia Hibbert, Christine S. Hopp, Tuan M. Tran, Shanping Li, Moussa Niangaly, Hamidou Cisse, Didier Doumtabe, Jeff Skinner, Dan Sturdevant, Stacy Ricklefs, Kimmo Virtaneva, Muhammad Asghar, Manijeh Vafa Homann, Louise Turner, Joana Martins, Erik L. Allman, Marie-Esther N’Dri, Volker Winkler, Manuel Llinás, Catherine Lavazec, Craig Martens, Anna Färnert, Kassoum Kayentao, Aissata Ongoiba, Thomas Lavstsen, Nuno S. Osório, Thomas D. Otto, Mario Recker, Boubacar Traore, Peter D. Crompton, Silvia Portugal. Increased circulation time of Plasmodium falciparum underlies persistent asymptomatic infection in the dry season. Nature Medicine, 2020; DOI: 10.1038/s41591-020-1084-0