Where does Toxoplasmosis come from and How do cats spread Toxoplasmosis?

You may have heard that you can get something called toxoplasmosis from your cat.

Toxoplasma gondii, the organism that causes the disease is a protozoan parasite that infects most species of warm blooded animals. It has a fairly complex life cycle consisting of three different stages.

The first is the tachyzoite, the form that causes acute infection by invading cells. The second is called a bradyzoite, and in latent infections this stage lives in tissue cysts. The third stage is the sporozoite, a form that lives inside an oocyst, a kind of egg shell, and can survive in the environment, where it can be picked up by animals and people.

Cats are often a link in the chain, they excrete uninfectious oocysts in their feces, where, given enough time and the right conditions, the oocysts sporulate (hatch, in a sense) and become infectious. The parasites like warm, moist soil, where they can survive for up to a year, although both cooling and heating will kill them. You don’t need a cat to become infected, although contact with infected cat feces is one route.

Birds, sheep, pigs, goats, and horses have also been found to carry T. gondii. If you eat inadequately cooked infected meat or something that has come in contact with infected meat, you can get the parasite that way.

You can get it from soil in your garden that has the oocysts in it. It’s impossible to know what percentage of disease is caused by cats, infected meat, or soil, but there is a statistically significant decrease in risk for infection if you’re a vegetarian. Most serious of all, for reasons we will explain below, an infected pregnant woman can transmit the disease to her fetus, which can be born infected.

It is likely that you already have the parasite, an estimated 23 percent of the U.S. population is infected with it, but it rarely causes any symptoms, so most people never have to worry about it. Even when it does cause symptoms, they are usually not very serious (they resemble symptoms of the flu), and they go away by themselves without any treatment.

But some people do have to be concerned about coming in contact with this protozoan, in particular pregnant women and people who are immunocompromised either because they have a disease that affects their immune systems or because they are undergoing certain types of chemotherapy.

Infants infected before birth can be born retarded or with other severe mental and physical problems. The immunocompromised can suffer damage to the eyes and brain. Infants are often born with no symptoms, but if untreated develop early learning disabilities and, sometimes decades later, visual disabilities. If babies are treated with pyrimethamine and a sulfonamide for a year, 70 percent of them will develop normally.

Estimates of the number of infants born with toxoplasmosis vary, but at least 400 a year, and by some calculations as many as 4,000, are born infected. Screening programs are not commonly used in the United States, but a program instituted in Massachusetts and New Hampshire during the years 1986 to 1992 found 52 cases of congenital toxoplasmosis in 635,000 infants screened. The cost of the screening program compared to the cost of raising visually or mentally impaired children demonstrated that the program was cost-effective as well. The CDC would like to see more such programs established elsewhere.

There are various blood tests for toxoplasmosis, and some are quite sensitive and accurate. They detect not the protozoan itself, but antibodies to it in the blood.

One problem with this is that the tests don’t distinguish between a current infection and one that has occurred in the past because the antibodies can be detected for as long as 18 months after an infection has entirely cleared up. Usually the disease doesn’t require any treatment, but pregnant women who have the illness can be treated with the antibiotic spiramycin or with sulfadiazine or with a combination of sulfadiazine and pyrimethamine. Which drug regimen is used depends on the gestational age of the fetus and whether the fetus itself is already infected. This doesn’t completely prevent infection at birth, but it reduces its probability by about 50 percent.

Toxoplasmosis is a serious health problem. Although few otherwise healthy people suffer from its effects, 750 people, most of them very young or not so healthy, die from it every year, and about half of them got the disease by eating contaminated food. This makes toxoplasmosis the third leading cause of food-borne deaths in the United States.

Preventing food-borne toxoplasmosis isn’t hard if you follow the procedures for food handling. Cooking beef, lamb, and veal to 145°F, pork, ground meat, and wild game to 160°F, and whole poultry to 180°F in the thigh is enough to make the parasite harmless. Freezing the meat will also destroy T. gondii. Washing vegetables and cleaning off the cutting board, as you would anyway (wouldn’t you?), will also help. Pregnant women should avoid gardening or wear gloves if they do garden, get someone else to change the cat litter, keep cats indoors where they can’t eat infected mice or other prey, and not adopt or handle stray cats.

Cats should be fed only commercial canned or dry cat food or well-cooked table food, and never raw or undercooked meat. These are fairly simple rules, and if you follow them, you won’t have to think about euthanizing the cat just because you’re having a baby.