Recent years have witnessed the discovery of a number of secreted proteins in Toxoplasma gondii that play important roles in host–pathogen interactions and parasite virulence, particularly in the mouse model. However, the role that these proteins play in driving the unique features of T. gondii compared to some of its nearest apicomplexan relatives (Hammondia hammondi and Neospora caninum) is unknown. These unique features include distinct dissemination characteristics in vivo and a vast host range. In this review we comprehensively survey what is known about disease outcome, the host response and host range for T. gondii, H. hammondi, and N. caninum. We then review what is presently known about recently identified secreted virulence effectors in these three genetically related, but phenotypically distinct, species. Finally we exploit the existence of genome sequences for these three organisms and discuss what is known about the presence, and functionality, of key T. gondii effectors in these three species.

Toxoplasma gondii oocysts are morphologically and antigenically similar to oocysts of another feline coccidian, Hammondia hammondi. The distinction between H. hammondi and T. gondii is important from an epidemiological perspective because all isolates of T. gondii are potentially pathogenic for humans and animals, whereas H. hammondi is not known to cause clinical disease in any naturally infected intermediate or definitive hosts. In the present report, H. hammondi (designated HhCatEt1 and HhCatEt2) oocysts were found microscopically in the feces of 2 of 36 feral domestic cats (Felis catus) from Addis Ababa, Ethiopia. Oocysts were orally infective to Swiss Webster and gamma interferon gene knockout mice; the inoculated mice developed tissue cysts in their muscles. Laboratory-raised cats fed mouse tissues of infected mice shed H. hammondi oocysts with a prepatent period of 5 days. The DNA extracted from sporulated oocysts reacted with H. hammondi-specific primers, and sequences were deposited in GenBank (accession nos. JX477424, and KC223619). This is the first report of isolation of H. hammondi from cats from the African continent.