Toxoplasma gondii is a ubiquitous protozoan with very low host specificity. This obligate intracellular parasite is capable of infecting all mammals and non-mammalian species such as birds and reptiles. Feline species and in particular domesticated cats, serve as the definitive hosts of T. gondii, and mammals and birds constitute the intermediate hosts of this organism . The life cycle of T. gondii is divided in two phases, the intestinal (enteroepithelial) and extraintestinal phases. The intestinal phase takes place in cats only and results in the formation of resistant oocysts. The extraintestinal phase occurs in all intermediate hosts as well as cats. Toxoplasmosis is transmitted through ingestion of oocysts excreted in cat feces or the bradyzoite form of the parasite in raw or undercooked meat. In most humans, toxoplasmosis is symptomatic; however, in immunocompromised individuals, such as AIDS patients, this disease can lead to blindness, neurological disorders, hepatitis, and pneumonia. Toxoplasmosis is also transmitted transplacentally. An estimated 40% of all infected pregnant women give birth to infected infants, but only 10% of those infants are symptomatic. This means that about one in every 2100 pregnancies in the United States today lead to congenital toxoplasmosis. If left undetected, acute infection during pregnancy may result in spontaneous abortion, stillbirth, or neonatal diseases such as encephalitis, chorioretinitis, hepatosplenomegaly, and in some cases, mental retardation.

IgM antibodies against T gondii appear within 2 weeks following infection, and usually peak between 4-8 weeks. These antibodies become undetectable in 6 months to one-year post infection. In contrast, IgG antibodies have a low titer during the first few weeks and peak within 1-2 months. Serum levels of anti-T. gondii IgG will then show a decline but remain stably positive. There are various techniques for the detection of T. gondii. These include the Sabian-Feldman dye test, which detects the live parasite, indirect hemagglutination, indirect Fluorescent antibody test, and EIAs.. The MDI Toxo G Test is an EIA, which allows for accurate detection of IgG antibodies against T. gondii. Measurement of IgG antibodies against T. gondii, when performed in conjunction with the IgM antibody tests, will allow for the diffrentiation of acute toxoplasmosis from a past infection with T. gondii.