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Toxoplasmosis -ˌplaz-ˈmō-səs

By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

Toxoplasmosis is infection caused by the single-celled protozoan parasite Toxoplasma gondii. Usually, the infection does not cause symptoms, but some people have swollen lymph nodes, fever, a vague ill feeling, and sometimes a sore throat, blurred vision, and eye pain.

  • People acquire the infection by transferring the parasite's eggs from a contaminated object to the mouth or by eating contaminated food.

  • Most infections cause few or no symptoms.

  • Women who become infected during pregnancy may transmit the parasite to the fetus, sometimes causing a miscarriage, stillbirth, or serious problems in the baby.

  • Typically, only people with a weakened immune system have severe symptoms, usually resulting from brain inflammation (encephalitis), which may cause weakness on one side of the body, confusion, or coma.

  • Less often, other organs are affected in people with a weakened immune system.

  • Doctors usually diagnose the infection by doing blood tests that detect antibodies against the parasite.

  • Cooking meat thoroughly or freezing it and washing the hands thoroughly after handling raw meat, soil, or cat litter help prevent spread of the infection.

  • Most healthy people do not require treatment, but adults with symptoms and newborns and infants with the infection are treated with sulfadiazine plus pyrimethamine and leucovorin.

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).

Did You Know...

  • The toxoplasmosis parasite produces eggs only in the intestine of cats.

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, after 1 to 5 days, become able to cause infection. Wild birds, rodents, deer, and many domestic animals (particularly pigs and sheep) may ingest the eggs in food or soil contaminated by cat stool. The eggs release forms of the parasite called tachyzoites. Tachyzoites spread in the animal's tissues and eventually form cysts.

Life Cycle of Toxoplasma gondii

  • 1a. Eggs are passed in cat stool. Many eggs are passed but usually for only 1 to 2 weeks. After 1 to 5 days in the environment, eggs become able to cause infection.

  • 1b. Cats can become reinfected by consuming food or other materials contaminated with the eggs.

  • 2. Other animals (such as wild birds, rodents, deer, pigs, and sheep) may consume the eggs in contaminated soil, water, plant material, or cat litter.

  • 3. Shortly after the eggs are consumed, they release forms of the parasite that can move (called tachyzoites).

  • 4. Tachyzoites spread throughout the animal's body and form cysts in nerve and muscle tissue.

  • 5. Cats become infected after eating animals that contain these cysts.

  • 6a. People can become infected by eating undercooked meat containing these cysts.

  • 6b. People can also become infected if they eat food, water, or other materials (such as soil) contaminated with cat stool or when they touch a pet cat's litter and then touch their mouth.

  • 7. Rarely, people are infected when they have a blood transfusion or organ transplant that contains the parasite.

  • 8. Rarely, the infection is spread from mother to fetus.

  • 9. In people, parasites form cysts in tissues, usually in muscle and the heart, brain, and eyes.

Transmission of Toxoplasmosis

People may acquire the infection in several ways:

  • Eating food, water, or other materials (such as soil) that are contaminated with cat stool containing Toxoplasma eggs

  • Eating meat that contains Toxoplasma cysts

  • Spread from a newly infected mother to a fetus

People may swallow Toxoplasma eggs after they touch contaminated cat litter, soil, or other objects and then touch their mouth or handle and eat food without washing their hands. People may swallow cysts when they eat raw or undercooked meat (usually pork or mutton) from infected animals. Rarely, the parasite is transmitted through blood transfusions or by an organ transplanted from an infected person.

Toxoplasmosis during pregnancy

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 birth defects or congenital toxoplasmosis.

A woman who was infected before the pregnancy does not pass the parasite on to her fetus unless her immune system has been weakened (for example by HIV infection), reactivating her infection.

Toxoplasmosis in people with a weakened immune system

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. If they have been infected in the past, developing a disorder that weakens the immune system or taking a drug that suppresses the immune system (immunosuppressants) may cause the infection to be reactivated.

A reactivated infection usually causes symptoms. The initial infection often causes no symptoms. A reactivated infection usually affects the brain, but it may affect the eye or spread throughout the body (disseminate).

In people with a weakened immune system, toxoplasmosis is very serious and may be fatal if untreated.

Symptoms of Toxoplasmosis

Most people with a healthy immune system have few toxoplasmosis 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 intellectual disability. Sometimes people develop chorioretinitis later in life when congenital toxoplasmosis is reactivated. Such people have blurred vision, eye pain, and sensitivity to light.

