It was once believed that crazy ladies acquired a lot of cats. Then it was discovered that a lot of cats instead created crazy ladies; cat poop is laden with an infectious parasite known as Toxoplasma gondii, a protozoan that has recently caused toxoplasmosis epidemics in people. At first it was just schizophrenia in older women, but then broadened out to pregnant women and all people with immune deficiencies. 

Once any link is established some will connect it to obsessive-compulsive disorder and kids' trouble in school, so maybe things have gone overboard in blaming, but Toxoplasma gondii can still be a real concern. Each year in the United States, cats deposit about 1.2 million metric tons of feces into the environment.

E. Fuller Torrey, who directs the Stanley Medical Research Institute, calls for better control of the cat population. Surveys have shown that backyards and communities may harbor three to 400 oocysts per square foot or more in places where cats frequently leave deposits. Each and every one of those oocysts has the potential to cause an infection.

As for the cats, they typically become infected upon hunting and eating an infected bird, mouse, or other small mammal. Then, they spread oocysts around into the soil, grass, water, and elsewhere.

For cat owners, there is little need to worry - if your cats stay indoors. But if that feline friend (or any cat they come in contact with, so virtually all cats) does spend time outside, take care with litter boxes, keep sandboxes covered, and wear gloves when gardening. One estimate shows that the dirt under ones fingernails could harbor up to 100 T. gondii oocysts.

Torrey and coauthor Robert Yolken of Johns Hopkins University Medical Center recommend extra care with young children, who may be at the greatest risk. But, at this point, there are still many unknowns.

"The accumulation of Toxoplasma gondii oocysts, found in cat feces, may be a much bigger problem than we realize because of their apparent long life and their association with some diseases," said Torrey.So is it worth getting tested? "No," Torrey says, except perhaps in the case of pregnant women. "Fifteen percent of us have antibodies, including me."

And, he adds, someone who tests positive at one point in time can later test negative.

Just get a dog.

Published in Trends in Parasitology.