Toxoplasmosis

Toxoplasmosis is one of the most widespread parasitic infections in the world, and cats sit right at the centre of its story — though usually as quiet carriers rather than sick patients. The single-celled parasite Toxoplasma gondii can infect almost any warm-blooded animal, but it can only complete its full reproductive cycle inside a cat. That biological quirk is why the disease is so closely associated with cats, and why it generates more worry among cat owners than it usually deserves.

Most cats that pick up Toxoplasma never show any sign of illness. Their immune system contains the parasite, which forms dormant tissue cysts, and the cat carries it harmlessly for life. The crucial public-health detail is that a cat sheds infectious oocysts in its faeces only briefly — typically just one to three weeks following its very first exposure — and never again afterwards. The oocysts also need one to five days in the environment before they become infectious, which is exactly why daily litter-box cleaning is such an effective safeguard.

Clinical toxoplasmosis — the form that actually makes a cat ill — is uncommon and tends to strike kittens or cats whose immune defences are compromised by feline leukaemia virus, feline immunodeficiency virus, or immunosuppressive drugs. In these cats the parasite can spread to the lungs, liver, eyes, or nervous system, causing fever, breathing difficulty, or eye inflammation.

Much of the anxiety around toxoplasmosis concerns pregnancy, because a first-time infection during pregnancy can harm the developing baby. The reassuring reality is that the main human risk comes from undercooked meat and gardening in contaminated soil far more often than from a pet cat. Sensible hygiene — cooking cat food, daily litter scooping, gloves, and handwashing — reduces the already-small risk to a negligible one, and is no reason to part with a beloved pet.