Frequently Asked Questions about Toxoplasmosis
What is Toxoplasmosis?
Toxoplasmosis is a disease caused by the protozoan parasite, Toxoplasma gondii. This disease is widespread in humans and many other warm-blooded animals. It is estimated that 30-40% of all Americans have been infected with the parasite. Cats, including wild species, are the only animals which harbor the adult parasite in their intestinal tract. These adult parasites produce oocysts (eggs) that are passed in the cat's feces. These eggs must develop for 1 to 5 days in the environment before they become infective, and they can remain infective for up to 18 months in the soil and in the cat's feces. When other animals and humans become infected with this parasite, cysts are formed in their tissues (muscle, brain, etc..,), but no adult parasites develop in their intestinal tract. Although animals other than cats do not shed eggs in their feces, the cycle continues when the tissues of an infected animal are eaten by another susceptible animal or person.
How is Toxoplasmosis Transmitted to People?
The 3 main ways of spreading the parasite are:
- Transplacental (from mother to unborn baby)
- Ingestion (swallowing) of infective cysts in tissues
- Ingestion of food or water contaminated with infective eggs from feces.
The two major sources for the transmission of toxoplasmosis are meat and cat feces. By handling raw meat or eating raw or undercooked meat, a person may ingest parasite cysts and develop the disease. Although any meat source may contain Toxoplasma tissue cysts, pork appears to be the main meat source of infection in the United States. Venison and meat from other wild animals can also be sources of T. gondii cysts, as can unprocessed goat's milk.
Infected cats usually shed eggs in their feces for only 1 to 2 weeks during their lives. Cats develop immunity against the parasite, which usually prevents reinfection. Because the immune cat is unlikely to become reinfected, it is unlikely that the cat will ever shed eggs again.
When eggs are swallowed by other animals or humans, they hatch and multiply within the muscle tissue. These cysts remain in the host's tissues for the lifetime of that animal or person, which also produces immunity to reinfection.
What are the Symptoms in People?
Ingestion of only a few eggs may result in no noticeable illness, but immunity may still be developed. Illness is more likely to result when a large number of eggs or tissue cysts are ingested. The illness may vary from flu-like symptoms to more severe symptoms such as enlarged, painful lymph nodes, fever or eye infection. Any organ may be involved and the condition may spread throughout the body.
When a previously non-infected woman becomes infected during pregnancy with T. gondii, the fetus (unborn baby) may become infected as well. The effect on the fetus is most severe during the first half of the pregnancy. The unborn child may develop permanent birth defects such as malformations, mental retardation, impaired vision, deafness, and death.
Should I get rid of my Cat?
No, people should not be afraid to own a cat. Current data suggests that ownership of pet cats does not increase the risk of toxoplasmosis. However, cats that are allowed to hunt wild rodents and birds are much more likely to become infected with T. gondii than cats that are kept indoors. Infected cats rarely shed eggs in their feces more than once in a lifetime, after which transmission to humans is unlikely. Cats can be tested by a veterinarian for evidence of exposure to the toxoplasma organism. Because cats may not develop antibodies to T. gondii during the egg-shedding period, a negative serologic response does not provide useful information regarding the ability of a particular cat to transmit toxoplasmosis. A cat with a positive blood test probably has already completed its episode of egg shedding. If a woman is pregnant or considering pregnancy, she can also be tested by her physician. By checking both the woman and the cat, proper recommendations can be made.
Toxoplasmosis and Pregnancy
Approximately 30% of the women of childbearing age in the U.S. have been exposed to T. gondii and are immune to toxoplasmosis. The remaining 70% are at risk of being infected with T. gondii during pregnancy. When a woman becomes infected with T. gondii during pregnancy, there is a 20 to 50% chance that her unborn child will be infected. It is estimated that only one in every 3,000 pregnancies is complicated by toxoplasmosis. Nevertheless, because the unborn baby is so vulnerable to this disease, pregnant women should take every precaution to prevent infection with the Toxoplasma gondii parasite.
Toxoplasmosis and the Immunocompromised
People who are immunocompromised (have a reduced function of the immune system) contract zoonotic infections (infections that are shared by people and pets) more often from contaminated food, water, soil, or other people than from pets. Toxoplasmosis in immunocompromised people usually results from reactivation of previous infections. There is no additional danger of transmission of the parasite from their cat. However, toxoplasmosis in the immunocompromised is a more serious disease, with the most common manifestation being encephalitis (inflammation of the brain).
How to Avoid Infection with T. gondii
To prevent infection of cats:
- Keep cats indoors to prevent them from hunting and eating wild rodents and birds.
- Feed cats only commercially prepared food or well-cooked meat; never raw meat or raw meat products.
To prevent infection of humans:
- Wash hands thoroughly with soap and water after handling undercooked meat and before eating.
- Wash all cutting boards, sink tops, knives, and other utensils that come in contact with uncooked meat with soap and water.
- Cook meat thoroughly to 151 degrees F (66 degrees C) to destroy any parasites that might be present. Avoid tasting while cooking.
- Wash vegetables thoroughly before eating to remove soil that may be contaminated with cat feces.
- Dispose of cat litter every day before any eggs have time to become infective and sanitize litter boxes with boiling water. (Many disinfectants are ineffective against T. gondii.) Cat boxes and litter should be handled by someone other than a pregnant woman.
- Wear gloves while gardening, especially where cats may have defecated.
- Cover children's sandboxes when not in use to prevent cats from defecating in them.
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