Toxoplasma gondii is a species of parasitic protozoa, belonging to the Apicomplexa, that can cause the disease toxoplasmosis in humans. The life cycle of Toxoplasma gondii has two phases. The sexual part of the lifecycle is taking place only in cats, the definitive host of Toxoplasma gondii. The asexual life cycle can take place in any warm-blooded animal, like other mammals and birds. In these intermediate hosts, the parasite invades cells forming a so-called parasitophorous vacuole. Within this vacuole T. gondii propagates by a series of binary fissions until the infected cell bursts. This fast replicating form of the asexually reproducing T. gondii is called tachyzoite. Additionally, a slow replicating form exists, the bradyzoite. Bradyzoite vacuoles can form tissue cysts in the infected animals/humans. These cysts are resistant to known antibiotics. The immune system of the host does not detect these cysts, while free tachyzoites are efficiently cleared by the immune response.

Tissue cysts can be ingested by a cat, e.g. by feeding on an infected mouse. The cysts survive passage through the digestive system of the cat and the parasites infect epithelial cells of the small intestine where they undergo sexual reproduction and oocyst formation. Oocysts are shed with the feces. Animals and humans that ingest oocysts (e.g. by eating unwashed vegetables etc.) become infected. The parasite enters macrophages in the intestinal lining and is distributed via the blood stream throughout the body. Infection is normally asymptomatic. However, in immune compromised patients toxoplasmosis can develop. During pregnancy Toxoplasma can lead to abortion or hydrocephalus.

It is currently discussed, that infected rats and mice are less fearful of cats, leading to a propagation of the lifecycle (see references).

Human prevalence

In the U.S. NHANES III national probability sample, 22.5% of 17,658 persons >12 years of age had Toxoplasma-specific IgG antibodies, indicating that they had been infected with the organism. There is some research to suggest that infection in humans can lead to slower reactions and an increase in risk taking

References

• M. Berdoy, J. Webster, D. Macdonald: "Fatal Attraction in Rats Infected with Toxoplasma gondii", Proceedings of the Royal Society of London, B267, pp 1591 - 1594, August 7, 2000.
• CDC document: Toxoplasma gondii Infection in the United States, 1999–2000