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.
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.
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.
Men and women of all races are equally affected by T. gondii, however, except for immunocompromised individuals, the implications are more serious for women, as they can pass the infection on to their offspring. The rate of infection in the United States varies considerably with location. Studies have found that the infection rate in women of childbearing age ranges from 30 percent in Los Angeles to 3.3 percent in Denver. Varying sanitary conditions and culinary habits, such as eating raw meat, account for some of this variation. The rate of infection increases with the age of the individual. About 3,500 to 4,000 children are born in the United States each year with congenital toxoplasmosis. Outside the United States, fetal infection rates tend to be higher, although the number of babies born with congenital toxoplasmosis was as of 2004 declining worldwide.
Causes and symptoms
In fetuses, the severity of infection is dependent on the time of transmission. Fetuses who acquire the infection during the first trimester of pregnancy often are still-born or die shortly after birth. Fetuses who acquire the infection late in pregnancy often show no symptoms when born.
Severe infections lead to seizure disorders, neurological disorders, abnormal muscle tone, deafness, partial or complete blindness caused by a condition called chorioretinitis, and mental retardation. These conditions may not be present at birth, especially if the infection occurred late in pregnancy. Vision deficits, especially, tend to show up later in life.
Young children can acquire toxoplasmosis in the same ways as adults. However, symptoms and complications when the disease is acquired after birth tend to be much milder than with congenital toxoplasmosis.
Children and adults with weakened immune systems have a high risk of developing serious symptoms, including cerebral toxoplasmosis, an inflammation of the brain ( encephalitis ), one-sided weakness or numbness , mood and personality changes, vision disturbances, muscle spasms , and severe headaches. If untreated, cerebral toxoplasmosis can lead to coma and death.
When to call the doctor
Women who believe they may have become infected shortly before conception or during pregnancy should call their doctor immediately. Treatment is possible during pregnancy. Symptoms in the newborn may be obvious during the newborn examination. If they are not, parents should consult their doctor if they feel their child has any neurological or vision complications or is not meeting appropriate developmental milestones.
A diagnosis of toxoplasmosis is made based on clinical signs and supporting laboratory results, including visualization of the organism in body tissue or isolation in animals. Blood tests for toxoplasmosis are designed to detect increased amounts of a protein or antibody produced in response to infection with T. gondii. Antibody levels can be elevated for years, however, even when the disease is in a dormant state. Amniocentesis (sampling amniotic fluid) between 20 and 24 weeks of gestation can detect toxoplasmosis in the fetus.
Most healthy individuals who contract toxoplasmosis do not require treatment, because the healthy immune system is able to control the disease. Symptoms are not usually present. Mild symptoms may be relieved by taking over-the-counter medications, such as acetaminophen (Tylenol) and ibuprofen (Motrin, Advil). Sore throat lozenges and rest may also ease the symptoms.
The benefits of treating women who contract toxoplasmosis during pregnancy almost always outweigh any risks involved. Treatment is with antibiotic and antimicrobial drugs. Transmission of toxoplasmosis from the mother to the fetus may be prevented or reduced if the mother takes the antibiotic spiramycin. Later in a pregnancy, if the fetus has contracted the disease, treatment with the antibiotic pyrimethamine (Daraprim, Fansidar) and folinic acid (an active form of folic acid ) may be effective. Babies born with toxoplasmosis who show symptoms of the disease may be treated with pyrimethamine, the sulfa drug sulfadiazine (Microsulfon), and folinic acid. Healthy children over the age of five usually do not require treatment. Infected individuals with weakened immune systems may require lifetime drug treatment to keep the infection from recurring.
The prognosis is poor when congenital toxoplasmosis is acquired during the first three months of pregnancy. Afflicted children die in infancy or suffer damage to their central nervous systems that can result in physical and mental retardation. Infection later in pregnancy often results in only mild symptoms, if any. The prognosis for acquired toxoplasmosis in adults with strong immune systems is excellent. The disease often disappears by itself after several weeks. However, the prognosis for immunodeficient patients is not as positive. These patients often relapse when treatment is stopped. The disease can be fatal to all immunocompromised patients, especially individuals with AIDS, and particularly if not treated.
There are no drugs that can eliminate T. gondii cysts in animal or human tissues. Humans can reduce their risks of developing toxoplasmosis by practicing the following measures:
- freezing foods (to 10.4°F/–12°C) and cooking foods to an internal temperature of 152°F/67°C to kill the cysts
- practicing sanitary kitchen techniques, such as washing utensils and cutting boards that come into contact with raw meat
- keeping pregnant women and children away from household cats and cat litter
- disposing of cat feces daily because the oocysts do not become infective until after 24 hours
- helping cats to remain free of infection by feeding them dry, canned, or boiled food and by discouraging hunting and scavenging
- washing hands after outdoor activities involving soil contact and wearing gloves when gardening
Fear of infection during pregnancy is the most common parental concern. When a fetus is found early in pregnancy to be infected, parents are faced with the decision of whether to continue the pregnancy given the likelihood of serious complications to the fetus.
Cyst —An abnormal sac or enclosed cavity in the body filled with liquid or partially solid material. Also refers to a protective, walled-off capsule in which an organism lies dormant.
Immunocompromised —A state in which the immune system is suppressed or not functioning properly.
Oocyst —A developmental stage of certain parasitic organisms, including those responsible for malaria and toxoplasmosis, in which the zygote of the organism is enclosed in a cyst.
Ambrose-Thomas, P., et al. Congenital Toxoplasmosis: Scientific Background, Clinical Management, and Control. New York: Springer, 2000.
Joynson, David H. M., et al. Toxoplasmosis: A Comprehensive Clinical Guide. Cambridge, UK: Cambridge University Press, 2001.
Jones, J. "Congenital Toxoplasmosis." American Family Physician 67 (May 15, 2003): 2131–8.
Leblebicioglu, Hakan, and Murat Hökelek. "Toxoplasmosis." eMedicine.com August 10, 2004. Available online at http://www.emedicine.com/ped/topic2271.htm (accessed October 15, 2004).
The Merck Manual of Diagnosis and Therapy , 17th ed. Edited by Mark Beers and Robert Berkow. Available online at http://www.merck.com/mrkshared/mmanual/home.jsp (accessed October 15, 2004).
Tish Davidson, A.M.