Toxoplasma gondii is a single-celled organism that causes an infection known as toxoplasmosis. It is found in cat faeces and the flesh of infected animals. Humans can contract the infection by ingesting cat faeces (perhaps from unwashed hands) or infected meat which is undercooked or raw.

T. gondii has a complex lifecycle involving an active phase (in which the organisms are known as tachyzoites), inactive phase (known as the bradyzoite phase) and an end-of-life phase (known as the sporozoite phase). Cats pass on active organisms in their faeces, whereas inactive bradyzoites are passed on in the flesh of infected animals. Pregnant women who experience active infection (with tachyzoites) can also transmit the infection to their foetus.

Toxoplasmosis or acute infection is caused by infection with tachyzoites (active organisms) which enter the body’s cells and replicate within them. In serious cases, the infection causes inflammation in the eyes and brain. Serious infections usually affect newborn babies born to an infected mother and adults who are immunocompromised, whereas healthy adults rarely display symptoms of toxoplasmosis. They are usually unaware they are infected.

Following acute infection, T. gondii organisms enter the bradyzoite or inactive phase and remain in the person’s body indefinitely. They may be reactivated if the person’s immune system becomes compromised.

For more information, see Toxoplasmosis.