What is it? Toxoplasma gondii is a protozoan parasite (like an amoeba) of cats. It rarely causes disease in cats, but is potentially dangerous to any other animal that comes into contact with cat faeces, including humans.
What causes it? The parasites live in the cat and lay eggs (or oocysts) which are passed out in the faeces. These are then eaten by rodents, sheep, dogs or people who handle cat faeces without sufficient hygiene! In this intermediate host, the parasites usually form cysts in the muscles, where they lie dormant until a cat eats the host (not terribly likely in the case of a human, but the parasite doesn't know that), when that cat becomes infected.
What cats are at risk? Any cat who ever eats live prey - about 30% of cats have been exposed at some time in their lives.
What are the symptoms? In the cat, usually nothing more than mild, transient diarrhoea. If the cat is pregnant when she becomes infected for the first time, her kittens may be severely disabled, or aborted. In other animals, symptoms are usually mild (fever, loss of appetite, lethargy) until their immune system clears the parasites. However, if they are pregnant, there is a strong possibility they will lose their puppy, lamb, kit - or, in humans, their baby. In any animal, if their immune system is weakened by another disease, there is the possibility of more severe disease affecting any organ system, especially the brain. In addition, we now know that that parasite acts to make mice and rats less afraid of cats (to increase the chances that they will be eaten), and some researchers think that Toxoplasma infection in humans increases the risk of depression and risk-taking behaviour.
How is it diagnosed? Blood tests for antibodies made by the immune system to fight off the parasites - a single test isn't useful though, your cat will need two a few weeks apart.
How can it be treated or managed? There are a range of drugs that can kill the parasite, in all species; the most commonly used is probably clindamycin.
Can it be prevented? Preventing cats from contracting the parasite requires preventing them from hunting, or catching fleas. To protect other animals and humans, avoiding contact with cat faeces is the best and most effective method, although there is a vaccine for use in sheep. There is no significant risk from handling cats (even if they are carrying Toxo) while pregnant, as long as good hygiene procedures are maintained (in other words, wash your hands thoroughly!).