Toxoplasmosis: Advances and Vaccine Perspectives

Toxoplasma gondii was first identified more than 100 years ago in the tissues of birds and mammals. In 1908 Nicolle and Manceoux described it for the first time in the gundi (Ctenodactylus gundi), a North African rodent, in tachyzoite forms. At the same time, Splendore in Brazil, identified the parasite in rabbit tissues. Due to its bow-like shape (Greek: Toxo = Arc) the genus was named Toxoplasma. However, only in the 1970’s was the complete life cycle known and the parasite recognized as a coccidian parasite (member of the phylum Apicomplexa). It is ubiquitous throughout the world and estimated to infect approximately half of the world's population. It is characterized by a polarized cell structure and two unique apical secretory organelles called micronemes and rhoptries. Toxoplasma has a complex life cycle consisting of a sexual cycle in its feline definitive hosts and an asexual cycle in its intermediate hosts. The latter, including humans, can be infected by ingestion of oocysts shed in cat feces. Unlike most other Apicomplexan parasites, Toxoplasma can be transmitted between intermediate hosts by either vertical (via placenta) or horizontal (carnivorism) transmission. Toxoplasma parasite is found in intermediate hosts in two interconvertable stages: bradyzoites and tachyzoites. Bradyzoites, a dormant form, are slow-growing, transmissible and encysted. Infections with bradyzoite-containing cysts occur upon ingestion of undercooked meat. The wall of these cysts is digested inside the host stomach and the released bradyzoites, which are resistant to gastric peptidases, subsequently invade the small intestine. There, they convert into tachyzoites, the rapidly growing, disease-causing form that can infect most nucleated cells, replicate inside a parasitophorous vacuole, egress, and then infect neighboring cells. These tachyzoites activate a potent host immune response that eliminates most of the parasites. Some tachyzoites, however, escape destruction and convert back into bradyzoites. In the absence of an adequate immune response, tachyzoites