Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. 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 few or no symptoms. On average, it is estimated that 10% to 20% of the population in North America is infected – that infection rate is higher in other parts of the world.
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. 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.
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. If you have AIDS, your doctor will determine if it is advanced enough to prescribe a daily antibiotic to help prevent toxoplasmosis infection.
People with ocular toxoplasmosis are also prescribed a corticosteroid (e.g., prednisone). 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) or wear gloves while doing so for the duration of your pregnancy.
To prevent getting infected with toxoplasmosis:
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. If you have AIDS, your doctor will determine if it is advanced enough to prescribe a daily antibiotic to help prevent toxoplasmosis infection.
People with ocular toxoplasmosis are also prescribed a corticosteroid (e.g., prednisone). 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) or wear gloves while doing so for the duration of your pregnancy.
To prevent getting infected with toxoplasmosis:
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