Toxoplasmosis is perhaps the parasitic disease most surrounded by myths and misunderstandings regarding domestic cats. Many pregnant women still believe they must get rid of their cat, and many owners are unaware of the real transmission mechanisms. This article analyzes the parasite's biology, the real risks and evidence-based preventive measures.
The Parasite and Its Life Cycle
Toxoplasma gondii is an obligate intracellular protozoan whose definitive host is the cat (and other felids). Intermediate hosts are virtually all warm-blooded animals, including humans. The life cycle has two main phases:
- Enteroepithelial phase (only in felids): the parasite reproduces sexually in the cat's small intestine and produces oocysts eliminated with feces. A cat infected for the first time sheds millions of oocysts for 1-3 weeks, after which the immune system controls the infection and shedding ceases or is minimal.
- Extra-intestinal phase (in all hosts): tachyzoites (active form) replicate in tissues and encyst as bradyzoites in muscle, brain and other organs, where they remain dormant for life.
Oocysts shed by cats are not immediately infective: they need to sporulate in the environment for 1-5 days to become contagious. This fact is key to understanding prevention.
How Humans Are Really Infected
The domestic cat is the perfect scapegoat, but the epidemiological reality is much more complex. Seroprevalence studies show that in Spain between 20 and 30% of the adult population has antibodies against T. gondii, but most of these infections were not acquired through contact with cat feces. The main routes of human infection are:
- Consumption of raw or undercooked meat (lamb, pork, game) containing tissue cysts: responsible for 30-50% of cases.
- Unwashed vegetables and fruits contaminated with oocysts from soil.
- Untreated water.
- Congenital transmission from mother to fetus during primary infection during pregnancy.
- Contact with cat feces (litter box): a real but smaller risk than perceived, and easily controllable.
The Real Risk During Pregnancy
Congenital toxoplasmosis only occurs when the mother suffers a primary infection during pregnancy. If the woman already has antibodies (previous infection), the fetus is protected. The risk of transmission to the fetus varies by trimester: it is lower in the first (15-25%) but fetal consequences are more serious; in the third trimester, transmission is more frequent (60-90%) but sequelae are usually less severe.
Consequences for the fetus can include hydrocephalus, cerebral calcifications, chorioretinitis, hepatosplenomegaly or miscarriage. However, in many developed countries, severe congenital toxoplasmosis is becoming less frequent thanks to prenatal serological screening.
Preventive Measures: What Works
For a pregnant woman with a cat at home, evidence-based recommendations are:
- Clean the litter box daily (oocysts take 1-5 days to sporulate; removing them before they become infectious eliminates the risk). If possible, have someone else do it during pregnancy.
- Wear gloves when handling the litter box or garden soil, and wash hands afterwards.
- Don't feed the cat raw meat: cats get infected mainly by eating prey or raw meat. An indoor cat fed kibble or cooked meat has virtually zero risk of being an active carrier.
- Cook meat thoroughly (internal temperature >66°C) and wash fruits and vegetables well.
- It is not necessary to get rid of the cat. International health organizations (CDC, WHO) are unanimous on this point.
Toxoplasmosis in Cats: Does the Cat Get Sick?
Most cats infected with T. gondii show no clinical signs. Clinical disease only appears in immunosuppressed cats (FIV, FeLV, corticosteroid treatment) or in young kittens with severe primary infection. Signs include fever, lethargy, anorexia, dyspnea (pneumonia) and uveitis. Diagnosis is confirmed by serology (IgM in acute phase, IgG in chronic phase) and treatment with clindamycin is effective in most cases.
