Life Under the Microscope: Toxoplasma gondii

Toxoplasma gondii is the causative agent of the disease known as toxoplasmosis. The parasite was first discovered by Charles Nicolle and Louis Manceaux in 1908. They initially named the parasite Leishmania gondii thinking that it belonged to the Leishmania genus. However, once they realized they discovered a new organism, they named it Toxoplasma gondii, from the Greek toxo, meaning arc or bow, and plasma, meaning something that is shaped or molded.  The parasite’s definitive host is the cat in which T. gondii can sexually reproduce.


Toxoplasma gondii

While approximately 30-50% of the population in developed countries are exposed to and possibly infected with the parasite, most infections are asymptomatic. However, some patients may experience flu-like symptoms, such as swollen glands and muscle aches and pains, and immunocompromised patients can experience fatal symptoms. The parasite has also been shown to affect psychomotor performance and potentially cause depression, suicide, and even schizophrenia.

There are various ways in which people can become infected with T. gondii. One of the most common is by eating meat (particularly pork or lamb) that is raw or undercooked. In addition, infections can also occur by ingesting vegetables, fruits, and even water that have come in contact with the parasite. Infection through blood transfusion and organ transplant are rare, however, since cats are the definitive hosts of the parasites, people can become infected from cleaning litter boxes. It is for this reason why pregnant women are advised against doing so.

The life cycle of T. gondii is separated into two stages: sexual and asexual. As mentioned earlier, the sexual stage only occurs in cats, and the asexual stage occurs in intermediate hosts, such as humans, cats and birds. The cycle begins whenever a cat eats an animal infected with parasitic cysts. The parasite then infects and reproduces in the epithelial cells of the small intestine of the cat, producing oocysts. These oocysts are then released into the intestine and are present in the cat’s feces, infecting others. In the asexual phase, humans ingest the oocysts, and the enzymes in the stomach and small intestine free the sporozoites in the oocysts. These sporozoites infect the epithelium of the intestine and differentiate into tachyzoites, the motile form of the parasite. These tachyzoites multiply inside vacuoles until the host cell bursts and dies, thus releasing the tachyzoites into the bloodstream, and into other tissues. Once the infection process occurs, the tachyzoites convert into bradyzoites. When clustered together, these bradyzoites are called tissue cysts.


Toxoplasma gondii oocyst with bradyzoites within

Laboratory testing for T. gondii includes serological, molecular, and staining methods. For serological analysis, IgM antibodies are produced first by the immune system when infected with the parasite, typically after one or two weeks. IgG antibodies are produced after several weeks of infection. Molecular testing using the Polymerase Chain Reaction (PCR) can be done to detect the DNA of T. gondii in blood, cerebrospinal fluid or amniotic fluid. Various staining methods can also be performed on tachyzoites found in tissue sections or body fluid. One stain that is very sensitive and specific to the parasite is the immunoperoxidase stain. Also, a Wright-Giemsa stain can be used to view the morphology of the parasite.