Toxoplasma is arguably the most successful animal parasite on earth: It infects hundreds of species of warm-blooded animals, most notably half of humanity. Its unusual ability to overcome the numerous challenges of infecting and reproducing inside such a wide range of creatures has long intrigued scientists, and now researchers at the Stanford University School of Medicine have identified two of the proteins critical to its ability to thrive.

The findings will be published in the Dec. 15 issue of Science by a team led by John Boothroyd, PhD, professor of microbiology and immunology. Working with mice, the researchers identified two genes that produce two proteins that Toxoplasma introduces into the cells of the host it infects. What's more, the researchers showed for the first time that certain changes in either of the proteins - called kinases - ramped up 10,000-fold the damage that Toxoplasma inflicted on the lab mice.

"This was a totally unknown phenomenon," said Boothroyd.

Although the majority of people infected by Toxoplasma have no symptoms, it can cause severe infections in individuals with compromised immune systems. In addition, women infected for the first time while pregnant can pass the organism to their fetuses, potentially resulting in sight and hearing problems as well as learning disabilities. The new findings have implications for determining how to treat these and other infected people: More aggressive therapy may be warranted if a strain that contains the proteins that increase virulence is the cause of the infection.

Humans can become infected by the parasite by accidentally consuming or inhaling the cysts from infected cat feces, by eating meat from an infected animal - especially pork, lamb or venison - or by drinking contaminated water. Cats are the primary carriers of Toxoplasma, though they rarely exhibit symptoms.

The significance of these newly discovered proteins is also highlighted in the same issue of Science by another paper with similar findings from a different group of researchers. Additional evidence of the proteins' critical role will be offered by a paper slated to appear in an upcoming issue of Nature, in which Boothroyd and his colleagues describe the molecular underpinnings of the mechanism used by these parasites to hijack cellular processes in their hosts.

"We think that the different versions of Toxoplasma strains evolved for optimal interaction with different hosts," said Boothroyd, noting that the wrong pairing of parasite and host can have dire consequences. "If a given strain gets into the 'wrong' host, the result is a system out of kilter and extreme disease. It's the bull in the china shop."

The origins of the more virulent strains of Toxoplasma were first documented in a 2001 Science paper from Boothroyd's group; the researchers found that the recombination of two relatively benign strains of Toxoplasma can result in a thousand-fold increase in their ability to cause serious disease. Over the last few years, the researchers have worked to track down exactly what happens to make some strains of Toxoplasma pack such an extra punch. By conducting a comprehensive scan of gene sequences of Toxoplasma strains, postdoctoral scholars Jeroen Saeij, PhD, and Jon Boyle, PhD, first authors of the paper in the Dec. 15 Science, searched for the gene or genes responsible for markedly increased virulence.

To do such a survey, however, the researchers needed to produce an array of strains. Because Toxoplasma strains only recombine in cats, the researchers provided mice infected with different Toxoplasma strains for the cats to eat. The researchers then took the resulting recombinant strains from the cats and infected other mice with them. Based on how these mice fared, the researchers could pinpoint stretches of DNA that contain the genes underlying severe virulence. Based on the DNA sequences, they could determine the proteins involved.

From 140 candidate proteins identified in the initial screen, they focused on those with qualities logically involved in differing levels of virulence, such as high variability of the sequence and the ability to be secreted from the organism to enter the host. This led them to two particularly compelling proteins - ROP16 and ROP18. Both are protein kinases, molecules that are used in the transmission of cellular messages. "If a parasite needs to co-opt a host cell for its own purposes, there is no better way than to introduce a kinase that can completely alter the entire physiology of that host cell," said Boothroyd.

The findings may play a role in helping pregnant women who become infected for the first time with the parasite. If doctors can test whether the women have one of the more virulent strains, they can then better assess the risks to the fetuses. The group's further exploration of the Toxoplasma virulence process may also have implications for immune-system modulating drugs that could specifically dampen the effects of toxoplasmosis.

This work was supported by grants from the National Institutes of Health, the Ellison Medical Foundation and the University of California Universitywide AIDS Research Program. Postdoctoral scholar Susan Coller, PhD, is the other Stanford researcher included as an author of this publication.

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