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Medically reviewed by Last updated on May 10, 2023.

What is Toxoplasmosis?

Harvard Health Publishing

Toxoplasmosis is a parasitic infection that infects a large proportion of the world's population, but rarely causes disease. Certain people, however, are at high risk of severe or life-threatening disease from this parasite. They include infants who are infected at birth, people with AIDS, people with cancer, and people who have had bone marrow or organ transplantation.

Toxoplasmosis is an infection caused by Toxoplasma gondii, a single-celled parasite that spends most of its life cycle inside cats. Because an infected cat can pass millions of Toxoplasma parasites daily in its feces, toxoplasmosis can spread easily to almost any other animal that shares the environment with cats. In humans, Toxoplasma parasites usually enter the body by being swallowed. This can happen when people touch their mouth with soiled hands, especially after changing cat litter, or if they eat pork, lamb or venison that has not been cooked thoroughly.

The Toxoplasma parasites multiply within cells that line the human digestive tract. Toxoplasma parasites can spread to almost any organ in the body, including the brain, skeletal muscles, heart muscle, eyes, lungs and lymph nodes. In healthy people, the body's immune system eventually stops the spread of Toxoplasma parasites, although some remaining parasites can lie dormant indefinitely in the brain or retina.

In people whose immune defenses are weakened because of AIDS, cancer or immunosuppressant medication, a new toxoplasmosis infection may spread out of control and become deadly, or dormant Toxoplasma parasites from an old toxoplasmosis infection may suddenly become active again and cause severe illness. This situation is especially dangerous for people with AIDS. In these people, dormant toxoplasmosis can reactivate and cause a severe brain infection (encephalitis), which can lead to seizures and other neurological problems. If left untreated, the death rate from encephalitis is very high. In addition to being swallowed, Toxoplasma parasites can enter the body through contaminated blood transfusions or through organ transplants taken from infected donors. Also, if a toxoplasmosis infection develops in a pregnant woman, the parasites can cross the placenta and cause toxoplasmosis in the infant. This is called congenital toxoplasmosis. These newborns are at high risk of toxoplasmosis-related eye problems and developmental disabilities.


In people with normal immune defenses, up to 90% of cases of toxoplasmosis do not cause any symptoms, so the infection often is not recognized. In the relatively few cases in which symptoms do develop, the most common symptoms are:

  • Painless swelling of the lymph nodes
  • Headache
  • Malaise (a general sick feeling)
  • Fatigue
  • Low-grade fever

In rare cases, patients also have experienced muscle aches, sore throat, abdominal pain, rash or neurological symptoms.

In people with weakened immune systems, especially those with AIDS, symptoms of toxoplasmosis are often brain-related and severe. These symptoms can include:

  • Disturbances in mental functions, especially disorientation, difficulty concentrating, or behavioral changes
  • Fever
  • Headache
  • Seizures
  • Disturbances in nerve function, especially abnormal movements, difficulty walking, difficulty speaking and partial loss of vision

Also, if toxoplasmosis affects the eyes of a person with a weakened immune system, there may be blurred vision, "spots" in the field of vision, eye pain and extreme sensitivity to light. If toxoplasmosis affects the lungs, there can be shortness of breath, fever, a dry cough, coughing up of blood and, eventually, respiratory failure.

If a woman develops toxoplasmosis during pregnancy or within six weeks before becoming pregnant, her child may be born with congenital toxoplasmosis. The child often does not have any symptoms at birth. However, a thorough exam usually will uncover signs of infection in the infant's eyes. Other symptoms in newborns can include:

  • Unusually small body size
  • Strabismus, an eye that wanders or is misaligned, or other eye problems
  • Head size that is unusually large or unusually small
  • Convulsions
  • Jaundice
  • Enlarged lymph nodes
  • Abnormal bruising
  • Rash
  • Developmental delays and, sometimes, mental retardation

In addition, congenital toxoplasmosis increases the risk of fetal death or premature birth.


Your doctor will ask about your medical history to find out whether you have any medical problem that would weaken your body's immune defenses against toxoplasmosis, including HIV or AIDS, cancer, an inherited immune deficiency or an organ transplant. In addition, your doctor will review your current medications to check for any drug that may suppress or damage your immune defenses, allowing dormant Toxoplasma parasites to become active. Your doctor also will ask about your exposure to cats, especially outdoor cats that kill and eat small prey. To evaluate your risk of food-related toxoplasmosis, your doctor will ask whether you often eat raw or very rare meat.

If you have symptoms of toxoplasmosis, your doctor will examine you to check for enlarged lymph nodes (swollen glands), signs of brain involvement and eye damage. To confirm the diagnosis, your doctor will order blood tests to check for antibodies (defensive proteins made by the immune system) against the Toxoplasma parasite. Depending on your blood levels of certain antibodies, the doctor may be able to tell whether you have active toxoplasmosis or whether you had a past episode of toxoplasmosis. Most healthy people do not remember a past episode, because 90% of them never have symptoms. If you have an acute toxoplasmosis infection, the diagnosis can be confirmed by identifying Toxoplasma parasites in samples of your blood, body fluids or infected tissues.

