We have a better idea of how the malaria parasite survives when no mosquitoes are around
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An estimated 200 million people develop malaria each year, and the disease killed nearly 400,000 people in 2018 alone – now we're one step closer to understanding why it's so difficult to stop. The main parasite that causes the disease has developed a strategy to hide undetected in carriers' bloodstream.
Malaria is caused by five species of the Plasmodium parasite, the deadliest being Plasmodium falciparum. The parasite infects human red blood cells and replicates within them, causing symptoms such as fever and muscle pain.
Most cases of malaria occur during the rainy season – which occurs between July and December in some parts of West Africa. It makes sense for P. falciparum to replicate during these months as the mosquitoes that can spread the disease from person to person are abundant.
What wasn't clear is how the parasite reacts during the nearly six-month dry season, when mosquitoes are rare. Silvia Portugal from the University of Heidelberg in Germany and her colleagues have found that P. falciparum changes its gene expression in such a way that it survives undetected in the human bloodstream and is ready to flash again when it rains again.
Portugal and its colleagues followed 600 people in Mali. During the 2017 and 2018 rainy seasons, they recorded 386 and 347 diagnosed cases of malaria with fever, respectively. In the dry season, only 12 cases were diagnosed with fever in 2017 and five cases in 2018.
"What the parasite found here is a kind of sweet spot," says Portugal. During the dry season, it stays at levels so low that it rarely causes symptoms of illness or provokes a response from the person's immune system.
To find out how the parasite does this, the researchers examined and analyzed P. falciparum, which was collected from carriers during both the rainy and dry seasons. During the rainy season, the parasites produced a molecule that increases the chance that red blood cells will stick to blood vessels. This makes the parasite-containing cells less likely to get into the spleen, essentially a blood filter that is used to remove damaged or diseased red blood cells.
However, in the dry season, the parasites stopped producing molecules, which meant that infected blood cells no longer seemed to adhere as strongly to the walls of the blood vessels. As a result, most of the infected cells migrated to the spleen, where they were broken down. This kept the parasite population at a low level that would not cause disease or trigger an immune response.
"In order to achieve something close to eradicating malaria, it is not enough to speak to people who get sick," says Portugal.
"If we could clear these reservoirs during the dry season and reduce the amount of parasites we have when mosquitoes return, this could be an intervention."
Journal reference: Nature Medicine, DOI: 10.1038 / s41591-020-1084-0
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