Toxoplasmosis is a widespread parasitic infection caused by the single-celled organism Toxoplasma gondii. Over 60 million people in the U.S. are thought to be infected, often without symptoms. While the infection is generally mild in healthy adults, it presents a significant public health concern for vulnerable populations, particularly pregnant women and immunocompromised individuals. The domestic cat is uniquely involved in the parasite’s life cycle, which raises a common question: what is the specific risk of contracting toxoplasmosis from a cat that lives exclusively indoors?
The Transmission Cycle of Toxoplasma
The life cycle of Toxoplasma gondii requires a definitive host, and cats are the only animals in which the parasite can undergo its full sexual reproductive cycle. When a cat consumes infected prey, such as a rodent carrying tissue cysts, the parasite reproduces in the cat’s intestine. This process results in the cat shedding millions of microscopic eggs, known as unsporulated oocysts, in its feces.
These oocysts are not immediately infective to other hosts, including humans. They require a process called sporulation, which takes between one and five days, depending on environmental conditions like temperature and moisture, to become infective. Once sporulated, these oocysts are remarkably hardy, capable of surviving in soil and water for months or even years.
Humans and other warm-blooded animals act as intermediate hosts. Infection occurs by ingesting sporulated oocysts from contaminated soil, water, or unwashed produce. The parasite then forms dormant tissue cysts, most commonly in the muscle and brain, which can persist for the host’s lifetime. The primary transmission routes to humans are eating undercooked meat containing these cysts or consuming oocysts from the environment, rather than direct contact with a cat.
Evaluating the Risk from Indoor Cats
The risk of contracting toxoplasmosis from an exclusively indoor cat is generally considered very low. A cat typically only sheds oocysts for a short period, usually one to three weeks, immediately following its first exposure to the parasite. Once a cat has shed the parasite, its immune system develops resistance, making it unlikely to shed oocysts again, even after re-exposure.
For an indoor cat to shed oocysts, it must have recently acquired a primary infection, which requires ingesting the parasite’s tissue cysts or sporulated oocysts. This usually happens when a cat eats an infected intermediate host, like a mouse or bird, or is fed raw or undercooked meat. An indoor cat that does not hunt, is fed only commercial or cooked food, and has no access to rodents is highly unlikely to be newly infected and shedding.
A positive antibody test in a healthy cat indicates past exposure and immunity, suggesting it is no longer shedding infectious oocysts. Testing is not a reliable way to determine if a cat is currently shedding, as oocyst shedding is brief and often over before symptoms appear. Therefore, the absence of outdoor access and raw food remains the best indicator that an indoor cat is not a source of infection. Living with a cat, even an indoor one, has not been shown to be a significant risk factor for human infection.
Minimizing Household Exposure Risks
Preventing infection primarily involves two strategies: minimizing the cat’s chance of exposure and interrupting the parasite’s life cycle in the home. Cleaning the litter box daily is essential. Since the oocysts take at least 24 hours to sporulate and become infective, removing feces promptly eliminates the risk of environmental contamination.
High-risk individuals, such as pregnant women, should delegate the task of litter box cleaning to another household member. If this is not possible, wearing disposable gloves and practicing thorough hand washing immediately afterward is recommended. Feeding cats only commercial dry or canned food eliminates the primary risk of infection via tissue cysts in meat.
Non-Feline Sources and Food Safety
Because human infection most frequently occurs through non-feline sources, personal hygiene and food safety are equally important. Precautions should be taken to avoid ingesting cysts or oocysts from the environment:
- All meat, especially pork, lamb, and venison, should be cooked thoroughly to the recommended safe temperatures to destroy any tissue cysts.
- Freezing meat for several days before cooking can greatly reduce the chance of infection.
- Wearing gloves when gardening or handling soil is a practical precaution, as outdoor soil can be contaminated with oocysts shed by stray or neighborhood cats.
- All fruits and vegetables should be rinsed thoroughly under running water before consumption.