If your doctor suspects that toxoplasmosis involves your brain, he or she will order a computed tomography (CT) scan or magnetic resonance imaging (MRI) scan of your head to check for evidence of encephalitis.

Congenital toxoplasmosis can be diagnosed before birth using ultrasound or a procedure called amniocentesis. After birth, the infant may have the following tests: eye exam, neurological exam, CT scan of the head, and laboratory analysis of cerebrospinal fluid taken during a lumbar puncture (spinal tap).

Expected Duration

If you have a healthy immune system, mild symptoms of toxoplasmosis probably will subside within a few weeks, even without medical treatment. Rarely, swollen lymph nodes go away more slowly, sometimes within months. After acute symptoms pass, some dormant Toxoplasma parasites may last in the body for decades but usually will not cause any symptoms, unless the immune system is compromised.

However, if you have a weakened immune system because of a disease such as AIDS, then you will need therapy for toxoplasmosis for as long as your immune system remains weak, because the disease usually returns when treatment is stopped. If your immune system is strengthened with the use of highly active antiretroviral therapy, it may be possible to stop toxoplasmosis therapy.


You can help prevent toxoplasmosis by taking the following precautions:

  • Do not eat meat that is raw or rare. If you have a meat thermometer, cook meat to an internal temperature of at least 140° Fahrenheit.
  • Wash your hands thoroughly after you handle raw meat, after you work in your garden, and after you change a cat's litter box.
  • If you are pregnant or have a weakened immune system, do not handle raw meat or change a cat's litter box. If you cannot avoid doing these things, use gloves.
  • If you own a cat, keep it indoors and feed it store-bought canned or dry cat food.
  • If you have HIV, you will be tested to see if you have antibodies against toxoplasmosis in your blood, which would indicate that you were infected in the past. If the antibody test is positive, and your immune system is severely weakened, you will be treated with medications, such as the antibiotic trimethoprim sulfamethoxazole (Proloprim, Trimpex) to prevent the disease from reactivating. If the antibody test is negative, you will be counseled to avoid infection by using the techniques described above.


In most cases, if you are a generally healthy person, no treatment is necessary unless your symptoms are severe or unusually persistent. If toxoplasmosis affects your eyes, your doctor may treat you with pyrimethamine (Daraprim) combined with either sulfadiazine (Microsulfon) or clindamycin (Cleocin).

If you have a weakened immune system, your doctor will treat you with a combination of drugs to kill the Toxoplasma parasite. The usual treatment of choice is pyrimethamine combined with sulfadiazine. Other drugs that may be used include trimethoprim-sulfamethoxazole (Bactrim, Septra), clindamycin and atovaquone (Mepron).

Newborns with congenital toxoplasmosis are treated for a least one year with combination therapy — pyrimethamine plus sulfadiazine or another combination that is equally effective.

If you develop toxoplasmosis during pregnancy, your doctor can prescribe medications that will reduce the risk that your child will develop congenital toxoplasmosis. These medications include spiramycin (Rovamycine), pyrimethamine, and sulfadiazine. To decrease the possibility of drug-related birth defects, the type and timing of medications depend on which trimester you are in.

People treated with pyrimethamine should receive folinic acid (leucovorin) to help prevent side effects.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

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When To Call A Professional

Call your doctor if you develop symptoms of toxoplasmosis, especially if you are pregnant or have any medical condition that weakens the immune system. If you are planning to become pregnant, ask your gynecologist about your need for a pre-pregnancy blood test for Toxoplasma.


People with AIDS who have recovered from acute toxoplasmosis are at high risk of future episodes, because the dormant parasite may be reactivated. To prevent this, an AIDS patient must begin a regimen of preventive drugs and continue to take the medications as long as his or her immune system remains weakened.

One popular prophylactic drug combination — trimethoprim-sulfamethoxazole — also helps to prevent Pneumocystis jiroveci (formerly called Pneumocystis carinii) pneumonia, an infection that targets AIDS patients with weakened immune systems. This drug combination may be responsible for the decrease in toxoplasmosis of the brain seen in AIDS patients.

Many congenital toxoplasmosis cases can be cured with medications. Even children who had severe infections at birth may never show signs of severe long-term damage if they are diagnosed and treated early. Delays in diagnosis and treatment can contribute to a poor prognosis.

If a pregnant woman develops toxoplasmosis, her child's risk of congenital toxoplasmosis decreases 60% if she is treated properly with medication.

Additional Info

National Institute of Allergy and Infectious Diseases (NIAID)


Learn more about Toxoplasmosis

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Further information

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