Toxoplasma gondii is a single-celled parasitic organism that can infect most animals. However, sexual reproduction of the parasite only occurs in cats (both wild and domestic), which are the organism's definitive host. T. gondii's life cycle begins when a cat eats infected prey, which is typically a mouse or bird. Cats can also become infected if they eat raw, contaminated meat or accidentally ingest a small amount of infected soil.

Once ingested, T. gondii burrows into the walls of the cat's small intestine. The parasite forms early-stage cells called oocysts, which the cat excretes in its feces, usually for about two to three weeks. A single stool may contain millions of oocysts. Most healthy cats will not shed oocysts after this initial stage, which typically lasts about two to three weeks.

Within a few days, the excreted oocysts develop into mature, highly infectious cells that can survive in warm, moist soil for up to one year.

If another animal ingests the parasite, they multiply rapidly inside the host and eventually form cysts (inactive eggs) that lodge in the brain or muscles. Cysts form as early as seven days after infection and remain inside the host for the rest of its life. They produce little or no inflammatory response. The new host animal usually experiences no symptoms and will not excrete oocysts in its feces. At this stage, the parasite can only be transmitted if a predator eats the host.

When humans become infected with T. gondii, the parasite forms cysts that may affect almost any organ, but they are most likely to lodge in the brain, bones and heart. Healthy individuals typically do not show symptoms because the immune system keeps the parasite infiltration under control. In such instances, the parasites remain in an inactive state in the body for life.

When the parasite enters an immunocompromised patient, the parasite is active causing symptoms such as headache, confusion, and seizures that may lead to serious complications.

When a woman is exposed to the parasite during pregnancy, the organism may spread to the placenta. When this occurs, the infection may be transmitted to the fetus through the placenta or during vaginal delivery. If the mother becomes infected and is not treated during the first trimester, about 17% of fetuses become infected, and disease in the infant is usually severe. If the mother becomes infected and is not treated during the third trimester, about 65% of fetuses become infected and experience mild symptoms at birth. The different rates of transmission are most likely related to placental blood flow, the virulence and amount of T. gondii acquired, and the overall health of the mother's immune system.

Healthy individuals typically experience no symptoms. In such instances, the parasites remain in an inactive state in the body for life. If the patient develops an immunocompromised condition in the future, symptoms of toxoplasmosis may arise.

Mild symptoms include flu-like symptoms, swollen lymph nodes, and muscle aches and pains.

HIV patients typically develop severe symptoms of toxoplasmosis, including headache, confusion, poor coordination, seizures, ocular toxoplasmosis (severe inflammation of the retina), as well as lung problems that are similar to tuberculosis or pneumocystis pneumonia (lung infections).

Infected babies may experience severe complications, including mental retardation, convulsions, spasticity (muscle tightness), cerebral palsy, deafness, and severely impaired vision. In addition, the infant's head may be abnormally small (microcephaly) or abnormally large (hydrocephalus) due to increased pressure on the brain. Fetal infection with Toxoplasma gondii may result in stillbirth or abortion.

General: If toxoplasmosis is suspected, blood tests may be conducted to check for antibodies to the parasite. Most pregnant women in the United States are not routinely screened for toxoplasmosis. Only New Hampshire and Connecticut regularly screen infants for the infection. If a pregnant mother tests positive for the disease, the fetus may be tested with a prenatal test called amniocentesis. Imaging studies may be used to determine whether a patient with severe toxoplasmosis has lesions or cysts in the brain.

Blood tests: If toxoplasmosis is suspected, several blood tests may be conducted to check for antibodies to the parasite. Antibodies are proteins produced by the immune system that detect and bind to foreign substances that enter the body, including parasites like T. gondii. Because these antibody tests may be difficult to interpret, the U.S. Centers for Disease Control and Prevention (CDC) recommends that all positive results are confirmed by a laboratory that specializes in diagnosing toxoplasmosis. Initial test results usually take about a week, although expert confirmation may take longer.

Patients who are tested soon after exposure may have not yet developed antibodies to the parasite. Such patients may have a false negative test result. This means that the patient has toxoplasmosis, but tests negative for the infection. If it is suspected that the patient has toxoplasmosis, but the test results are negative, the patient will be retested in two to three weeks.

Amniocentesis: Prenatal testing for toxoplasmosis is possible with amniocentesis, which may be conducted at 15-18 weeks of gestation. During the procedure, a long, thin needle is inserted into the abdominal wall and into the uterus. A small amount of fluid is then removed from the sac surrounding the fetus. The sample is tested in a laboratory for evidence of toxoplasmosis. There is a risk of miscarriage, ranging from one out 200-400 patients. Some patients may experience minor complications, such as cramping, leaking fluid, or irritation where the needle was inserted.

Ultrasound scan: An ultrasound scan uses sound waves to produce images of the fetus inside the womb. Although an ultrasound scan cannot diagnose toxoplasmosis, it may help a healthcare provider detect signs of hydrocephalus (swelling of the head due to an accumulation of cerebrospinal fluid in the scull). Since most infants do not show signs of toxoplasmosis at birth, a negative ultrasound scan does not rule out infection.

Magnetic resonance imaging (MRI): A magnetic resonance imaging (MRI) test may be conducted to detect cysts or lesions in the brain. This test uses a magnetic field and radio waves to create images of the head and brain.

Brain biopsy: In rare cases, especially if patients do not respond to treatment, a neurosurgeon may take a small sample of tissue from the brain. The sample is then analyzed in a laboratory to check for the presence of toxoplasmic cysts. A brain biopsy is an invasive procedure that is associated with serious health risks, and therefore, should only be performed in extreme cases.