Parasite not cat-astrophic, says WSU researcher

By Linda Weiford, WSU News

FritzPULLMAN, Wash. – Mention “toxoplasma gondii” and most people will give you a blank stare. Others will blame a cat.

The truth is, this cunning parasite infects one-third of the world’s population and is more likely to be spread by eating undercooked meat than by stroking a feline.

Though many people have never heard of toxoplasma and the disease it causes, toxoplasmosis, the protozoan is surprisingly common, said Heather Fritz, a veterinary parasitologist at Washington State University who studies the unique organism. As for people who claim that cats are prowling time bombs of infection, “that’s just not the case,” she said.

Not only is toxoplasma found in an estimated 60 million Americans, but it’s also capable of infecting nearly every warm-blooded animal on the planet.

“It’s what you’d call a very successful parasite,” said Fritz, who is probing the secrets of its success in hopes of developing ways to control its spread in the environment.

Whiskers, dangerous?

Toxoplasmosis causes few, if any, symptoms among most people who are healthy. But it can be dangerous to those with weak immune systems or pregnant women who can pass it to their unborn babies. At worst, it leads to blindness, brain damage, dementia and even death.

Veterinary parasitologist Heather Fritz, nine months pregnant, with her cat Phil. (Photo by Henry Moore, WSU Biomedical Communications Unit)

And yes, cats do shed the parasites in their stool. But the chances of the pathogens being passed on to humans are slimmer than most people have been led to believe by hyped-up media reports.

“Beware of the cat: Britain’s hidden toxoplasma problem,” blared a headline in the United Kingdom’s Independent. “How your cat is making you crazy,” boomed an article in The Atlantic.

Cat lovers, take a deep breath. There’s no need to place Whiskers in another home, said Fritz.

“You stand a greater chance of contracting toxoplasmosis by drinking contaminated water or eating contaminated meat that’s raw or inadequately cooked than getting it from your cat – especially if it’s an indoor cat,” she explained.

Here’s why. Although the parasite is found in all kinds of birds and mammals, there’s only one place where it can sexually reproduce: Inside a cat’s gut – whether a tabby, calico or cougar – so long as it’s a member of the felidae family.

Typically, cats contract toxoplasma from infected rodents and birds when they hunt. Once inside the intestines, the parasite produces millions of egg-like cysts called oocysts, which the cat sheds in its feces, said Fritz.

When excreted by outdoor cats, the oocysts can survive more than 18 months in soil and water. From there, the parasite is typically picked up by animals such as rodents, sheep and pigs grazing or feeding on the ground, she said. As for indoor cats, unless they snag an infected rodent in the house, “it’s nearly impossible for them to contract the parasite.”

Let’s say Whiskers is an outdoor cat who contracts the parasite after catching an infected mouse.

“The cat will only excrete the oocysts in its feces for three weeks or so – and typically only once during its lifetime,” Fritz explained.

So don’t throw out the cat with the kitty litter. But do practice good hygiene, handling meat properly, cleaning the litter box often and wearing gloves while doing it, Fritz advised.

Puppeteer parasite

Countless domestic and wild felines may rule, or rather, roam the earth, but that alone doesn’t explain why an estimated 2 billion people worldwide are infected with toxoplasma.

Studies show that toxoplasma makes infected mice and rats unafraid of cats, who in turn can easily eat them. (Photo by Wendy Ingram, Adrienne Greene, U of California)

“It goes back to the organism being so successful,” said Fritz. “It’s able to invade almost every type of cell, inside what’s likely every type of warm-blooded animal. Even sea otters can contract it,” she said, pointing out that the parasite has managed to make its way to the ocean.

In part, what makes this protozoan so successful is that once it enters the host animal, it spreads swiftly by manipulating the very fleet of cells that should be killing it. Akin to getting a lift on city taxis, the parasites “actually use immune system cells to travel around the body,” she said.

In another remarkable ploy, toxoplasma can alter the brains of mice and rats, causing them to be unafraid of their arch-enemy, the cat. This means the cat gets an easy meal and toxoplasma gets a new home.

“The parasite infects many, many mammals, but it can only complete its life cycle in the intestinal tract of a cat,” said Fritz.


Heather Fritz, WSU veterinary parasitologist,
Linda Weiford, WSU News, 509-335-7209,