A study by Karolinska Institutet in Sweden and Makerere University in Uganda finds that the malarial parasite hides in the placenta of pregnant women and this is the reason for pregnant women and children to suffer severe malaria.
Plasmodium falciparium is the worst of the four malaria parasites that infect man. It is particularly dangerous in that it also infects the placenta of pregnant women, with fatal consequences for both her and the foetus. This, combined with the often feeble medical resources of malaria-stricken countries, can lead to such serous complications that the mother dies during delivery.
For some reason, women in their first pregnancy lose the semi-immunity that is normally found in adults, explains Niloofar Rasti, a KI graduate student who has been working with the study. The placenta seems to be an anatomically favourable environment for a subpopulation of the parasites.
During one particular phase of its lifecycle, the parasite enters human red blood cells, where it produces proteins that attach themselves to receptors in the wall of the blood vessels. This causes the red blood cells to accumulate in organ capillaries, and gives rise to life-threatening symptoms. Adults who have been infected several times can become partly immune as their defence system gradually starts to recognise the parasite's proteins. When the placenta is formed, however, a new environment is introduced with a different set of receptors. This means that a new growth niche is made available to a subpopulation of the parasites.
Earlier studies have suggested that each protein from the parasite attaches to only one specific protein, a receptor, in the placenta. Ms Rasti and her colleagues suspected, however, that the natural mechanisms are more complex than laboratory studies have shown. They therefore collected and analysed placentas on site in Uganda.
'Most of the parasites we studied could bind to three different receptors in the placenta,' she says. 'This would mean that a future vaccine cannot be based on the principle of one protein-one receptor, as was previously believed.'
Now that scientists know that several placental receptors are involved in the binding mechanism, attention will be shifted to the parasite itself, and whether it produces many different surface proteins or if one and the same protein is able to bind to many host receptors.