Toxoplasmosis is caused by infection with the microscopic parasite Toxoplasma gondii. This common protozoan can infect most warm-blooded animals, including humans, but its transmission is often misunderstood. A cat scratch is not a route for human infection with Toxoplasma gondii. Concerns about cats stem from their unique role in the parasite’s life cycle, but the organism must be ingested to cause disease in people.
The Cat’s Role in Toxoplasmosis Transmission
Cats are the only known definitive hosts where Toxoplasma gondii can complete its life cycle. A cat typically becomes infected by eating an intermediate host, such as a rodent or bird, that contains the parasite’s tissue cysts. Once inside the feline’s small intestine, the parasite multiplies and produces millions of environmentally resistant structures called oocysts.
These oocysts are shed through the cat’s feces for a short period, usually one to three weeks following initial infection. The oocysts are not immediately infectious upon excretion; they require one to five days in the environment to sporulate and become infective. The parasite is not found in the cat’s saliva or claws, confirming that a scratch or bite does not transmit the infection.
Common Ways Humans Contract Toxoplasmosis
Humans are infected through accidental ingestion of the parasite, primarily through foodborne contamination and environmental exposure. The most common way people contract toxoplasmosis is by eating undercooked meat containing the parasite’s tissue cysts. Pork, lamb, and venison are common sources of infection.
Foodborne infection also occurs if a person handles raw meat and then touches their mouth before washing their hands, or if cross-contamination occurs via contaminated cutting boards or utensils. Environmental exposure involves ingesting sporulated oocysts from sources like unwashed produce grown in contaminated soil or untreated water.
Cats contribute to environmental exposure, as oocysts shed in feces can contaminate soil, sandboxes, and water sources. Ingestion can occur after gardening without gloves or when cleaning a cat’s litter box if proper hand hygiene is not followed. Since oocysts need time to become infectious, changing the litter box daily minimizes the risk.
Symptoms and High-Risk Populations
In healthy individuals, infection with Toxoplasma gondii is usually asymptomatic. If symptoms appear, they are typically mild, resembling a flu. These symptoms can include muscle aches, fever, and swollen lymph nodes that may persist for several weeks or months before resolving.
Once the initial infection is controlled, the parasite does not leave the body but enters a latent state, forming cysts mainly in muscle and brain tissue. The disease becomes a serious concern for two primary high-risk groups: pregnant individuals and people who are immunocompromised. If a woman acquires the infection for the first time during or just before pregnancy, the parasite can pass to the fetus, potentially causing congenital toxoplasmosis.
Congenital infection can lead to severe consequences for the developing baby, including miscarriage, stillbirth, or neurological damage. Infants infected before birth may show no symptoms initially but can later develop serious issues like vision loss, mental disabilities, or seizures. For those with weakened immune systems, such as HIV/AIDS patients or organ transplant recipients, a latent infection can reactivate, leading to life-threatening conditions like encephalitis or severe ocular disease.
Essential Prevention Strategies
Prevention involves attention to food safety and environmental hygiene. When preparing food, always use a food thermometer to ensure meat is cooked to the appropriate internal temperature, which kills the parasite’s tissue cysts. Avoid tasting meat before it is fully cooked, and wash all cutting boards, utensils, and countertops thoroughly after they have come into contact with raw meat.
To avoid environmental exposure, thoroughly wash all fruits and vegetables before eating them, particularly if they were grown outdoors. When working outdoors in gardens or handling soil, wear gloves to prevent contact with potentially contaminated earth. For cat owners, the litter box should be changed daily to remove oocysts before they have time to sporulate and become infectious. If a pregnant person lives in the home, another household member should be responsible for this task.