Cats in the U.S. deposit about 1.2 million metric tons of feces into the environment every year, and that mountain of kitty poop may be harboring an overlooked public health menace, scientists say.

You may have read stories about Toxoplasma gondii, the “cat lady” parasite that has been linked to an illness called toxoplasmosis, as well as a range of psychological disorders and a raised risk for suicide. Cats pick it up usually from eating an infected bird or rodent, and can pass it to humans through their excrement -- say, if an owner doesn’t wash his or her hands after changing a litter box.

You might figure, if you’re not keeping 20 cats in the house, what’s to worry about? But Johns Hopkins University Medical Center researchers E. Fuller Torrey and Robert Yolken say in a paper for the journal Trends in Parasitology that feline fecal material could be a vector for the disease to pass more widely. Even if you’re not a cat person, you are probably coming into contact with cat poop without realizing it.

The travel-ready form of the T. gondii parasite is at a hardy stage of its life cycle called an oocyst, a tiny bundle of a few cells. Oocysts can remain viable for more than a year if deposited in the right conditions, and a single infected cat can shed anywhere from 3 to 810 million oocysts over a little more than a week. A 2007 survey of residential areas around California’s Morro Bay estimated that residential areas contained anywhere from nine to 434 T. gondii oocysts per square foot.

The potential for infection may be even higher in places likely to attract cats looking for a place to relieve themselves -- sandboxes, playgrounds, parks and gardens. Children may be the most vulnerable to such contamination. Torrey and Yolken noted that scientific evidence shows toddlers put their hands in their mouths every two to three minutes, and routinely eat soil throughout the day.

“Although there are no measurements of how many T. gondii oocysts are required to infect a child, for obvious reasons, a study that was conducted with pigs found that a single oocyst was sufficient to cause infection in 13 of 14 experimentally infected pigs,” the authors wrote.

Torrey and Yolken acknowledge that much more research needs to be done to understand just how serious a threat cat poop poses. Many humans already possess antibodies to T. gondii, and the more serious reported cases of toxoplasmosis tend to be found in the elderly or people with weakened immune systems. In the meantime, the authors recommend keeping a cover on your backyard sandbox and wearing gloves while gardening, just to be safe.

The bugs in cat poop aren’t just threatening to humans. T. gondii has been blamed for a spate of sea otter deaths off the California coast. It’s thought that the parasite might pass from cat poop to storm-water runoff into the ocean, where it can get into the otter’s dinner of mussels and other shellfish. There’s even a new strain of T. gondii called Type X linked to the majority of otter infections (it’s not yet clear if it’s any more virulent or serious than the more familiar strain in humans, Type II). Something to think about before you let Fluffy out for the night, perhaps.

SOURCE: Torrey et al. “Toxoplasma oocysts as a public health problem.” Trends in Parasitology published online July 9, 2013.