Toxoplasma Gondii in Humans: Infection, Symptoms & Risks

Toxoplasma gondii is a single-celled parasite found worldwide, infecting up to one-third of the global population. While it can infect many warm-blooded animals, its life cycle depends on felines, like domestic cats, which are the only hosts where the parasite can sexually reproduce. The parasite is then shed in the cat’s feces, becoming a source of the disease toxoplasmosis for other animals and humans.

The prevalence of toxoplasmosis varies by region. In the United States, about 11% of the population aged six and older has been infected. The infection is more common in hot, humid climates and at lower altitudes, where the parasite survives longer in the environment.

How Humans Get Toxoplasma gondii

The most common way humans become infected is through foodborne transmission by consuming undercooked or raw meat containing the parasite’s tissue cysts. These are small, dormant sacs that can remain in muscle tissue for the life of the animal. Pork, lamb, and venison are frequent sources, but cysts can also be found in beef, chicken, and contaminated shellfish.

Another primary route involves accidentally ingesting oocysts, a microscopic form of the parasite passed in cat feces. Cats become infected by eating rodents or birds and can then shed millions of oocysts for up to three weeks. These oocysts become infectious after one to five days in the environment, where they can contaminate soil, water, and produce. People can get infected by cleaning a litter box without washing their hands or eating unwashed fruits or vegetables.

In rare cases, the parasite can be transmitted through an organ transplant or blood transfusion. Toxoplasmosis is not passed from person to person, except from mother to child.

Symptoms of Toxoplasmosis in People

The presentation of toxoplasmosis depends on the person’s immune status. For most people with a healthy immune system, a T. gondii infection is asymptomatic. When symptoms do occur, they are mild and flu-like, including swollen lymph nodes, muscle aches, and fever, which generally resolve on their own.

A specific manifestation is ocular toxoplasmosis, which affects the retina and can lead to inflammation and scarring, causing reduced vision or permanent eye damage. It can result from a newly acquired infection or the reactivation of a congenital one.

For individuals with compromised immune systems, such as people with HIV/AIDS or organ transplant recipients, the infection can be severe. A latent infection can reactivate and cause serious illness. While the brain is the most common site of severe disease, it can also affect other organs, leading to conditions like pneumonitis (lung inflammation).

Congenital toxoplasmosis occurs when a fetus is infected during pregnancy, with consequences being most severe when the mother is infected in the first trimester. The infection can result in miscarriage, stillbirth, or significant health issues for the newborn, such as eye damage or neurological problems. Some infants may appear healthy at birth but develop symptoms, particularly vision problems, years later.

Toxoplasma gondii and the Brain

Toxoplasma gondii has an affinity for the central nervous system and can form dormant tissue cysts within the brain. For most people, these cysts remain inactive, but in individuals with severely weakened immunity, particularly those with AIDS, they can reactivate. This leads to toxoplasmic encephalitis, a serious neurological condition characterized by multiple brain lesions that can cause headache, confusion, seizures, and poor coordination.

Beyond encephalitis, research is exploring links between latent T. gondii infection and an increased risk for conditions such as schizophrenia, bipolar disorder, and depression. Proposed mechanisms include subtle neuroinflammation, alterations in neurotransmitter levels like dopamine, and direct effects of the parasite on neurons.

Animal studies have shown that infected rodents lose their innate fear of cat odors, a behavioral change that makes them more likely to be caught by cats. It is important to approach findings on human behavioral effects with caution, as many studies show correlations rather than direct causation, and the scientific community continues to investigate the impact of latent toxoplasmosis.

Preventing Toxoplasma gondii Infection

Preventing Toxoplasma gondii infection primarily involves practicing careful food safety and good hygiene. One of the most effective measures is to cook meat to safe internal temperatures, which kills the parasite’s tissue cysts.

To prevent foodborne illness, you should:

• Cook whole cuts of meat to at least 145°F (63°C) and ground meats to 160°F (71°C).
• Freeze meat for several days at sub-zero temperatures (0°F or -18°C) to reduce the chance of infection.
• Wash all fruits and vegetables thoroughly under running water.
• Wash cutting boards, utensils, and countertops with hot, soapy water after they have been in contact with raw meat.
• Avoid drinking unpasteurized goat’s milk or eating raw shellfish.

Precautions related to cats are also important. Cat litter boxes should be changed daily, as the parasite’s oocysts only become infectious one to five days after being shed. If you are pregnant or have a weakened immune system, it is best to have someone else handle this task. If you must do it yourself, wear disposable gloves and wash your hands thoroughly afterward. Keeping cats indoors and feeding them only commercial cat food prevents them from acquiring the parasite. Covering outdoor sandboxes prevents cats from using them as litter boxes. Individuals in high-risk groups who have concerns about exposure should speak with a healthcare provider.