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The malaria parasite kills millions every year, mostly in sub-Saharan Africa

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Scientists tracking fluorescent malaria parasites as they move through the bodies of mice and rats have discovered that the parasite can spend part of its life in the lymph nodes.

Lead researcher Robert Ménard of the Pasteur Institute in France says they are unsure whether this helps the parasite or helps the host protect itself from infection.

The study was published online by Nature Medicine yesterday (22 January).

On the one hand, parasites in the lymph nodes could alert the body to the presence of an invader and activate a protective immune response, says Ménard.

On the other, the parasites' presence in lymph nodes might desensitise the immune system and weaken its response to the infection.

Ménard's team made their surprise discovery after infecting mice and rats with malaria parasites that had been genetically modified to produce a fluorescent protein.

This allowed the researchers to make the first ever real-time study of what happens to the parasites after infected mosquitoes bite rodents.

The team expected the parasites to move from the skin into blood vessels, which would take them to the liver where they develop into another form that later triggers the symptoms of malaria.

But about a quarter of the parasites entered the lymphatic system, the network of thin vessels that carries white blood cells of the body's immune system.

The parasites ended up in lymph nodes close to where the rodents were bitten. Although the white blood cells appeared to attack some of the parasites, others were able to begin developing into forms thought only to exist in the liver.

No parasites were left in the lymph nodes 52 hours after infection, suggesting they cannot fully develop there.

Ménard says that even parasites that partially develop in the lymph nodes could significantly affect how the immune system responds to infection.

Link to abstract of paper in Nature Medicine

Reference: Nature Medicine doi:10.1038/nm1350 (2006)