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OUR immune system fights malaria infections differently from the way most
people imagined. This may mean that scientists trying to design a malaria
vaccine will have to change tack.

Four species of the Plasmodium parasite cause malaria. Until now
scientists thought that immunity to the disease was specific to individual
species. But findings by Marian Bruce of the Wellcome Trust Centre for the
Epidemiology of Infectious Diseases in Oxford contradict this idea. Our immune
system also seems to regulate overall parasite numbers, regardless of the
species.

Bruce and her team studied 34 children in Papua New Guinea to see how the
body mounts its own defences to the Plasmodium parasites. Blood tests
taken over 60 days revealed that most of the children were infected by more than
one species of Plasmodium, even though none of them had any symptoms of
malaria. But the researchers were surprised to find that while the
levels of individual species fluctuated over time, the total number of parasites
in the children鈥檚 blood stayed constant at around 1000 parasites per
microlitre.

鈥淭here is something new here that is controlling the parasite level, and
preventing these children from becoming diseased,鈥� says Bruce. 鈥淲e can鈥檛 explain
it by any immune mechanisms we already know about.鈥�

The enigma could complicate efforts to find a vaccine against the disease.
Attempts to design a malaria vaccine have tended to concentrate on one species
only, and most vaccine researchers are targeting P. falciparum, the
most widespread and dangerous strain of malaria parasite.

Bruce is worried that this approach could backfire in areas where malaria is
endemic. If a vaccine successfully targets P. falciparum infections,
other parasite species might simply multiply in the blood in its place. Vaccines
need to include targets specific for different species, she says.

Andrew Taylor-Robinson from the Parasitology Research Group at Leeds
University points out that in sub-Saharan Africa, where the majority of malaria
deaths occur, only one species of parasite is usually present.

But he doesn鈥檛 downplay the significance of the results. 鈥淭his challenges the
importance of species-specific immunity, which has been a central tenet of
malaria immunology over the years,鈥� he says. 鈥淲ith this type of information from
the field, we will perhaps reanalyse which antigens we are using.鈥