Toxoplasmosis is an infectious disease caused by the parasite, Toxoplasma gondii, which is found throughout the world, including in the United States and other developed nations.

Cats (both wild and domestic) are the main hosts of this parasite. Cats excrete millions of these parasites in each stool for up to several weeks after becoming infected. Contact with an infected cat’s feces is the primary method of transmitting the parasite. Many different mammals (e.g., cows, pigs) and birds (including chickens) can become infected with the parasite by ingesting soil or feed contaminated with an infected cat’s feces. One way cats may become infected is by eating infected rodents, birds or other small animals.

Cats are called the definitive host because they are the only hosts that can produce the adult, mature stage of the parasite. Other animals, including humans, are called intermediate hosts.

People may become infected with the Toxoplasma parasite in a variety of ways, including:

• Contact with cat feces
  
  
• Consuming infected meat that is raw or undercooked
  
  
• Consuming unwashed fruits or vegetables grown in contaminated soil
  
  
• Transmission by an infected pregnant woman to her fetus

Infection by the Toxoplasma parasite is common and it does not pose a threat to most healthy people. A healthy, functioning immune system is able to combat the parasite and prevent symptoms of illness. However, toxoplasmosis can be extremely dangerous for certain populations, including infants born to women who become infected during pregnancy, and individuals with compromised immune systems.

In the rare cases where generally healthy people develop symptoms of infection, the symptoms are typically mild, and may resemble the flu or mononucleosis (e.g., muscle aches, fever). People with weakened immune systems may exhibit more severe signs of infection, such as confusion and seizures. Babies born to mothers who were exposed to the Toxoplasma parasites during pregnancy may have symptoms including an enlarged head, liver or spleen, seizures, eye infections, yellowing of the skin and the whites of the eyes (jaundice), and developmental delays.

In people with healthy immune systems, medical treatment for toxoplasmosis is usually not necessary because minor symptoms typically subside within a few weeks. However, when severe symptoms are present, medical treatment is urgent. Left untreated, toxoplasmosis can cause organ damage (e.g., brain, heart, muscles) and may be potentially life-threatening. People with weakened immune systems should consult their physicians about a blood test for Toxoplasma antibodies.

In addition, pregnant women who may have come into contact with the Toxoplasma parasites need to undergo testing to ensure that parasites were not passed to the fetus via the placenta. Toxoplasmosis acquired by the fetus (congenital toxoplasmosis) may result in miscarriage, stillbirth or birth defects.

Antibiotics specifically designed to kill the parasites are available to treat toxoplasmosis, though they can sometimes have adverse side effects (e.g., suppression of bone marrow). The best prevention method is to avoid contact with potentially contaminated sources (e.g., cat feces, raw meat, unpasteurized milk) that may cause infection.