Genetic diversity of Toxoplasma gondii in animals and humans

Abstract

Toxoplasma gondii is one of the most widespread parasites of domestic, wild, and companion animals, and it also commonly infects humans. Toxoplasma gondii has a complex life cycle. Sexual development occurs only in the cat gut, while asexual replication occurs in many vertebrate hosts. These features combine to create an unusual population structure. The vast majority of strains in North America and Europe fall into three recently derived, clonal lineages known as types I, II and III. Recent studies have revealed that South American strains are more genetically diverse and comprise distinct genotypes. These differences have been shaped by infrequent sexual recombination, population sweeps and biogeography. The majority of human infections that have been studied in North America and Europe are caused by type II strains, which are also common in agricultural animals from these regions. In contrast, several diverse genotypes of T. gondii are associated with severe infections in humans in South America. Defining the population structure of T. gondii from new regions has important implications for transmission, immunogenicity and pathogenesis.

Figure 1.

Life cycle of T. gondii. Sexual replication occurs only in the intestine of the cat, resulting in shedding of (a) diploid oocysts that undergo sporogony in the environment to yield sporozoites. A–E refer to stages of development within enterocytes. Oocysts are extremely environmentally resistant and give rise to infections in a variety of intermediate hosts. During acute infection, rapidly proliferating (b) tachyzoites disseminate within the host. Semidormant tissue cysts containing (c) bradyzoites give rise to long-term infection and are transmitted asexually following oral ingestion by a variety of hosts. Adapted from Frenkel (1973).

Figure 2.

Toxoplasma gondii infects a wide range of placental mammals including at least some members of the major orders highlighted here. Toxoplasma gondii also infects bird and marsupials, making it one of the most widespread parasites of vertebrates. Adapted from Romer & Parsons (1977).

Figure 3.

Diversity of T. gondii in North and South America. (a) Relationship of major lineages of T. gondii as defined by phylogenetic analysis of intron sequences. Major groups in North (blue) and South (red) are indicated. Group numbers are given in boxes; bootstrap values indicted in shaded circles. White circles, greater than 50 per cent; grey circles, greater than 75 per cent; black circles, greater than 90 per cent. (b) Ancestry of T. gondii lineages with respect to the spread of Chr1a*. Adapted from Khan et al. (2006a).