Toxoplasmosis – Parasitic Disease

Spectrum of Human Disease Caused by the Cat Parasite, T. gondii

Apr 12, 2007

Toxoplasma gondii, a tissue and intestinal parasite of cats, is a tissue parasite of humans and other animals. Infection causes various types of disease.

Toxoplasma gondii, a protozoan parasite, occurs worldwide and infects more than 500 million people. Fortunately, most people don’t know they have the parasite. Toxoplasmosis symptoms in humans are variable.

Acute toxoplasmosis

Acute toxoplasmosis occurs when a person is first exposed to the parasite. Many people have no symptoms—Toxoplasma gondii invades, multiplies, and encysts in tissues. Tissue cells are destroyed throughout the body, including lung, liver, brain, heart and eye cells, but destruction is usually relatively minor and soon ceases. Some people experience temporary flu-like symptoms with acute infection. Only a few develop serious toxoplasmosis symptoms.

The unfortunate individual with overt toxoplasmosis symptoms usually has headache and fever, sore muscles, and swollen lymph nodes above the collarbone and in the groin. There may also be anemia and lung complications. Death is very rare in healthy adults. Subacute toxoplasmosis—acute infection that takes a long time to clear up—results in greater cell destruction. Again, most cases resolve; encysted T. gondii parasites remain in the tissues for life, causing no problems.

Chronic toxoplasmosis

Chronic toxoplasmosis—long-term infection—occurs in virtually everyone infected with the parasite. Parasites remain encysted in tissues, controlled by host immunity and unable to multiply or cause illness. Most people have no symptoms.

Occasionally, tissue cysts break down releasing parasites that invade new cells and multiply, forming new cysts. If there are many cysts present, this process results in inflammation and symptoms associated with tissue damage in locations such as the eyes (see ocular toxoplasmosis, below), heart, lungs, or brain. Chronic encephalitis (inflammation of the brain) occurs occasionally, with spastic paralysis (involuntary muscle spasms and paralysis) in some cases.

New research on T. gondii and toxoplasmosis shows that the parasite causes behavioral changes in mice and rats, and may cause subtle behavioral/personality changes in people also. Though intriguing and alarming, these suggested effects in humans are not well studied yet.

Ocular toxoplasmosis

When Toxoplasma gondii invades cells in the retina of the eye, blindness can result. Multiplication of parasites, causing cell destruction and expanding lesions in the retina, destroys the delicate tissues of the retina and macula. This is most often seen in chronic infections when encysted parasites reactivate and multiply. The more retinal cells infected, the greater the cell destruction and damage to vision.

Toxoplasmosis during pregnancy

When a woman acquires her first (acute) case of toxoplasmosis during pregnancy, the consequences to her baby can be grave. In about 45 percent of cases, the parasite passes to the fetus; 9 percent of infected fetuses die from the infection; 30 percent suffer brain damage, vision impairment, and/or mental disability; 60 percent have no symptoms initially but the parasite can reactivate later causing ocular toxoplasmosis or other problems (data from Roberts and Janovy).

While the highest risk of transmission of toxoplasmosis during pregnancy occurs when the mother contracts acute toxoplasmosis in the third trimester, the risk of serious disease in the fetus is highest in the first and second trimester.

Toxoplasmosis in the immunosuppressed

Individuals with immunity problems, such as those with AIDS and organ donor recipients, have an impaired ability to fight off T. gondii; therefore, the parasite can multiply unchecked in an acute infection or reactivate from encysted parasites. Drugs are available for treatment; however, toxoplasmosis in such patients is life-threatening.