"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.
Source:Stanford University Medical Center