Toxoplasmosis is a relatively rare illness that may be transmitted to a pregnant woman by means of cat feces, to the potential risk of the fetus.
Most people have probably heard that pregnant women should not be around cats. This is, to some degree or another, good advice; this organism is the reason. You see, Toxoplasma gondii spreads itself predominantly in its egg form, and these eggs can only be produced in a cat's lower digestive tract; cats are, essentially, the organism's favorite host. While cats, like humans, fight the intruders off after about two weeks, the eggs can survive in the feces (or in conditions of relatively moisture, such as normal soil) for as long as a year and a half. This can lead to the subsequent contamination of food, water, and virtually anything that comes into contact with infected material. Proper water sterilization and food washing can lower the likelihood of infection, but constant contact with infected cats and their feces is a sure-fire method of infection.
Now, don't take this to mean that cats are all carriers of the disease. As I did say, cats fight off the disease within a matter of weeks. As such, only about one in one hundred cats, worldwide, is estimated to be infected, depending upon the geographic prevalence of the organism, determined mostly by its conditions for survival and the availability of hosts. Human infection in some places is as much as half the population (having ever contracted the disease), and in others is statistically negligible.
Human infection usually results in no symptoms, or at the worst, very minor flu-like symptoms, lasting a few weeks to a few months, sometimes accompanied by a rash. Severe symptoms (such as eye damage, potentially resulting in blindness, called chorioretinitis) occur mostly in immunodeficient individuals, but may occur very sporadically in those of good health.
AIDS and other immune disorders aren't the only cause of immunodeficiency, however. Remember the warning for pregnant women? That's not important because pregnant women can't deal with flu symptoms; the real concern is for the fetus, which, depending upon the time of infection, will not have developed its immune system sufficiently to defend against the infection, should it pass through the placental barrier. Infection during the earliest weeks of pregnancy lends a great risk to miscarriage, stillbirth, and the potential for birth defects. More severe symptoms at birth include chorioretinitis (which can result in blindness) and brain damage. Some such symptoms may take many years to develop.
Rates of fetal infection from the mother depend heavily upon the stage of pregnancy. In many ways, toxoplasmosis during pregnancy is a gamble: infection is unlikely to occur early in the pregnancy, but is more likely to result in severe symptoms if it occurs in the first two trimesters. In the last trimester, infection rates are as high as 50%, but result in less severe symptoms. Somewhere between one in 1,000 and one in 10,000 fetuses in the United States are infected upon birth, though how many were put at risk by parental infection is not entirely known; as such it is difficult to determine the precise rate of cross-placental infection.
Being around a cat is not necessarily dangerous while pregnant, so long as exposure to feces is limited. Do not change the litter of a cat while pregnant without thoroughly washing your hands, if you wish to limit potential risk. Avoid breathing in dust from the litter, as well, as it may make some quantity of the eggs airborne, in effect. Gardening barehanded or without washing may cause a similar effect, as cat feces may have been deposited in the soil unbeknownst to the future mother. Unwashed food or unsterilized water (including well water) is also a potential risk for infection.
