Toxoplasmosis

People may become infected with toxoplasmosis by accidentally ingesting cat feces.

• Toxoplasmosis is a disease caused by the parasite Toxoplasma gondii.
• In most instances, human infection occurs after the parasite is ingested.
• The majority of people infected have no symptoms, but the disease has the potential to cause serious problems in some people, especially in those who are immunocompromised and in pregnant women.
• If symptoms develop, they are flu-like (for example, muscle aches, swollen lymph nodes, malaise) and may last for a few weeks.
• Less frequently, severe infections can lead to eye problems, brain impairments, seizures, and rarely, death.
• Certain drugs, alone and in combination, can be used to treat toxoplasmosis. Many people in the U.S. and in other developed countries develop infections from eating infected meat or inadvertent ingestion of cat or kitten feces.
• Prevention of this disease mainly centers on avoiding human contact with undercooked, contaminated meat and contact with cat or kitten feces.
• The organism was first observed in rodents in 1908.
• Toxoplasma was noted to cause congenital infection (meaning passed from mother to fetus during pregnancy) in the 1930s and became widely recognized as a cause of disease in immunocompromised people in the late 1960s.
• More infections were noted beginning in 1983 when people with HIV/AIDS developed Toxoplasma encephalitis (inflammation of the brain).
• The CDC considers toxoplasmosis to be the third most common cause of food-borne deaths in the U.S. and estimates about 60 million people in the U.S. carry the parasite.
• Most infected people have an immune system that suppresses the parasites, so the vast majority of people show no symptoms. However, if the immune system becomes depressed, the parasites can cause serious disease.

Toxoplasma gondii is a protozoan parasite that infects most species of warm-blooded animals (for example, cats, pigs, sheep, and humans) and causes the disease toxoplasmosis. The only known host animal that allows the parasite to complete its life cycle is the cat (domestic cats and other relatives in the family Felidae). After primary infection, cats shed millions of oocysts in their feces for about one to three weeks; the oocysts take one to five days to sporulate, which then can infect mice and birds (termed intermediate hosts) when these animals ingest water, plants, or soil containing the sporulated oocysts. The oocysts can remain viable in the environment for about a year. These sporulated oocysts become tachyzoites when ingested and migrate into muscle and neurological tissues where they further develop into bradyzoites. When a cat ingests an infected mouse or bird, the ingested bradyzoites develop into either tachyzoites or oocysts. The life cycle of Toxoplasma is completed when oocysts are shed in the cat's feces. Humans and other animals are not part of the complete life cycle (unless eaten by a cat). The majority of infections occur when humans, domesticated or wild animals ingest food, soil, or other animals that contain either sporulated oocysts or animal tissue containing Toxoplasma bradyzoites. Humans usually become infected by ingesting undercooked infected meat, food, or water. Infection may also be transmitted by contaminated blood transfusions, transplantation of infected organs, or from an infected mother to fetus. Finally, the disease may be acquired by directly ingesting cat feces, which may occur when cleaning out litter boxes.

Most people infected with Toxoplasma are asymptomatic. Those who develop symptoms usually have cervical lymph node swelling and flu-like symptoms that resolve in a few weeks or months without treatment. The organism remains in the body in a latent state and may reactivate if the person becomes immunocompromised. For example, patients with AIDS can develop lesions in the brain due to Toxoplasma reactivation. Chemotherapy patients can develop eye, heart (myocarditis), lung or brain involvement when parasites become reactivated. Congenital Toxoplasma infections can cause serious eye, ear, and brain damage at birth. However, congenital infections may be asymptomatic until the first few years of life or even until the second or third decade when eye (decreased vision or blindness), ear (hearing loss), or brain damage symptoms (encephalitis, seizures, mental-status changes) develop. Toxoplasmosis is the leading cause of chorioretinitis (inflammation of the retina and choroid of the eye) in the United States.

Because the majority of people do not get symptoms with toxoplasmosis, most infected individuals do not seek medical care. However, people who develop enlarged cervical lymph nodes and develop a flu-like syndrome should consider seeking medical care if they have known or suspected contact with cats or cat-contaminated food. If women who are planning a pregnancy or are pregnant develop these symptoms, they should seek medical care. Immunocompromised individuals, especially those with HIV infection, should also seek medical care if the above-mentioned symptoms develop or if they develop new eye symptoms or mental-status changes.

Most infected people will have no physical findings, but on physical examination, some will have enlarged cervical lymph nodes (most common physical finding), or an enlarged spleen or liver. People with moderate to severe infections may exhibit jaundice (especially infants) or increased bruising due to liver involvement, eye problems (decreased vision or blindness), meningoencephalitis (inflammation of the brain and linings of the brain), seizures, pneumonitis, and mental-status changes. Unfortunately, many other diseases can cause similar mild and severe symptoms (for example, Chagas disease, giardiasis, malaria, cat scratch disease, brain abscesses, sepsis, cytomegalovirus, and many others). Fortunately, there are a number of tests that can help differentiate toxoplasmosis from other diseases and provide evidence for a presumptive or definitive diagnosis.

