Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Mar 2023
Topic Resources

Toxoplasmosis is infection caused by the protozoan parasite Toxoplasma gondii. Infection occurs when people unknowingly ingest toxoplasma cysts from cat feces or eat contaminated meat. Usually, the infection does not cause symptoms, but some people have swollen lymph nodes, fever, a vague ill feeling, and sometimes a sore throat or blurred vision and eye pain. In people with a weakened immune system due to AIDS or another condition, toxoplasmosis can reactivate, usually affecting the brain. A reactivated infection can cause weakness, confusion, seizures, or coma or spread throughout the body. Pregnant women can pass the infection to a fetus(called congenital infection), and infected babies can have birth defects, loss of vision, seizures, intellectual disability, and other abnormalities.

  • People acquire the infection by transferring the parasite's eggs from an object contaminated with infected cat feces 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 Overview of Immunodeficiency Disorders Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more 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.

  • Most healthy people with toxoplasmosis do not require treatment, but adults with eye involvement, severe or persistent symptoms, or weakened immune systems, and pregnant women, and newborns with congenital infection are treated.

  • People with AIDS or other conditions that weaken the immune system are treated until the disease is under control and are then placed on preventative therapy until their immune deficiency is reversed by anti-retroviral therapy (for those with AIDS) or other measures.

  • 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.

Toxoplasma gondii is a protozoa (single-cell infectious organisms) 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 a disorder that weakens the immune system (such as AIDS Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause... read more Human Immunodeficiency Virus (HIV) Infection or cancer Overview of Cancer A cancer is an abnormal growth of cells (usually derived from a single abnormal cell). The cells have lost normal control mechanisms and thus are able to multiply continuously, invade nearby... read more ), or who use medications that suppress the immune system (immunosuppressants), particularly those used to suppress rejection of an organ transplant (see tables and ).

Did You Know...

  • Pregnant women may become infected with the toxoplasmosis parasite by exposure to infected food or cat litter, and this can pass to a fetus and cause miscarriage, stillbirth, or serious problems in the baby.

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. Eggs in the soil are able to cause infection for months. 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. Toxoplasma 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 and become infected.

  • 3. Shortly after the eggs are consumed, the eggs 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. The cysts may remain for the rest of the person's life without causing symptoms. They may become active and cause symptoms if the person's immune system is weakened by a disorder or medication.

Life Cycle of Toxoplasma gondii

Transmission of Toxoplasmosis

People may acquire toxoplasmosis 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

  • Rarely, having a blood transfusion or organ transplant that contains the parasite

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 lamb) 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. Infection is more severe if the fetus is infected early in the pregnancy. The result may be a fetus that grows slowly, preterm birth, a miscarriage, stillbirth, or a baby born with birth defects. Congenital toxoplasmosis can cause problems with vision, seizures, and intellectual disabilities later in life.

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

A reactivated infection is most likely to occur in 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 or no toxoplasmosis symptoms and recover fully. About 10 to 20% of these people have swollen but painless lymph nodes. A few of these people also have intermittent low fevers, a vague ill feeling, muscle aches, and sometimes a sore throat. The symptoms disappear on their own, usually after several weeks.

Congenital toxoplasmosis

Children born with congenital toxoplasmosis may be severely ill and die before or shortly after birth, or they may have birth defects. Some never become ill. Others appear healthy at first but develop symptoms (such as seizures, intellectual disability, or vision problems) months or even years later.

Typical symptoms in newborns can include

Chorioretinitis can cause blurred vision, eye pain, sensitivity to light, and blindness.

Symptoms in people with a weakened immune system

In some people (usually those with a greatly weakened immune system), toxoplasmosis causes inflammation of the lungs (pneumonitis), the heart (myocarditis Myocarditis Myocarditis is inflammation of the muscle tissue of the heart (myocardium) that causes tissue death. Myocarditis may be caused by many disorders, including infection, toxins and drugs that affect... read more ), or, less commonly, the liver (hepatitis Overview of Hepatitis Hepatitis is inflammation of the liver. (See also Overview of Acute Viral Hepatitis and Overview of Chronic Hepatitis.) Hepatitis is common throughout the world. Hepatitis can be Acute (short-lived) read more ). The affected organ may stop functioning adequately (called organ failure). Without treatment, these types of toxoplasmosis are usually fatal.

