Decoding of malaria parasite's genome could lead to vaccine

— -- Scientists have unraveled the genetic code of a malaria parasite that sickens hundreds of millions of people each year, a step that may lead to better treatment and a vaccine.

The bug, known to researchers as Plasmodium vivax, is the main cause of malaria in Latin America and Southeast Asia, accounting for as much as 40% of up to 500 million cases worldwide each year. It was once entrenched in the USA. In 2002, a small cluster of cases were reported in Loudoun County, Va., near Washington, D.C.

The mosquito-borne parasite doesn't arouse the same level of scientific concern as its African cousin, Plasmodium falciparum, because it rarely kills. Falciparum malaria, found in Africa, is the leading cause of death of children younger than 5.

But vivax malaria is, in its own way, as much a scourge as the African variety. Like falciparum malaria, it causes excruciating spells of recurrent disease, with high fever, chills, body aches, diarrhea, nausea, vomiting and anemia.

"It makes people very sick," says lead researcher Jane Carlton, of New York University's Langone Medical Center. "It can come out of the liver weeks or months after the initial mosquito bite. That makes it a very serious risk to human health."

Vivax malaria is so debilitating that sufferers, most of whom are poor, can't support themselves or their families. "Vivax is one of the stealth reasons that poor people can't escape poverty," says Peter Hotez, president of the Sabin Vaccine Institute at George Washington University and an expert in tropical diseases.

The research, out today in the journal Nature, reflects a growing effort to roll back the disease. Six years ago, scientists in Britain and the USA reported that they had deciphered the genetic code of falciparum malaria. On Wednesday, two teams of researchers reported that they had also decoded vivax and Plasmodium knowlesi, a malaria parasite found mostly in monkeys and first diagnosed in a human 40 years ago.

By comparing vivax with falciparum, Hotez says, researchers may be able to figure out what makes the African variety so much more lethal.

Researchers found that the two malaria parasites share much of the same genetic sequence, with about 5,500 genes. Genes are blueprints for proteins that perform millions of tasks needed to sustain life.

The new evidence shows vivax has evolved clever mechanisms for sustaining itself in the human body, including the ability to disguise itself so that it can evade the immune system. It also has multiple ways of invading red blood cells, making it much harder to block. Finally, it can lie dormant in the liver, where only one drug can kill it.

That drug, primaquine, can't be taken by pregnant women and people who suffer from a blood disorder common in regions where vivax is found.

Carlton says the researchers are hoping to find a "genetic switch" that governs the drug's dormant phase, so they can design a drug to shut the parasite down.