The way in which the malaria parasite infects pregnant women is more complex than previously thought, with implications for vaccine research, say scientists.

A joint study between the Karolinska Institutet in Sweden and Makerere University in Uganda looked at how blood cells infected with the malaria parasite Plasmodium falciparum invade the placenta.

Malaria affects many millions of pregnant women each year, mostly young African women during their first pregnancy when they lose the semi-immunity normally found in adults.

In these young mothers, the parasite accumulates in the placenta, causing them to become anemic. The children of infected mothers often have lower birth weights.

Until recently, scientists believed that red blood cells infected with the malaria parasite attached to the placenta through a single protein on the blood cell surface, which bound to the CSA protein on placental cells.

This one-to-one binding would have made it easier to design a vaccine for pregnant mothers.

But the research using blood samples taken from pregnant mothers in Uganda shows that things are not so simple.

"Most of the parasites we studied could bind to three different receptors in the placenta," says author Niloofar Rasti of the Karolinska Institutet. "This would mean that a future vaccine cannot be based on the principle of one protein to one receptor, as was previously believed."

The researchers say their findings suggest that the success of malaria vaccines for pregnant mothers will depend on an in-depth understanding of the binding interactions between infected blood cells and the placenta.

Stephen Rogerson, a professor at the Department of Medicine, Melbourne University, Australia says, “Wahlgren’s finding goes rather against the prevailing dogma, which suggests that only adhesion to CSA is of major importance in placental malaria.”

"We urgently need new medications to treat and prevent malaria in pregnant women, as resistance to the few drugs that are safe for pregnancy has risen to dangerous levels," he says.

And although testing such vaccines can be controversial, Rogerson says that "future strategies should include both pregnancy-specific vaccines, and possibly vaccines aimed at the whole malaria-exposed population".

The study was led by Mats Wahlgren of the Karolinska Institutet, and was published online by Proceedings of the National Academy of Sciences last week (28 August).

Link to abstract of paper in Proceedings of the National Academy of Sciences

Reference: Proceedings of the National Academy of Sciences doi:10.1073/pnas.0601519103 (2006)