Recent Genetic Change Enhanced Toxoplasma's Survival

Researchers have found that a genetic change occurring only about 10,000 years ago allowed the Toxoplasma parasite to directly infect a wide range of animals, a survival advantage that explains why there are only three main strains of the parasite today. These discoveries were reported in the January 17, 2003, issue of Science.

The Toxoplasma parasite is thought to infect up to a quarter of the world's population. However, only people with weakened immunity typically develop severe toxoplasmosis, a potentially serious disease that can lead to birth defects, brain inflammation, and vision problems. The infection usually is acquired by ingesting spores from contaminated soil, water, cat litter, or objects that have had contact with cat feces. The infection also can be acquired from eating raw or partially cooked meat, especially chicken, pork, lamb, or venison.

Research has demonstrated the existence of three main strains of the parasite that are closely related genetically, whereas related parasites typically have many distinct strains. To explain this fact, investigators from the Washington University School of Medicine (St. Louis, MO, USA; www.wustl.edu) estimated the rate at which mutations arise in Toxoplasma. They then sequenced a select set of genes from the three strains to determine how many mutations were present. That data, along with estimates of the mutation rate, indicated that the three strains arose from a common ancestor no more than 10,000 years ago.

The new strains were able to by-pass sexual reproduction in an intermediate host animal and reproduced asexually in the first animal to be infected. The strains with this trait had a great reproductive advantage, and it is these strains that are found today.

"Direct oral infectivity after eating tissue cysts is seen only in Toxoplasma and this trait is exemplified by these young strains,” explained senior author Dr. L. David Sibley, associate professor of molecular microbiology at Washington University. "This strongly suggests that the unique combination of genes passed along during that one mating event endowed the three young strains with an ability to more effectively spread throughout the food chain.”

Dr. Sibley warned, "Our findings raise the possibility that other parasites may also radically change their lifestyle by a similar mechanism and hence present new threats of infection.”



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Washington University School of Medicine