Toxoplasmosis

Introduction

Toxoplasmosis, caused by a one-celled parasite called Toxoplasma 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, Toxoplasmosis is of grave concern to two groups: pregnant women and those with suppressed immune systems diseases, particularly people with HIV or those undergoing chemotherapy.

Lifecycle

The parasite Toxoplasma 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 lifecycle. 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 toxoplasmosis continue to harbor the organism in their body for the rest of their life 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 toxoplasmosis rarely know when they became infected.

Studies on risk factors for infection have shown that the main cause of infection is from eating undercooked meat, particularly sheep or lamb. The French tend to eat more dishes using uncooked or undercooked 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 toxoplasmosis 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 re-infected (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. Early infection in pregnancy can lead to miscarriage or severe birth defects such as blindness, mental retardation, or cerebral palsy, occurring in 10% of cases of congenital toxoplasmosis. Infection occurring later in pregnancy is not as severe, and in fact 90% of babies born with congenital toxoplasmosis look normal at birth, only to possibly develop serious vision or hearing problems, or seizures years later.

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

Toxoplasmosis Testing

The most common method of detecting Toxoplasma infection is through antibody testing. Blood tests detect the presence of antibodies to the Toxoplasma parasite called IgG or IgM. Positive IgG indicates past exposure and is evidence of immunity. Positive IgM is supposed to indicate 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 U.S.

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, (1-650-853-4828) is a U.S. Centers for Disease Control (CDC) reference laboratory. They have been able to reduce pregnancy terminations by 50 percent, by determining that a previous positive blood test was actually a false positive.

Prevention

  • 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.
  • Cook all meat, particularly sheep, beef, and pork, to a minimum of 150 degrees Fahrenheit (66 degrees C). Cook poultry to 180 degrees F.
  • 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.
  • Do not drink unpasteurized milk or milk products, especially goat's milk products.

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