Toxoplasmosis

September 29, 2011

Toxoplasmosis, caused by a one-celled parasite called Toxoplasma gondii (T gondii), is one of the most widespread infections in the world, affecting roughly 50% of the world’s population, regardless of gender. Generally a mild, harmless infection, tox-oplasmosis is of grave concern to two groups of patients: pregnant women and those with suppressed immune systems, due to HIV infection or chemotherapy treatments.

LIFE CYCLE

The parasite T. gondii is capable of infecting all warm-blooded mammals and birds. It is found in cats, sheep, rodents, swine, cattle, and humans. Cats hold the unique distinction of being the only definitive host for this organism, for it is only in the cat that the organism can progress through all of the stages of its life cycle. Up to 60% of all domestic cats harbor toxoplasma in their body.

The organism enters the cat through the digestive tract, typically when the cat eats an infected rodent or bird. The cat’s immune response attacks the organism, but rather than being killed, the organism enters a new phase where it is protected from the immune system and continues to survive. The cat (and only the cat) then excretes in its feces a form of the organism that is highly contagious if ingested. But this phase of the infection only lasts about 20 days in the cat’s life, after which the cat (while still harboring the parasite) is no longer capable of transmitting infection.

All other animals infected with toxoplasma continue to harbor the organism in their body for the rest of their lives but they are not contagious and the organism is not excreted in their feces, which is why all other hosts are referred to as intermediate hosts.

HUMAN INFECTION

About 30% of Americans and Britains have had toxoplasmosis, but in France as many as 65% (in some regions as high as 95%) have had it. The symptoms are generally so mild that people testing positive for toxo-plasmosis rarely know when they became infected.

Studies on risk factors for infection have shown that the main cause of infection is from eating under-cooked meat, particularly sheep or lamb. The French tend to eat more dishes using uncooked or under-cooked meat, thus their higher rate of toxoplasmosis exposure. Unwashed vegetables can transmit the organism as it can survive in soil. Cat ownership, in some studies, has not been shown to be a risk factor for tox-oplasmosis infection, presumably because there are so few days in the cat’s life where their feces actually contain infectious organisms.

If someone has had toxoplasmosis once, they are forever immune afterwards, but if later in life they develop an immunodeficiency condition (such as HIV or cancer chemotherapy), then they can become rein-fected (called recrudescence) and can develop a life-threatening infection in the brain called encephalitis.

TOXOPLASMOSIS AND PREGNANCY

Toxoplasmosis, while harmless to the mother, can have disastrous consequences for the fetus. In early pregnancy, infection is less common but has more severe consequences. These include miscarriage or severe birth defects such as blindness, mental retardation, or cerebral palsy which occur in 10% of cases of congenital toxoplasmosis. Fetal infection more commonly occurs in the latter half of pregnancy but fetal manifestations are not typically evident. In fact, 90% of babies born with late-onset congenital toxoplasmosis look normal at birth, with only some of them developing serious vision or hearing problems, or seizures many years later.

Documented infection can and should be treated during pregnancy. This has been shown to help reduce fetal harm and prevent some cases of congenital toxo-plasmosis. It is estimated that from 1 to 10 out of 10,000 babies born in the United States have congenital toxo-plasmosis, about 400-4,000 cases per year.

TOXOPLASMOSIS TESTING

The most common method of detecting toxo-plasma infection is through antibody testing. Blood tests detect the presence of IgG or IgM antibodies to the toxoplasma parasite. Positive IgG indicates past exposure and is evidence of immunity. Positive IgM usually indicates current infection, but there have been problems with commercial testing for IgM resulting in a high level of false positives. Universal screening is not practiced in the United States.

Therefore, it is recommended that all pregnant women testing positive for IgM have repeat tests run at a toxoplasma reference laboratory. The Toxoplasma Serology Laboratory in Palo Alto, California (www.pamf. org, 1-650-853-4828) is a U.S. Centers for Disease Control (CDC) reference laboratory. Testing done by these highly specialized laboratories can determine whether a previous positive blood test was actually a false positive. By having such data available for counseling, health care providers have been able to reduce anxiety and possible consideration of pregnancy terminations for perceived toxoplasmosis by 50%.

PREVENTION

  1. Avoid contact with cat litter or wear gloves. Wear gloves while gardening because the organisms can live in the soil. Cats also like to defecate in children’s sandboxes.
  2. Cook all meat, particularly sheep, beef, and pork, to a minimum of 150°F (66°C). Cook poultry to 180°F.
  3. Thoroughly wash all fruits and vegetables, or peel them. Using soap and hot water, wash all surfaces that have come into contact with raw meat or poultry.
  4. Do not drink unpasteurized milk or milk products, especially goat’s milk products.

See Also: Acquired immunodeficiency syndrome, Miscarriage, Pregnancy

Suggested Reading

  • Howard, B., Chow, G., & Fairchok, M. (2002). Congenital toxoplas-mosis. In S. Ransom, M. Dombrowski, M. Evans, & K. Ginsburg (Eds.), Contemporary therapy in obstetrics and gynecology (pp. 328-329). Philadelphia: W.B. Saunders.
  • Lopez, A., Dietz, V., Wilson, T., & Jones, J. (2000). Preventing congenital toxoplasmosis. Morbidity and Mortality Weekly Report, 49, 57-75.

 

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