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April 17, 2003

Malaria may have been traveling with humans much longer than previously believed

UW Health Sciences/UW Medicine

An international team of scientists has gathered evidence that suggests the parasite that causes malaria may have emerged as a human disease agent much earlier in history than previously thought. The malaria parasite is Plasmodium falciparum.

Relatively recently, analysis of certain chromosomes in the mosquitoes that transmit malaria has lead to the hypothesis that the African parasite gained a stronghold on humans about 6,000 years ago, when agricultural societies came into being. Stagnant water in crop fields offered nearby breeding ground for mosquitoes, and the density of the population allowed the parasite to spread.

Another related hypothesis, called “Malaria’s Eve,” suggests most of the malaria parasite population died out between 5,000 to 6,000 years ago and left behind a small seed population. This remnant remained until human living conditions changed and allowed the parasite population to expand.

A report in the April 11, 2003, Science magazine is adding to the controversy on the origin and expansion of malaria. Scientists from laboratories in the United States, Brazil, Canada, and Thailand worked together to examine the mitochondrial DNA from 100 worldwide samples of the parasite.

Their genetic studies point to a sudden surge in the malaria parasite population in Africa about 10,000 years ago. This expansion was followed by a migration of the parasite to other parts of the world. The populations of malaria parasites in South America and Asia seemed to be older than the African expansion of malaria described as occurring about 6,000 years ago. The genetic data suggest that the malaria parasite could have been around up to 130,000 years ago. The parasite could have migrated out of Africa during the Pleistocene age, when the human population exploded and began moving to other parts of the world. Human (and mosquito) population size and human migration patterns appear to have had a major influence on the demography of the malaria parasite.

The scientists cautioned that their study of the evolution of the malaria parasite was based on a single locus, the mitochondrial DNA, and therefore is only one observational point in its evolutionary history.

Every year malaria kills about 2.7 million people and sickens more than 200 million. Evolutionary biologists hope their research on the history of the organism may point to new strategies for its control. Such research may also show how deadly diseases might emerge during major changes in human society and in the living conditions of people.

Scientists on the study published in Science as “Early Origin and Recent Expansion of Plasmodium falciparum” were Drs. Deidre A. Joy, Xiarong Fang, Jianbing Mu, Tetsuya Furuya and Xin-zhuan Su, all of the Laboratory of Malaria and Vector Research at the National Institute of Allergy and Infectious Diseases, National Institutes of Health in Bethesda, Md.; Kesinee Chotivanich and Nicholas J. White, both of the Wellcome Trust-Mahodil University-Oxford Tropical Medical Research Programme in Bangkok; Antonia U. Krettll of the Malaria Laboratory, Centro de Pesqulsas, in Brazil; May Ho, of the Department of Infectious Disease at the University of Calgary, Canada; Alex Wang and Edward Suh of the Center for Information Technology at the National Institutes of Health; and Peter Beerli of the Department of Genome Sciences at the University of Washington in Seattle.