The Perils of Poop—Part I

by Dianna Stirpe 

“[It] can only reproduce sexually in felids.” Felids (felines) are T. gondii’s only known definitive hosts—the parasite must complete its life cycle in cats, where it produces infective oocysts in their intestines, which are then shed in their feces. 

Within the first week or two after being infected, a cat can shed hundreds of millions of these oocysts. With, as the journal of the American Veterinary Medical Association reports, at least 44 percent of domestic cats defecating outdoors, amounting to over 76 tons of waste every year--that puts quite a burden on the environment. 

These oocysts are tenacious too. They can remain infective for up to 18 months and in moist soils for years, while birds, insects, and earthworms mechanically carry them on to potential food sources and into water. 

Infected birds, rodents, dogs, sheep, pigs, or other warm-blooded animals may then act as intermediate hosts; though they do not produce and shed oocysts as felids do, skeletal muscles of an affected animal carry the parasite. When another animal eats that muscle tissue, T. gondii can be acquired. 

If your cat roams outside, she can pick it up by eating scavenged meat or an infected rodent, bird, or rabbit, but also from soil or surface water. And she may never display apparent symptoms of the infection, since these can be as subtle as a mere increase in affectionate behavior. 

Thankfully, many affected cats recover and develop antibodies against additional infections, and the chances your cat will shed oocysts a second time are slim. But in the U.S. there are an estimated 78 million domestic cats and as many as 90 million feral cats, compared to the wild felids, such as cougars and bobcats, which number in the tens of thousands.

Therefore, our furry best friends are likely the primary source of ocean contamination, with up to 2 percent of them shedding oocysts at any given time. Since toxoplasmosis is becoming more common in marine mammals, Dr. Gulland says, “felid fecal material must be entering the marine ecosystem.” If you flush your cat’s feces down the toilet, you may be spreading the parasite without realizing it. 


Wastewater treatment plants cannot eradicate oocysts. Neither chlorine nor ultraviolet light can fully destroy them. Even if sewage treatment systems could one day be designed to neutralize them, overflows occur, especially in the rainy season—which we in the Northwest are all too familiar with!

Some amount of untreated sewage invariably ends up in streams and rivers feeding either directly or ultimately into the ocean, no matter what part of the country you live in. And if you compost your cat’s waste or allow her to bury her feces outside, both rain and sprinkler runoff carry oocysts and any other contaminants into storm drains, which in turn carry them into freshwater systems and ocean saltwater, where oocysts can survive for two or more years.

How marine mammals all over the planet are becoming infected with toxoplasmosis continues to be studied, but sea otters appear to ingest the parasite through migratory filter-feeder fish (such as anchovies), gastropods (such as turban snails), arthropods (such as crabs), or bivalves (such as oysters). All these food sources absorb and accumulate contaminants in their tissues. 

Because the highest prevalence of toxoplasmosis in marine mammals has been found in those feeding closest to where rivers and drainage pipes outflow, and because sea otters—a federally listed threatened species especially hard hit by this parasite—do not prey on known intermediate hosts of T. gondii, scientists are certain the source is felids. 

Sea otters live exclusively in near-coastal environments and are considered sentinels of environmental change. Recent studies show that 17 percent of all sea otter deaths have been directly caused by toxoplasmosis; 62 percent of all the dead and 42 percent of all the living sea otters tested have been found to carry the parasite in their system; and 76 percent of sea otters dwelling primarily near storm drains and river mouths carry T. gondii antibodies.

When a marine mammal is infected, the parasite usually causes encephalitis (infection of the brain) resulting in muscular tremors, ataxia, paralysis, and seizures. The condition remains difficult to treat and is almost always fatal. Also, the animal’s vulnerability makes it an easier target for sharks. 

All marine mammals struggle against an ocean of adversity: oil spills, chemical runoff, plastic and non-biodegradable human garbage, boat collisions, entanglements in fishing gear and marine debris, a diminishing food supply from decades of human overfishing, and a host of other parasites, diseases, and pollutants on a list far too long. The ocean is their sustenance, their home. 

Whether or not your cat has toxoplasmosis, here’s how you can help marine life weather their troubled waters: 

• Always bag and trash your feline’s poop, and make sure it’s deposited in approved sanitary landfills where runoff is controlled. Compostable litters have excellent environmental benefits, but it’s better to play it safe and never flush or compost your pet’s waste.
• Bag any cat feces you find in your yard from neighborhood cats too. Every little bit helps!
• Keep your cat indoors. With the right toys and stimulating interactive playtime with you, many cats adjust to indoor life without a hitch. Doing this keeps them safer overall, may allow them to live longer, and will also help keep our small bird populations from declining as rapidly as they are currently.
• Don’t allow your cat to eat rodents that get into your home.
• If you feed your cat a raw diet or any raw treats, make sure to fully freeze such meats and/or organs before thawing and serving, since T. gondii is much less likely to survive being frozen.
• Volunteer with spay/neuter events in your community and support Trap-Neuter-Return programs. TNR allows stray and feral cats to live out their lives but can help keep their populations from growing.