Genetic analyses of Chinese isolates of Toxoplasma gondii reveal a new genotype with high virulence to murine hosts

A great deal of evidence demonstrates that a strongly clonal population structure of Toxoplasma gondii strains exists in humans and animals in North America and Europe, while the strains from South America are genetically separate and more diverse. Potential differences in virulence between different strains mean that an understanding of strain diversity is important to human and animal health. However, to date, only one predominant genotype, ToxoDB#9 (Chinese I), and a few other genotypes, including ToxoDB#205, have been identified in China. By using DNA sequence-based phylogenetic analyses, we have re-evaluated the population structure of T. gondii strains collected from China and compared them with other global strains. Based on phylogenetic analysis of restriction fragment length polymorphisms, multilocus sequence typing and intron sequences from T. gondii, we propose that the Chinese isolates described as Chinese I are divided into two groups called Chinese I and Chinese III. Our results demonstrate that significant differences were found in mouse mortality caused by some Chinese strains, and also the archetypal I, II, III strains in mice. Furthermore, a comparison of cyst loading in the brains of infected rats showed some Chinese strains to be capable of a high degree of cyst formation. Furthermore we show that genotyping using neutral genetic markers may not be a useful predictor of pathogenic phenotypes.