Medical Conditions - Toxoplasmosis

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The Facts

Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. On average, it is estimated that 20% to 40% of the population in North America is infected – that infection rate is even higher in other parts of the world.

This single-celled parasite is capable of living in a wide range of birds and mammals, but only produces eggs in the lining of the intestines of cats. In humans it usually causes no symptoms. However, once infected, the infection remains for life.

Toxoplasmosis usually lies dormant, but occasionally it reactivates to cause disease. Usually this happens when some other disease weakens the immune system. Toxoplasmosis is considered an opportunistic infection, one that shouldn’t harm healthy people but can be very serious if your immune defenses are down (e.g., people with AIDS or cancer, or who are taking medications that suppress the immune system). It also threatens the fetus if an expectant mother is infected during pregnancy.


The most common way people get toxoplasmosis is by ingesting toxoplasma eggs (oocysts). Cats are the primary source of these eggs. While Toxoplasma gondii can reproduce asexually in a variety of animals, only in the intestines of cats does it undergo sexual reproduction to generate eggs, which are then passed out in the cat’s feces. The eggs that are passed by the cat can remain viable in moist soil for 18 months or longer (when not exposed to direct sunlight). This means it is possible to ingest these eggs by directly contacting the cat or its litter box or by ingesting food (vegetables) from contaminated soil.

Another way in which people get toxoplasmosis is by eating undercooked meat. Animals such as cows, pigs, and sheep may ingest soil contaminated with the eggs shed by the cats. In these animals the eggs hatch, and the Toxoplasma penetrate through the intestines and form tiny cysts within their tissues. If we eat meat from these animals that is not thoroughly cooked, the cysts break apart in our stomach and the Toxoplasma inside the cysts are then released to invade our tissues. Pork and lamb are much more likely than beef to contain these cysts.

Only rarely can toxoplasmosis be transmitted by a blood transfusion or organ transplantation. Toxoplasmosis is not spread from human to human.

Toxoplasmosis is everywhere, and many of us are infected without knowing it. The good news is that most infections occur without causing any symptoms and eventually the parasites lie dormant in their cyst form in humans, just as they do in cows and sheep. They do not usually cause harm unless the immune system is not functioning properly.

Symptoms and Complications

For most of us, the immune system never reaches a state where toxoplasmosis can get enough of a foothold to cause symptoms. However, a few healthy people suffer mild symptoms from toxoplasmosis infection.

About 80% to 90% of people show no symptoms when infected with toxoplasmosis. About 10% to 20% of people will develop swollen glands and some of these people will develop symptoms that are similar to the flu or infectious mononucleosis (e.g., low-grade fever, muscle aches, sore throat, an enlarged spleen and liver, and sometimes mild diarrhea). There may also be a mild anemia. These symptoms can last for weeks or longer, but will go away without treatment.

Of people with AIDS, about 30% to 40% develop disease from toxoplasmosis, usually because of the reactivation of an old infection. Most of these infections involve the central nervous system. Symptoms can include:

  • coma
  • loss of vision or other senses
  • partial paralysis
  • weakness
  • tremors
  • headache
  • confusion
  • difficulty speaking
  • seizures
  • fever

For the majority of people with AIDS who have toxoplasmosis, the onset of symptoms is quite slow and subtle. There may be changes in emotions, behaviour, or cleanliness. Later, there may be numbness or weakness in the arm or leg.

Rarely, areas outside the central nervous system are affected and can lead to inflammation of the brain (encephalitis), its lining (meningitis), the heart (myocarditis), the lungs (pneumonitis), and various other organs. Other symptoms such as high fever, chills, sweating, and rash can occur with these infections.

Women who already have toxoplasmosis infection and become pregnant have little to worry about. But if a woman becomes infected while pregnant, there’s a risk the fetus will be infected. The risk is about 6% in the first trimester. Fetuses infected at this stage often miscarry. The risk of infection of the fetus rises to 30% if infection occurs in the second trimester and rises up to 60 – 81% if infection occurs in the third trimester. Without treatment, there is a 20 – 30% risk of passing the toxoplasmosis infection from mother-to-fetus during pregnancy.

Babies infected late in pregnancy rarely miscarry but instead have symptoms such as:

  • seizures
  • brain damage
  • enlarged spleen and liver
  • inflammation of the eye
  • jaundice (yellowing of the skin and whites of eyes)
  • poor motor coordination
  • unusually small head
  • rash
  • easy bruising

Less severe infections may not be obvious at birth but can show up months or years later. The baby often grows up into a healthy young adult. But around age 20 or 30, the eyesight starts to degenerate as toxoplasma attacks the retina, the light-sensitive membrane at the back of the eye. Ocular toxoplasmosis can cause pain, blurred vision, and permanent damage, including blindness, and can occasionally occur in adults.

Making the Diagnosis

Normally, Toxoplasma infection is easily detected by looking for antibodies to the parasite in a blood sample. The only exception to this rule is people with AIDS who have severe disease, and whose immune systems are too weak to produce detectable antibodies. Usually the disease is in the brain, and the doctor will use a computed tomography (CT) scan or magnetic resonance imaging (MRI) scan to locate small round lesions that are highly suggestive of toxoplasmosis. To diagnose toxoplasmosis during pregnancy, a sample of amniotic fluid may be used to detect the parasite.

Treatment and Prevention

People with healthy immune systems usually do not need treatment for toxoplasmosis. For people with symptoms, the treatment most frequently used is a combination of pyrimethamine* and sulfadiazine or clindamycin. There are a few minor variations, but treatment is basically similar for babies, healthy adults, and people with AIDS. People taking pyrimethamine are usually also prescribed leucovorin (also called folinic acid) to prevent toxic effects on the bone marrow.

People with ocular toxoplasmosis are also prescribed a corticosteroid (e.g., prednisone). Initial treatment is usually for 4 to 6 weeks after which re-evaluation for future treatment is done by the ophthalmologist. Even then, eye problems are likely to recur, and it takes constant vigilance to catch new flare-ups early and prevent slow deterioration of eyesight.

Pregnant women who become infected are usually treated with spiramycin as long as the fetus is not infected. If the fetus is infected and the pregnancy is beyond the first trimester (i.e., after the first 3 months), then pyrimethamine and sulfadiazine are used. Pregnant women should avoid cats if possible. If you have a cat and you are pregnant, it is recommended that you not handle cat feces (i.e., change the litter box) for the duration of your pregnancy.

To prevent getting infected with toxoplasmosis:

  • Wear gloves while working in the garden, especially if you’re handling soil, and wash your hands well with soap and warm water after touching soil.
  • Cook meat thoroughly, to at least 74°C to 77°C (164°F to 170°F), and wash hands well after touching raw meat.
  • Wash or peel fruits and vegetables.
  • Wash all kitchen tools and utensils and hands thoroughly with hot, soapy water after contact with raw meat, seafood, or unwashed fruits or vegetables.
  • Cover sandboxes to reduce the chance of cat feces getting in the sand.
  • Avoid unpasteurized milk and milk products.
  • Keep cats indoors.

*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.

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