Toxoplasmosis is infection caused by the single-celled protozoan parasite Toxoplasma gondii. Symptoms may include swollen lymph nodes, fever, a vague ill feeling, and sometimes a sore throat, blurred vision, and eye pain.

Toxoplasma gondii is present worldwide wherever there are cats. The parasite infects a large number of animals as well as people. Many people in the United States have been infected, although few ever develop symptoms. Severe infection usually develops only in fetuses and people with an immune system weakened by AIDS, cancer, or drugs used to suppress rejection of an organ transplant (immunosuppressants).

Although the parasite can grow in the tissues of many animals, it produces eggs (oocysts) only in cells lining the intestine of cats. Eggs are shed in a cat's stool and can survive for up to 18 months in the soil.

People may acquire the infection by transferring Toxoplasma eggs from contaminated soil or other objects to their mouth or by eating contaminated food. Occasionally, animals such as pigs acquire toxoplasmosis from contact with contaminated soil by eating eggs. People can become infected by eating raw or undercooked meat from infected animals. The parasite is rarely transmitted through blood transfusions or by an organ transplanted from an infected person.

A woman who acquires the infection during pregnancy can transfer Toxoplasma gondii to her fetus through the placenta. The result may be a miscarriage, stillbirth, or a baby born with congenital toxoplasmosis (see Some Infections of Newborns). A woman who was infected before the pregnancy does not pass the parasite on to the fetus.

People with a weakened immune system, primarily those who have AIDS or cancer or who take drugs to suppress rejection of an organ transplant, are especially at risk of toxoplasmosis. Symptoms usually develop in these people when a previously acquired Toxoplasma infection is reactivated but can develop when an organ is transplanted from an infected person. The infection usually affects the brain, but it may affect the eye or spread throughout the body (disseminate). In people with a weakened immune system due to AIDS, cancer, or immunosuppressants taken after organ transplantation or for other reasons, toxoplasmosis is very serious and may be fatal if untreated.

Symptoms

Most people with a healthy immune system have few symptoms and recover fully. Children born with congenital toxoplasmosis may be severely ill and die shortly after birth, or they may have no symptoms until months or years later. Some never become ill. Typical symptoms in newborns can include inflammation of the eyes (chorioretinitis), which can result in blindness, as well as enlargement of the liver and spleen, jaundice, rash, easy bruising, seizures, a large or small head, and mental retardation.

Toxoplasmosis acquired after birth in people with a healthy immune system seldom causes symptoms. When symptoms occur, they are usually mild and include swollen but painless lymph nodes, intermittent low fevers, a vague ill feeling, and sometimes a sore throat. Some people develop only chorioretinitis, with blurred vision, eye pain, and sensitivity to light. Chorioretinitis usually results from reactivation of congenital toxoplasmosis.

Symptoms of toxoplasmosis in people with a weakened immune system depend on the site of infection. Toxoplasmosis of the brain (encephalitis) produces symptoms such as weakness on one side of the body, trouble speaking, headache, confusion, seizures, and coma. Acute disseminated toxoplasmosis can cause a rash, high fever, chills, trouble breathing, and fatigue. In some people, infection causes inflammation of the liver (hepatitis), lungs (pneumonitis), or heart (myocarditis). The affected organ may stop functioning adequately (called organ failure). These types of toxoplasmosis can be life threatening.

Diagnosis

The diagnosis is usually based on blood tests that detect antibodies against the parasite. However, if the person's immune system is impaired by AIDS, the blood test may be falsely negative. To determine whether a fetus has been infected, a doctor usually takes a sample of the fluid around the fetus (amniotic fluid) to be analyzed. If toxoplasmosis of the brain is suspected, computed tomography (CT) or magnetic resonance imaging (MRI) of the brain is done. Less commonly, a piece of infected tissue is removed and examined under a microscope (biopsy) to identify parasites or characteristic proteins (antigens) released by the parasite.

Prevention and Treatment

Pregnant women should avoid contact with cats. If contact is unavoidable, pregnant women should at least avoid cleaning cat litter boxes or wear gloves when doing so. Meat should be cooked thoroughly, to a temperature of 165 to 170° F (74 to 77° C), and hands should be washed thoroughly after handling raw meat, soil, or cat litter. Freezing to a temperature of 9° F (13° C ) or below also destroys the parasite.

Potential organ donors should be tested to prevent the spread of the parasite through transplanted organs. Trimethoprim-sulfamethoxazole may be used to prevent toxoplasmosis. People who cannot take this drug may be given pyrimethamine with dapsone. Another option is atovaquone with or without pyrimethamine. Because pyrimethamine can damage bone marrow, leucovorin is given with it to help protect the bone marrow. People with AIDS may be given highly active antiretroviral drugs to reduce the risk of toxoplasmosis.

Infected adults without symptoms and with a healthy immune system do not require treatment. Adults with symptoms and infants with congenital toxoplasmosis are treated with sulfadiazine plus pyrimethamine and leukovorin. Higher does of pyrimethamine are typically used in people with AIDS or other conditions that weaken the immune system. If people cannot take sulfadiazine, clindamycin can be used with pyrimethamine instead. Women who acquire toxoplasmosis during pregnancy may be treated with spiramycin to prevent transmission to the fetus. In addition to these drugs, people with chorioretinitis are given prednisone or another corticosteroid to reduce inflammation. In people with AIDS, toxoplasmosis tends to recur, so drugs are often continued indefinitely.

Last full review/revision March 2007 by Richard D. Pearson, MD