Toxoplasmosis - Human

Description

T gondii is an important zoonotic agent. In some areas of the world, up to 60% of the human population have serum IgG titers to T gondii and are likely to be persistently infected.

Signalment

Toxoplasmosis is a major concern for people with immune system dysfunction (eg, people infected with human immunodeficiency virus). In these individuals, toxoplasmosis usually presents as meningoencephalitis and results from the emergence of T gondii from tissue cysts located in the brain as immunity wanes rather than from primary T gondii infection. Toxoplasmosis is also a major concern for pregnant women because tachyzoites can migrate transplacentally and cause birth defects in human fetuses. Infection of women with T gondii may occur after ingestion of undercooked meat or accidental ingestion of oocysts from cat feces.

Diagnosis

Clinical Signs

Most people with a healthy immune system have few symptoms and recover fully. Children born with congenital toxoplasmosis may be severely ill and die shortly after birth, or they may have no symptoms until months or years later. Some never become ill. Typical symptoms in newborns can include inflammation of the eyes (chorioretinitis), which can result in blindness, as well as enlargement of the liver and spleen, jaundice, rash, easy bruising, seizures, a large or small head, and mental retardation.

Toxoplasmosis acquired after birth in people with a healthy immune system seldom causes symptoms. When symptoms occur, they are usually mild and include swollen but painless lymph nodes, intermittent low fevers, a vague ill feeling, and sometimes a sore throat. Some people develop only chorioretinitis, with blurred vision, eye pain, and sensitivity to light. Chorioretinitis usually results from reactivation of congenital toxoplasmosis.

Symptoms of toxoplasmosis in people with a weakened immune system depend on the site of infection. Toxoplasmosis of the brain (encephalitis) produces symptoms such as weakness on one side of the body, trouble speaking, headache, confusion, seizures, and coma. Acute disseminated toxoplasmosis can cause a rash, high fever, chills, trouble breathing, and fatigue. In some people, infection causes inflammation of the liver (hepatitis), lungs (pneumonitis), or heart (myocarditis). The affected organ may stop functioning adequately (called organ failure). These types of toxoplasmosis can be life threatening.

mmune system dysfunction (eg, people infected with human immunodeficiency virus). In these individuals, toxoplasmosis usually presents as meningoencephalitis and results from the emergence of T gondii from tissue cysts located in the brain as immunity wanes rather than from primary T gondii infection. Toxoplasmosis is also a major concern for pregnant women because tachyzoites can migrate transplacentally and cause birth defects in human fetuses.

Laboratory Tests

The diagnosis is usually based on blood tests that detect antibodies against the parasite. However, if the person's immune system is impaired by AIDS, the blood test may be falsely negative. To determine whether a fetus has been infected, a doctor usually takes a sample of the fluid around the fetus (amniotic fluid) to be analyzed.

Diagnostic Imaging

If toxoplasmosis of the brain is suspected, computed tomography (CT) or magnetic resonance imaging (MRI) of the brain is done. Less commonly, a piece of infected tissue is removed and examined under a microscope (biopsy) to identify parasites or characteristic proteins (antigens) released by the parasite.

Treatment

To prevent infection, the hands of people handling meat should be washed thoroughly with soap and water after contact, as should all cutting boards, sink tops, knives, and other materials. The stages of T gondii in meat are killed by contact with soap and water. T gondii organisms in meat can also be killed by exposure to extreme cold or heat. Tissue cysts in meat are killed by heating the meat throughout to 67°C or by cooling to -13°C. Toxoplasma in tissue cysts are also killed by exposure to 0.5 kilorads of gamma irradiation. Meat of any animal should be cooked to 67°C before consumption, and tasting meat while cooking or while seasoning should be avoided. Pregnant women should avoid contact with cat litter, soil, and raw meat. Pet cats should be fed only dry, canned, or cooked food. The cat litter box should be emptied daily, preferably not by a pregnant woman. Gloves should be worn while gardening. Vegetables should be washed thoroughly before eating because they may have been contaminated with cat feces. At present there is no vaccine to prevent toxoplasmosis in humans.