Toxoplasmosis is an infection caused by the microscopic parasite Toxoplasma gondii, which is found globally. This parasite can infect nearly all warm-blooded animals, including humans and various bird species. Cats, both wild and domestic, are the definitive hosts for Toxoplasma gondii, meaning they are the only animals where the parasite can sexually reproduce and produce eggs (oocysts). While Toxoplasma gondii infection is common in cats, it often goes unnoticed because most infected cats do not show clinical signs.
How Cats Contract Toxoplasma gondii
Cats primarily become infected with Toxoplasma gondii by consuming infected prey, such as rodents or birds, or by eating raw or undercooked meat that contains tissue cysts. These tissue cysts contain bradyzoites. Once ingested, bradyzoites are released in the cat’s digestive tract, enter intestinal cells, and reproduce sexually. This leads to the formation of oocysts, which are then shed in the cat’s feces.
Cats typically begin shedding oocysts 3 to 10 days after consuming infected tissue, continuing for 10 to 14 days. Shedding usually occurs only once in a cat’s lifetime, but in large numbers. Oocysts are not immediately infectious when shed; they require 1 to 5 days to sporulate and become infective in the environment. Environmental contamination from infected cat feces is another route of infection for cats, though less common than ingesting infected meat.
Signs and Treatment in Cats
Most infected cats do not display clinical signs, making identification challenging without specific diagnostic tests. However, in rare instances, particularly in young kittens or cats with weakened immune systems due to conditions like feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV), symptoms can develop.
When symptoms occur, they vary depending on the organs affected by tachyzoites. Common signs may include fever, lethargy, decreased appetite, and weight loss. More severe presentations can involve pneumonia, leading to difficulty breathing, liver disease, or neurological issues such as seizures, poor coordination, or imbalance. Eye inflammation can also be observed.
Veterinarians diagnose toxoplasmosis in cats based on history, clinical signs, and laboratory tests. Blood tests measure IgG and IgM antibodies. High IgG levels indicate past infection and immunity, while elevated IgM levels suggest active infection. If treatment is necessary, the antibiotic clindamycin is commonly prescribed for two to four weeks. Supportive care, such as intravenous fluids, may be needed for severely affected cats.
Transmission to Humans and Risk Factors
Direct transmission from cats to humans is uncommon, primarily occurring through ingestion of infective oocysts from cat feces. This can happen if individuals contact contaminated cat litter or soil and then touch their mouth without proper handwashing. Oocysts in cat feces must sporulate for 1 to 5 days after being shed before becoming infectious; daily litter box cleaning significantly reduces this risk.
The most common routes of human infection are foodborne, including consuming undercooked meat (particularly pork, lamb, and venison) that contains parasite tissue cysts. Contaminated fruits, vegetables, or water that have contacted infected cat feces or soil are also significant sources of human infection. Rarely, transmission occurs through organ transplantation or blood transfusion.
Certain groups face a higher risk of severe complications from Toxoplasma gondii infection. Pregnant individuals are particularly vulnerable because a newly acquired infection during pregnancy can lead to congenital toxoplasmosis, where the parasite can cross the placenta and cause serious birth defects in the fetus, including brain damage or vision impairment. Immunocompromised individuals (e.g., those with HIV/AIDS, chemotherapy patients, organ transplant recipients) are also at increased risk. In these individuals, Toxoplasma gondii infection, even a reactivated latent one, can lead to severe, potentially life-threatening conditions like encephalitis (inflammation of the brain), myocarditis, or pneumonia.
Preventing Infection in Cats and Humans
Preventing Toxoplasma gondii infection in cats involves reducing exposure. Cat owners should feed cats commercially prepared dry or canned food, or thoroughly cooked meat, rather than raw or undercooked meat. Keeping cats indoors helps prevent them from hunting and consuming infected rodents or birds. Avoiding stray cats or kittens, especially during pregnancy, also reduces exposure risks.
For humans, maintaining good hygiene and safe food handling practices are paramount in preventing infection. Litter boxes should be cleaned daily to remove feces before oocysts sporulate and become infectious; wearing disposable gloves and washing hands thoroughly afterward is recommended. Cooking meat to safe internal temperatures (typically 160°F/71°C) kills the parasite. Freezing meat for several days at sub-zero temperatures (0°F/-18°C) also greatly reduces infection risk, though it may not eliminate all parasites.
Washing fruits and vegetables thoroughly under running water before consumption helps remove contamination from soil. Washing cutting boards, utensils, and countertops with hot, soapy water after preparing raw meat is important to prevent cross-contamination. Avoiding untreated water and unpasteurized goat’s milk further minimizes risk. While Toxoplasma gondii is widespread, adhering to these precautions makes routine cat ownership generally safe for most healthy individuals.