Toxoplasma gondii makes headlines whenever a medical journal publishes the latest findings about the cat-carried parasite, and the studies often leave cat owners wondering if their pet is a health risk.
Research has looked into a possible link between T. gondii and everything from mental disorders to neurological disorders, but is your cat really dangerous?
Here's everything you need to know about T. gondii.
What is Toxoplasma gondii?
T. gondii is the most common parasite in developed nations, and it can infect any warm-blooded species. Cats are the parasite's definitive host, meaning they're the only hosts in which it can reproduce.
How many people have it?
Up to a third of the world's population carries the protozoan parasite, and the CDC estimates that more than 60 million people in the U.S. may have it.
Although it's widespread, T. gondii is more common in certain parts of the world. Americans have an infection rate of 10 percent to 20 percent, while the French infection rate is as high as 55 percent, likely because undercooked meat is a standard part of their diet.
Cats can become infected by eating wild animals that carry the parasite. (Photo: Sreejith K/flickr)
How do cats get it?
Cats typically become carriers of T. gondii by hunting and eating wild animals that are infected. However, they can also contract it by coming into contact with an infected cat's feces.
Once infected, the parasite reproduces in the cat's intestines until immature eggs called oocysts are excreted in feces. Infected cats can shed millions of oocysts for up to two or three weeks after infection.
How do you know if your cat has it?
Infection with T. gondii rarely causes the disease known as toxoplasmosis in cats. Most felines become immune to it through exposure to the parasite.
According to the ASPCA, cats that aren't immune may experience diarrhea or loss of appetite. However, the disease can also affect a cat's lungs, liver and nervous system. Kittens exposed in the womb are the most vulnerable to toxoplasmosis and more likely to exhibit systems.
If you suspect your cat has the disease, you can talk to your veterinarian about having the animal tested.
How do you get it?
If a cat is infected with T. gondii, a cat owner can conceivably contract it by cleaning a litter box or touching a contaminated surface and then ingesting it. You can also become infected by eating undercooked meat or unwashed fruits and vegetables.
Infected cats excrete oocysts in their feces during the weeks after infection, and these eggs can remain in warm soil and water for a year, so it's also possible for a person to come in contact with the parasite in a garden or sandbox, especially in areas with outdoor or feral cats.
People don't contract T. gondii simply by living with a cat or petting a cat.
What are the symptoms of infection?
Most people will never recognize symptoms; however, some may experience flu-like symptoms such as fever, swollen lymph nodes and muscle aches. In rare cases, infected people can develop other conditions such as brain or heart inflammation.
In those with weakened immune systems — such as people with cancer, AIDS or an autoimmune disease — T. gondii can cause a disease called toxoplasmosis, which can result in fetal development disorders, loss of eyesight, brain damage, premature birth and even death.
Can you be tested?
A simple blood test can tell you if you're infected with T. gondii, but most health care professionals say we shouldn't be too concerned. For most healthy children and adults, the immune system prevents the parasite from causing illness.
What's the risk to pregnant women?
Infections that develop in the early stages of pregnancy can lead to miscarriage or stillbirth, and babies can contract toxoplasmosis from their mothers and suffer eye or brain damage.
Janet Morel, a mother of two, contracted toxoplasmosis while pregnant with her first child. She was never tested for T. gondii, and her daughter was born with severe brain and eye damage.
"I hate what toxoplasmosis has done to my daughter. It has stolen the dreams I once had for her, and from me," she said while testifying before the Illinois State Senate in favor of a bill that would require doctors to educate pregnant women about the parasite and test them for it.
According to Dr. Rima McLeod, medical director at the University of Chicago's Toxoplasmosis Center, pregnant women in France are tested monthly until the baby is born, and other countries conduct such tests as well. However, testing for T. gondii during pregnancy isn't ingrained in U.S. medical practice.
"Hopefully we'll begin to do that in this country," she said. "This is a serious disease that's not that infrequent. It causes a lot of suffering and costs for care, and it can be diagnosed and prevented."
How is it treated?
Most healthy people don’t require treatment for toxoplasmosis, but there are antibiotics available to treat symptoms of acute toxoplasmosis and to prevent the spread of the disease to infants by an infected mother.
"We are working to develop medicines to definitively cure all the forms of the infection, the active and dormant forms, and to make a vaccine to prevent it," McLeod said. "Our long-term goal has been to understand this disease in order to improve treatment and outcomes."
Dividing T. gondii parasites under a microscope (Photo: CC BY 4.0/Wikimedia)
Does T. gondii cause mental disorders or other issues?
Many studies have linked chronic Toxoplasma infection to psychiatric conditions, but researchers say there is not proof to confirm that T. gondii is the cause of them.
A study by Stanley Medical Research Institute and Johns Hopkins University compared two previous unpublished surveys from 1982 and concluded that owning a cat in childhood is "more common in families in which the child later becomes seriously mentally ill. If true, an explanatory mechanism may be Toxoplasma gondii."
A second study from the Academic Medical Centre in Amsterdam analyzed 50 published studies and concluded that "findings suggest that T. gondii infection is associated with several psychiatric disorders."
"There may be an association but that doesn't mean there's causation," Dr. Hayden Schwenk, a pediatric infectious disease specialist at Stanford, told the SF Gate. "Lots of people own cats and schizophrenia is a relatively uncommon illness."
"Practically no people we work with have any sign of significant neurobehavioral problems such as schizophrenia, bipolar disorder, depression or suicide. If it happens, it must be very rare," said McLeod who cares for many persons with toxoplasmosis. "When it comes to mental disorders, it's very hard to say what came first. Maybe people who have the disease might be people who simply have more ways to contract Toxoplasma. An association does not mean there is a cause and effect."
A recent study found links the parasite with several brain conditions, including Alzheimer’s disease, Parkinson’s, epilepsy and some cancers.
Can T. gondii affect your behavior?
We know that rats infected with the parasite lose their fear of felines, which increases the likelihood they'll be killed and eaten by a cat, providing T. gondii with a host where it can reproduce. Because it has this effect on rats, several researchers have looked at how it might change human personality or behavior.
Jaroslav Flegr, an evolutionary biologist at Charles University in Prague, suspects that T. gondii modifies connections between neurons in the brains of infected people, which can cause them to behave in strange or even self-destructive ways.
He's conducted numerous studies to test this hypothesis, and the first involved administering personality tests to infected people and parasite-free people. His research revealed that there were many sex-specific changes in personality.
He reported that males with the parasite were more introverted, suspicious and likely to disregard rules while infected women were more outgoing, trusting, rule abiding and concerned with their image. You can read more about his research here.
Other studies that have looked at the effects on personality and behavior have found that infected people have slower reaction times and are more likely to be involved in traffic accidents.
How can you protect yourself?
Even if you're pregnant or have a compromised immune system, experts agree that it's perfectly safe to share your home with a cat if you follow some simple safety precautions such as the ones outlined below:
- Keep your cat indoors.
- Don't feed your cat raw or undercooked meat.
- Wear gloves while changing the litter box and while gardening.
- Change your cat's litter box daily and wash your hands afterward. Use boiling water to clean litter boxes and allow the water to sit for 5 minutes to kill the parasite.
- Cover sandboxes when they're not in use so cats don't defecate in them.
- Cook meat until it's well done.
- Wash fruits and vegetables thoroughly with clean water.
Editor's note: This file has been updated since it was published in June 2015.