The definitive diagnosis of toxoplasmosis is made by identifying Toxoplasma gondii organisms in blood, body fluids (for example, spinal or amniotic fluid), or tissue (biopsy samples). In addition, body fluids can be injected into mice; the animals will develop the disease if the parasites are in the injected body fluid. Also, body fluids can be inoculated into cell cultures where the parasites can proliferate. These tests are usually done in specialized laboratories by experienced personnel.

Other tests can yield a presumptive diagnosis and are based on the person's immune response to the parasite. Body fluids can be tested by PCR and there is an enzyme-linked immunosorbent assay (ELISA) technique that can indicate acute infection. Another test, the Sabin-Feldman test, measures the patient's IgG antibody directed against the parasites and is a standard reference test for toxoplasmosis. IgG antibodies indicate that infection with Toxoplasma has occurred in the past but do not tell whether the current infection is due to T. gondii. Other tests detect IgM antibodies directed against the parasite and may detect these antibodies as early as the first week of infection. Most often, these tests are done by specialized laboratories. The timing of these tests is important as is the interpretation of the results. An incorrect diagnosis can occur when a person has a positive toxoplasmosis test result due to another unrelated disease causing the symptoms. Consultation with an infectious diseases expert may help determine the diagnosis when only presumptive evidence of Toxoplasma infection is available.

Pregnant individuals and those planning to become pregnant can get tested with immunological tests similar to those listed above for presumptive diagnosis to determine if there is a risk for the mother to transmit Toxoplasma infection to the fetus. If the woman has no antibodies in her bloodstream, she is susceptible to getting the disease and could be monitored more closely and educated.

Toxoplasmosis can be treated medically. There are several agents, usually used in combination, to treat infection by this parasite. The individual circumstances of each patient determine the optimal drug combination, dosage and duration. For example, patients who are pregnant or with HIV/AIDS require special treatment considerations. The best way to determine individual medical treatments, based on the patient health situation, is in consultation with an infectious diseases expert.

Patients who have been diagnosed with toxoplasmosis require follow-up with their treating physicians. People with mild infection may need little follow-up if no medical treatment was needed; however pregnant individuals and their delivered infants may require close follow-up to determine if additional treatments are necessary. Immunocompromised patients, especially HIV patients, require lifelong ongoing treatment and regular follow-up evaluations. People who are known to have had toxoplasmosis in the past and has become immunocompromised (for example, HIV, cancer or undergo chemotherapy) need to inform their caregivers about the parasite infection since immunosuppression can allow reactivation of the parasites. These patients will require close follow-up.

Prevention of toxoplasmosis centers on avoiding ingestion of the parasites. The following is suggested by the CDC and other public-health officials to prevent or reduce the chance of getting toxoplasmosis:

• Thoroughly cook all meat (freezing meat for several days may also reduce the chance of ingesting viable Toxoplasma).
• Carefully wash hands and utensils after handling raw meat.
• Wash fruits and vegetables before eating them.
• Do not drink unpasteurized milk or drink untreated water.
• Feed cats only commercially prepared cat food or thoroughly cooked food.
• Do not adopt or handle stray cats.
• Do not obtain a new cat while pregnant.
• Pregnant women should wear gloves while gardening, thoroughly wash their hands afterward, and avoid contacting any cat feces; they should have others change cat litter boxes (change litter boxes daily).
• Keep outdoor sandboxes covered when not in use.

Toxoplasma-infected pregnant individuals can infect their fetus; treatment of the mother can reduce the chances of infecting the fetus. Organ and blood donors infected with Toxoplasma can transmit the parasite to recipients; testing donors for the parasite can prevent this rare type of infection. Studies are ongoing to produce a vaccine against Toxoplasma, but to date, none are available or commercially produced for humans or cats.

Most people who get toxoplasmosis will have an excellent outcome with no significant short- or long-term problems. However, an infected fetus or infant has a prognosis that may range from good to poor, depending on when in development they become infected, how rapidly the disease progresses and is diagnosed, and the response to treatment. However, the prognosis is usually poor if the fetus is infected in the first trimester; many such fetuses die or develop severe physical and mental problems seen at birth. Immunocompromised individuals have a good to poor prognosis, depending on how quickly the diagnosis is made and now well the patient responds to treatment. For example, if encephalitis develops due to toxoplasmosis in a patient with HIV, the prognosis can be good if the patient responds to treatment, but treatment usually needs to be continued for life.