Toxoplasmosis is a parasitic infection caused by the single-celled organism Toxoplasma gondii. This parasite has a worldwide distribution, with estimates suggesting that up to half of the global population has been exposed. For most healthy people, the infection remains asymptomatic. However, the parasite’s ability to persist in the body and its potential for severe disease in certain groups make it a subject of public health interest.
Routes of Human Infection
Humans acquire T. gondii through three primary routes, all involving the ingestion of the parasite in one of its infectious forms. The most common route is through consuming food or water contaminated with the parasite’s oocysts. These microscopic eggs are shed exclusively by members of the cat family. Cats typically shed oocysts in their feces for only one to two weeks following infection. Once shed, oocysts require one to five days to mature and become infectious, meaning environmental sources like soil, untreated water, or unwashed produce can transmit the infection if contaminated with cat feces.
The second route is consuming meat containing tissue cysts. These cysts are found in the muscle and neural tissues of many animals, including livestock like pigs and sheep. Eating raw or undercooked meat that has not been heated sufficiently to destroy the cysts allows the parasite to enter the human host.
The third transmission pathway is congenital, occurring when a woman acquires a primary infection during pregnancy. The parasite can cross the placenta and infect the developing fetus. Less common methods include receiving an infected organ transplant or, rarely, a blood transfusion.
Illness in Different Populations
The effects of T. gondii infection vary depending on the immune status of the infected person. In healthy individuals, the immune system rapidly controls the parasite, and the initial infection is often unnoticeable. When symptoms do occur, they are typically mild and non-specific, resembling a common flu. These manifestations may include a low-grade fever, muscle aches, and the painless swelling of lymph nodes. These acute symptoms usually resolve within a few weeks to months, but the parasite remains in the body, forming dormant tissue cysts that persist for the host’s lifetime.
Individuals with compromised immune systems face a higher risk of severe, life-threatening illness. For those with conditions like HIV/AIDS, or who are undergoing chemotherapy or organ transplantation, the dormant cysts can reactivate. This often leads to toxoplasmic encephalitis, a severe neurological disease characterized by confusion, seizures, poor coordination, and neurological damage.
Congenital infection presents a serious health concern. The timing of the mother’s infection during pregnancy influences the outcome; infection earlier in gestation is associated with more severe fetal damage. Complications for the fetus include:
- Hydrocephalus.
- Severe eye infections known as chorioretinitis.
- Abnormalities in brain tissue.
Many infants appear healthy at birth but may develop symptoms such as vision loss, hearing loss, or developmental delays later in life.
Detection and Treatment
Diagnosing Toxoplasma gondii infection relies on serologic testing, which detects specific antibodies in the blood. Laboratory tests look for two main types: Immunoglobulin G (IgG) antibodies indicate a past infection, while Immunoglobulin M (IgM) antibodies suggest a recent or acute infection. Specialized laboratories may also test for the IgA antibody or use an avidity test to help determine how recently the infection occurred.
In cases of suspected congenital infection, diagnosis can involve testing the amniotic fluid for the parasite’s genetic material using polymerase chain reaction (PCR). For severe manifestations, imaging techniques like magnetic resonance imaging (MRI) or computed tomography (CT) scans may be used to identify lesions in the brain.
Treatment is not required for healthy people with mild or no symptoms, as their immune system manages the acute phase. Medication is reserved for people with severe symptoms, those who are immunocompromised, and pregnant women or newborns with confirmed infection. Treatment involves a combination of two drugs: pyrimethamine and sulfadiazine. Folinic acid is administered alongside this combination to counteract potential side effects like bone marrow suppression. For pregnant women, a different antibiotic, such as spiramycin, may be prescribed to reduce the risk of fetal transmission. No known drug can completely eliminate the dormant tissue cysts from the body.
Practical Prevention Strategies
Prevention involves vigilance concerning food and environmental exposure. All meat should be cooked thoroughly to the recommended safe internal temperatures, with ground meats reaching at least 160°F (71°C). Freezing meat at sub-zero temperatures for several days before cooking reduces the chance of infection by destroying tissue cysts, though this does not guarantee safety. Thoroughly wash all fruits and vegetables before consumption, as they can be contaminated with oocysts from the soil. Avoid cross-contamination by washing cutting boards, utensils, and countertops with hot, soapy water after contact with raw meat.
Individuals should wear gloves when gardening or working with soil or sand, as these materials may be contaminated with infectious oocysts from cat feces. Thorough hand washing is required after contact with potentially contaminated surfaces.
Cat owners can minimize risk by taking specific precautions. Since oocysts are not infectious for the first 24 to 48 hours after being shed, the cat litter box should be cleaned daily to remove feces before the parasite matures. It is recommended to keep cats indoors and feed them only commercial cat food or well-cooked table food, preventing them from hunting infected prey.