Treating malaria and other diseases caused by parasites requires a good understanding of the parasite. A new video of a malaria parasite at work should help researchers develop better treatments.

The malaria parasites, Plasmodium falciparum and three close, less lethal, relatives, have dodged the human immune system for centuries.

The protozoa are transmitted by mosquitoes, and the malignant form of malaria kills more than a million people every year.

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Scientists know that P. falciparum first infects the liver and then somehow gets into the bloodstream, where it causes red blood cells to rupture.

It is during this stage of the disease that the classic symptoms — high fever and chills — occur.

A paradox

In the liver, P. falciparum takes the form of a merozoite, one of its life-cycle stages. Studies have shown that liver's immune cells gobble up free-moving merozoites.

"This was a paradox," explained Robert Menard, a researcher at the Howard Hughes Medical Institute. "We could not understand how the rate of infection could be so successful."

By tracking malaria infections in live mice, Menard and his colleagues discovered a sneaky trick.

After infecting the liver, P. falciparum wraps itself in dead liver cells, called merosomes, to cloak and transport itself back into the bloodstream, where it can do further harm.

Since the biology of mice is similar to that of humans, the researchers hope the new insight will lead to improved methods for warding off the disease.

Trojan horse

The scheme is reminiscent of the ancient Greeks who hid in a hollow horse to sneak into Troy.

"The parasite has evolved this complex structure. The best image to describe it is the Trojan horse, because it both transports the parasites and camouflages them," Menard said.

P. falciparum has evolved another neat trick. It releases cloaking chemicals while inside the dead liver cell that prevent the cell from broadcasting its own death signal, which would normally be the call for immune cells to seek it and destroy it.

The results were reported Aug. 3 in the online version of the journal Science. Volker Heussler at the Bernhard-Nocht Institute for Tropical Medicine in Hamburg, Germany and Rogerio Amino, at the Institut Pasteur in Paris, contributed to the work.

About malaria

Malignant malaria — there are two non-fatal kinds as well — can cause fever, chills and flu-like symptoms. Left untreated, it can kill.

Each year 300 million to 500 million cases occur worldwide, and more than one million people die, most of them young children in sub-Saharan Africa.

Malaria was eradicated from the United States in the early 1950s. But it is common in many developing countries, where travelers can get it.

Returning travelers and arriving immigrants could also reintroduce malaria in the United States.

Mosquitos of the genus Anopheles, which transmits malaria, are found throughout much of the United States. If local mosquitoes bite an infected person, those mosquitoes can, in turn, infect local residents.

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