Diagnosis of Toxoplasmosis

The diagnosis of toxoplasmosis is usually based on blood tests that detect antibodies against the parasite. (Antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies are proteins produced by the immune system to help defend the body against attack, including that 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).

Sometimes tests to check for the parasite's genetic material (DNA) are also done on the samples of blood, tissue from a biopsy, or cerebrospinal fluid (the fluid that surrounds the brain or spinal cord) obtained from a spinal tap Spinal Tap Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Imaging tests commonly used to diagnose nervous system (neurologic) disorders... read more Spinal Tap .

If people report eye problems, doctors check the eyes for the damage typically caused by toxoplasmosis and do blood tests to check for antibodies to the parasite.

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 Amniocentesis Prenatal testing for genetic disorders and birth defects involves testing a pregnant woman or fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including... read more ). The fluid is tested for antibodies to the parasite and for the parasite's genetic material. Because diagnosing toxoplasmosis during pregnancy or in the fetus or newborn is difficult, doctors often consult with an expert.

Treatment of Toxoplasmosis

  • Pyrimethamine and either sulfadiazine, clindamycin, or atovaquone, plus leucovorin; or alternatively, trimethoprim-sulfamethoxazole

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

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

People with symptoms of toxoplasmosis can be treated with pyrimethamine, sulfadiazine, and leucovorin. Leucovorin is given to help protect against decreased blood cell production in the bone marrow, which is a side effect of pyrimethamine. If people cannot take sulfadiazine, clindamycin, or atovaquone can be used instead. When pyrimethamine is not available, trimethoprim-sulfamethoxazole is used.

If people have a healthy immune system, they are usually treated for a few weeks.

People with AIDS or other conditions that weaken the immune system are given the same medications but are treated longer (usually for at least 6 weeks) until all signs of infection are gone. Relapses are common, and there are several options for chronic maintenance therapy, which is continued until their immune system improves.

In people with AIDS, toxoplasmosis tends to recur, so medications to control toxoplasmosis are continued indefinitely unless the immune system improves (indicated by an increase in the CD4 count Monitoring Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause... read more Monitoring to an acceptable level). Doctors make sure that the most effective antiretroviral medications are being used.

People with an eye infection can be given pyrimethamine plus sulfadiazine (or clindamycin) plus leucovorin. Trimethoprim-sulfamethoxazole is an alternative. Prednisone or another corticosteroid is usually given at the same time to reduce inflammation inside of the eye.

Women who acquire toxoplasmosis during pregnancy should see a doctor who specializes in toxoplasmosis during pregnancy. The choice of medications is complicated and depends on when the pregnant woman acquired the infection (which trimester) and whether the fetus has already been infected. Pyrimethamine can cause birth defects and is not used during the 1st trimester of pregnancy. Spiramycin (an antibiotic) may be used during the 1st trimester to help prevent the spread of toxoplasmosis from the woman to the fetus. Spiramycin is not commercially available in the United States.

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

Did You Know...

  • Pregnant women should avoid becoming infected with toxoplasmosis by washing hands, eating only fully cooked meat and seafood, avoiding contact with cat feces, and wearing gloves while gardening.

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. Gloves should also be worn while gardening to avoid contact with soil.

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.

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 medication may be given pyrimethamine (an antiprotozoan medication) plus sulfadiazine or clindamycin (antibiotics). Other options are atovaquone (an antiprotozoan medication) with or without pyrimethamine, or dapsone with pyrimethamine. Because pyrimethamine can decrease blood cell production in the bone marrow, leucovorin (also called folinic acid) is given with it to help protect against this side effect.

More Information

The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.

Drugs Mentioned In This Article

Generic Name Select Brand Names
No brand name available
Cleocin, Cleocin Ovules, Cleocin Pediatric, Cleocin T, CLIN, Clindacin ETZ, Clindacin PAC, Clindacin-P, Clinda-Derm , Clindagel, ClindaMax, ClindaReach, Clindesse, Clindets, Evoclin, PledgaClin, XACIATO
No brand name available
Primsol, Proloprim, TRIMPEX
Deltasone, Predone, RAYOS, Sterapred, Sterapred DS
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