An estimated 500 million people worldwide are at risk from at least one of the three tropical diseases linked to the parasite gene discovery. And the limited number of drugs available to fight the illnesses either have toxic side effects or lack efficiency in some stages of infection.

The discoveries, reported in several papers Friday in the journal Science, could significantly impact world health by pointing to drugs or vaccines that could be used against common weak points in the protozoa that cause Chagas disease, African sleeping sickness and leishmaniasis.

"This common core of genes is extremely important, because it may provide targets for a new generation of drugs that might fight all three parasites," said Najib El-Sayed, first author of two of the studies and senior author of a third paper. He is a molecular biologist at The Institute for Genomic Research in Rockville, Md., one of four institutions involved in the sequencing and analysis.

The parasites examined were Trypanosoma brucei, which causes the sleeping sickness; Trypanosoma cruzei, which causes Chagas, a devastating disease in the tropics of Latin America; and Leishmania major, which has been found in nearly 90 countries.

All three are carried by different insect hosts to reach humans, and cause very different diseases, but share a core of about 6,200 genes. Among the key findings are shared genes involved in the host-parasite relationship.

All of the parasites are particularly adept at evading hosts' immune systems, and the gene sequences offer the chance to get around those traits.

"Originally, we believed that the gene organization among all the parasites would be very different, but 70 percent of the genes occur in the same order," said Peter Myler of the Seattle Biomedical Research Institute, another one of the sequencing centers.

"So if we focus on the genes that are the same in all three, but different from humans, we have the potential to develop a class of drugs that can target all three diseases," Myler added.

An estimated 500 million people worldwide are at risk from at least one of the diseases. And the limited number of drugs available to fight the illnesses "either have toxic side effects or lack efficiency in some stages of infection, such as chronic Chagas disease," said Alberto Frasch, director of the Institute for Research in Biotechnology in Buenos Aires, Argentina, and a co-author for two of the papers.

In addition, he said, "drug resistance in diseases caused by trypanosomatids complicates their treatment. The information obtained from the new genome analysis might help us understand the resistance mechanisms to those drugs."

African sleeping sickness, which is spread in sub-Saharan Africa by tsetse flies, is marked by an early fever and progresses to heart and kidney failure, neurological destruction and eventual death.

Chagas, spread by blood-sucking insects called "kissing bugs," also causes a fever and impairs the heart, liver and brain, and reflex movements such as swallowing. Death from the disease most commonly is the result of heart failure.

Leishmaniasis, which can also be caused by several related parasites, is marked by a skin infection initially and is spread by sand flies. It causes fevers and liver disorders, and can result in internal infections.

Collectively, the Science reports have nearly 250 authors from 46 organizations in 21 countries on six continents. The US National Institute of Allergy and Infectious Diseases and The Wellcome Trust in Britain provided money for the sequencing projects. All the researchers posted details on a common Web site as their gene mapping progressed.

"The collaborative nature of this project shows what can be done when we combine the latest technologies with the brightest minds from around the world," said Ken Stuart, president and founder of the Seattle institute.