The main parasite that causes malaria, Plasmodium falciparum, can persist at low levels in the human bloodstream without causing disease during the dry season by altering its gene expression. This study, published in Nature Medicine, explains how the parasite can remain in a person without causing observable symptoms and allow malaria to re-emerge when mosquito populations resurge during the rainy season.
Malaria is a major cause of death in Africa, killing around 400,000 people in 2018, most of whom were under the age of 5 years. Although the majority of cases occur during the rainy season, when mosquitoes that spread P. falciparum are abundant, asymptomatic infected individuals can be found year round. The ability of the parasite to persist in the human host allows it to bridge the several-month dry period between rainy seasons. However, how the parasite is able to remain in the human host without causing observable symptoms is poorly understood.
Silvia Portugal and colleagues followed 600 people in Mali, 3 months to 45 years of age, during 2017 and 2018. They found that the P. falciparum in these people during the end of the dry season 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.
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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