Toxoplasma gondii Comprises Three Clonal Lineages: Correlation of Parasite Genotype

The population genetic structure of Toxoplasma gondii was determined by multilocus restriction fragment length polymorphism analysis at 6 loci in 106 independent isolates from humans and animals. Phylogenetic and statistical analyses indicated a highly unusual population structure con-sisting of 3 widespread clonal lineages. Extensively mixed genotypes were only apparent in 4 strains, which indicated that, while not separate species, sexual recombination between the 3 lineages is exceedingly rare in natural populations. T. gondii is a major cause of subclinical human infection and an important opportunistic pathogen that causes severe disease in immunocompromised pa-tients. While strains from all 3 lineages were isolated from humans, the majority of human toxoplas-mosis cases were associated with strains of a type II genotype. The correlation of specific clonal lineages with human toxoplasmosis has important implications for development of vaccines, drug treatments, and diagnostic protocols. The protozoan Toxoplasma gondii is an obligate intracellular parasite that infects a wide range of warm-blooded vertebrates [1, 2]. In cats, sexual reproduction of T gondii occurs only within intestinal epithelial cells, culminating in the productio