Toxoplasmosis Health Article

Definition

Toxoplasmosis is an infectious disease caused by the one-celled parasitic organism Toxoplasma gondii. Although most individuals do not experience any symptoms, the disease can be very serious and even fatal in fetuses, newborns, and individuals with weakened immune systems.

Description

Toxoplasmosis is caused by a one-celled parasite Toxoplasma gondii. This parasite is found worldwide. It causes infections that can be either acute or chronic. In about 60 percent of healthy adults who become infected, the organism causes no symptoms (asymptomatic). Most of the remaining 40 percent experience mild, flu-like symptoms, low-grade fever, and fatigue that resolve without intervention in a few weeks. Once exposed, reinfection does not occur in healthy individuals. However, in immunocompromised individuals, such as those with HIV/AIDS, symptoms can be severe, life threatening, and recurring. T. gondii infection of a fetus or newborn can also cause severe neurological impairment, blindness, mental retardation, and death. When a fetus acquires the infection through its mother, this is called congenital toxoplasmosis.

Transmission

The organism that causes toxoplasmosis can be transmitted in four ways. The most common way is through contact with feces of an infected cat. Cats, the primary carriers of the organism, become infected by eating rodents and birds infected with T. gondii. Once ingested, the organism reproduces in the intestines of the cat, producing millions of eggs known as oocysts. These oocysts are excreted in cat feces daily for approximately two weeks. In the United States, approximately 50 percent of cats have been infected with T. gondii.

Oocysts are not capable of producing infection until approximately 24 hours after being excreted in warm climates and longer in cold climates. However, they remain infective in water or moist soil for about one year. Humans become infected when they come in contact with and accidentally ingest oocysts when changing cat litter, playing in contaminated sand, working in the garden or similar activities, or by eating unwashed vegetables and fruit irrigated with untreated water that has been contaminated with cat feces.

The second way humans become infected with T. gondii is through eating raw or undercooked meat. When cattle, sheep, or other livestock forage through areas contaminated with cat feces, these animals become carriers of the disease. The organism forms cysts in the muscle and brain of the livestock. When humans eat raw or undercooked infected meat, the walls of the cysts are broken down in the human digestive tract, and the individual becomes actively infected. The encysted organism can be killed by freezing or cooking the meat well.

The only form of direct person-to-person transmission occurs from mother to fetus during pregnancy. This transmission occurs only if the mother is in the acute, or active, stage of infection when the organism is circulating in the mother's blood. It is estimated that about one third of women with active infections pass the infection along to their fetus. Women who have become infected six months or more before conception do not pass the infection on to their fetus, because the organism has become dormant (inactive) and formed thick-walled cysts in muscle and other tissues of the body. Reactivation of the infection in healthy individuals is extremely rare. Women who give birth to one infected child do not pass the infection to their fetus during subsequent pregnancies unless they are immunocompromised (for example, with AIDS) and the infection recurs.

Finally, individuals can also become infected through blood and organ transplant from an infected person.