Toxoplasmosis acquired after birth causes symptoms in only 10 to 20% of people with a healthy immune system. When symptoms of toxoplasmosis occur, they are usually mild and include swollen but painless lymph nodes, intermittent low fevers, a vague ill feeling, and sometimes a sore throat. The symptoms disappear on their own, usually after several weeks, although sometimes they last for months.

Toxoplasmosis symptoms in people with a weakened immune system depend on the site of infection, as in the following:

  • Toxoplasmosis of the brain (encephalitis): Symptoms such as weakness on one side of the body, trouble speaking, headache, confusion, seizures, and coma

  • Toxoplasmosis that has spread throughout the body (acute disseminated toxoplasmosis): 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 of Toxoplasmosis

  • Blood tests to detect antibodies against the parasite

  • If the brain may be affected, computed tomography or magnetic resonance imaging, followed by a spinal tap

The diagnosis is usually based on blood tests that detect antibodies against the parasite. (Antibodies are proteins produced by the immune system to help defend the body against attack, including by parasites.) The blood tests can be used to diagnose a new infection.

Doctors may do these blood tests in people who have a weakened immune system but have no symptoms of toxoplasmosis. The tests are done to look for evidence of a previous infection, which could reactivate if their immune system weakens further. However, if the person’s immune system is impaired by AIDS, the blood tests may indicate no infection when one is present (false-negative results).

If people report eye problems, doctors check the eyes for the damage typically caused by toxoplasmosis.

To determine whether a fetus has been infected, a doctor can take a sample of the fluid around the fetus (amniotic fluid) to be analyzed (a procedure called amniocentesis) to check for the parasite's genetic material (DNA). The test is usually done after the 18th week of pregnancy.

If toxoplasmosis of the brain is suspected, computed tomography (CT) or magnetic resonance imaging (MRI) of the brain is done, usually followed by a spinal tap (lumbar puncture). Less commonly, a piece of infected brain tissue is removed and examined under a microscope (biopsied) to identify parasites, characteristic proteins (antigens) released by the parasite, or genetic material of the parasite.

Prevention of Toxoplasmosis

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 (an antibiotic) may be used to prevent reactivation of toxoplasmosis in certain people with AIDS or other conditions that cause the immune system to be weakened. People who cannot take this drug may be given pyrimethamine (used to treat malaria) plus sulfadiazine or clindamycin (antibiotics). Other options are atovaquone (an antiprotozoal drug) with or without pyrimethamine and dapsone with pyrimethamine. Because pyrimethamine can decrease cell production in the bone marrow, leucovorin (also called folinic acid) is given with it to help protect against this side effect.

People with AIDS are also given antiretroviral drugs, which helps strengthen their immune system and reduce the risk that toxoplasmosis will be reactivated.

Treatment of Toxoplasmosis

  • Sulfadiazine (an antibiotic) plus pyrimethamine and leucovorin

  • Clindamycin (an antibiotic) or atovaquone plus pyrimethamine

  • For eye infections, drugs effective against toxoplasmosis and a corticosteroid

Most infected people without symptoms and with a healthy immune system do not require treatment.

People with symptoms can be treated with sulfadiazine plus pyrimethamine and leucovorin. If their immune system is normal, they are usually treated for a few weeks. People with AIDS or other conditions that weaken the immune system are treated longer (usually for 6 weeks), and the drugs are continued until their immune system improves. If people cannot take sulfadiazine, clindamycin or atovaquone can be used with pyrimethamine instead.

People with an eye infection can be given pyrimethamine plus sulfadiazine (or clindamycin) plus leucovorin. Prednisone or another corticosteroid is often given at the same time to reduce inflammation.

Women who acquire toxoplasmosis during pregnancy should see a doctor who specializes in toxoplasmosis during pregnancy. The choice of drugs is complicated and depends on when the pregnant woman acquires the infection (which trimester) and whether the fetus has already been infected.

Newborns who were infected before birth are usually given pyrimethamine, sulfadiazine, and leucovorin for a year after birth.

Did You Know...

  • Women who acquire toxoplasmosis during pregnancy should see a doctor who specializes in toxoplasmosis during pregnancy.

In people with AIDS, toxoplasmosis tends to recur, so drugs to control toxoplasmosis are continued until the immune system improves (indicated by an increase in the CD4 count) and symptoms resolve. Doctors also make sure that the most effective antiretroviral drugs are being used.

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