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A Brief History and Overview of Toxoplasma gondii


Elisabeth A. Innes. Moredun Research Institute, Pentlands Science Park, Edinburgh EH26 OPZ, UK. Tel.: + 44 (0) 131 445 5111; Fax: + 44 (0) 131 445 6111; E-mail:


Toxoplasma gondii was discovered by scientists working in North Africa and Brazil around 100 years ago. The parasite has since been found to be capable of infecting all warm-blooded animals including humans making it one of the most successful parasitic organisms worldwide. The pathogenic potential of T. gondii was recognized in the 1920s and 1930s, in congenitally infected children presenting with the classic triad of symptoms, namely hydrocephalus, retinochoroiditis and encephalitis. In addition, around the same time T. gondii parasites were found to be associated with severe intraocular inflammation. In the 1980s, T. gondii emerged as a major cause of death in patients with acquired immunodeficiency syndrome, illustrating the importance of the immune system in controlling T. gondii infection. T. gondii was reported as a major cause of abortion in sheep in New Zealand in the 1950s, which raised questions about potential new transmission routes for the parasite. The discovery of the cat as the definitive host in the 1960s was a very important finding as it helped to complete our understanding of the parasite’s life cycle, and the oocyst stage of T. gondii shed in the faeces of infected cats was found to be an important source of infection for many intermediate hosts and helped to explain infection in herbivorous animals and people with a vegetarian diet. In addition, this stage of the parasite was very robust and could survive in the environment, depending on the climatic conditions, for up to 12–18 months. Knowledge of the parasite’s lifecycle, transmission routes, risk groups and host immune responses has helped in the development of strategies to control the disease, reduce transmission of the parasite and limit environmental contamination.