(Editor's note: this page was written with help from Sniff and Cat Scan, indoor cats who really wanted to be indoor-outdoor cats until I told them all about toxoplasmosis.)
Toxoplasmosis is an infection with Toxoplasma gondii, a single-cell parasite that is not a bacterium. T. gondii is found around the world, and can live in the human body or in the body of any warm-blooded animal. However, the parasite can only reproduce in the intestines of cats.
The parasites actually produce their "eggs" within the cat's intestine. The oocysts are excreted in the cat's stool, and are not immediately infectious; they must mature outside the cat for 1-2 days before they can infect other animals. Cats acquire the parasite when they catch and eat mice or other animals that have been infested with the eggs,
or when they eat uncooked meat from an infested source. People and animals are usually infested by eating uncooked or undercooked meat containing the oocysts, or from oocysts in soil, contaminated food, or droppings from infested cats. There has been a case in which the parasite has been spread through a contaminated municipal water supply. It's also possible to be infested with T. gondii after being transfused with blood donated by an infested person, or after receiving a transplanted organ or bone marrow from an infested donor. The infestation in a person exposed after birth usually produces no symptoms at all; if there are symptoms, they usually include fever, malaise (feeling lousy), a sore throat, myalgia (muscle pain), and/or swollen glands. A few patients may have a rash and an enlarged liver or spleen. The symptoms are usually pretty minimal and resolve in a few days without any treatment. It seems to take from 4 days to 3 weeks after exposure to become infested with T. gondii. In some animals and people, the oocysts can develop into cysts that can be found in the brain, muscle (either skeletal or heart muscle), and sometimes in other organs, and which can stay infectious for as long as the animal or person lives. The biggest problems with T. gondii in humans are in babies whose mothers are first infested during pregnancy, and in people with human immunodeficiency virus (HIV) infection.
Babies who develop congenital toxoplasmosis usually have no symptoms when they are born. However, the infection in these babies is often some form of meningitis or encephalitis, and there may be calcifications (small masses of hard, dead tissue) in the brain. With a severe infection the baby may be stillborn, or may live for only a few days. Other possible problems include microcephaly (an unusually small head, which is often accompanied by mental retardation and developmental abnormalities), seizures, deafness, chorioretinitis (damage to the retina -- the light-sensitive part of the eye -- which may make the patient blind), and hydrocephalus (increased pressure inside the brain from blocked cerebrospinal fluid flow, which can also cause brain damage). Problems with development, including learning disabilities, and with vision may not be recognised until a child is older. Most T. gondii-related eye problems are seen in newborn babies, but eyesight can be affected by a new infestation in older children or adults.
People with normal immune systems can often clear, or at least control, T. gondii infestations. People with AIDS may be able to produce antibodies to T. gondii, and are harder to diagnose since the tests for T. gondii measure antibody levels in the bloodstream (T. gondii can't be cultured, since it doesn't reproduce without cats). AIDS patients are more likely to develop T. gondii encephalitis than people without HIV.
T. gondii infestation can be treated with several antibiotics, including pyrimethamine and sulfadiazine. People with HIV should take regular doses of trimethoprim/sulfamethoxazole, which can prevent infestation and should be taken for life. It is possible to treat a pregnant woman for a T. gondii infestation to reduce the chances that her child will be infested; this should be done by a specialist in infectious diseases.
As with every disease, prevention is better than trying to cure. Meat (especially pork, lamb, and deer meat) should be cooked until it's no longer pink, unless it has been preserved by smoking or pickling in brine (concentrated salt water). Fruits and vegetables should be washed before eating and kept away from raw meats, and all foods should be kept away from soil. You should wash your hands thoroughly after any contact with soil, such as when gardening, or after working with raw meat, vegetables, or fruits.
People with cats should take particular care to avoid exposure to T. gondii oocysts. Pregnant women should not handle litter boxes; if they must clean the litter, they should wear gloves while doing so and wash their hands thoroughly afterwards. Since the oocysts cannot infect people until 1-2 days after being pooped out by the cat. scooping the litter box every day helps reduce potential exposure. Outdoor cats are very likely to be T. gondii carriers; cats who have lived all of their lives indoors can be protected from infestation by feeding them only fully-cooked meats and commercial cat food, and not allowing them to attack and eat wild mice or other wild animals. (This should also protect them from other diseases, such as infection with toxin-producing E. coli.)