What Is Toxoplasmosis and Should You be Concerned?

The “Cat Ripper” in Croydon, south London, is very keen to make sure that their acts of cruelty are made public. REUTERS/Thomas Peter

Each year, a handful of medical journals publish studies highlighting a very specific health risk to humans who love their feline friends. Toxoplasma gondii (t. gondii) is a common parasite that makes a happy home in cats and therefore poses a risk to pet owners. A person who ingests the parasite can develop toxoplasmosis, a chronic lifelong infection that many studies link to psychiatric problem. The risk is documented in countless studies, including an analysis of existing research just published in Acta Psychiatrica Scandinavica that finds a strong link between toxoplasmosis and schizophrenia.

But really, how dangerous is your cat?

What is toxoplasma?

The t. gondii parasite is a type of protozoan parasite that has the potential to infect all warm-blooded mammals. Cats are the "definitive host" of this parasite, meaning they are the only hosts in which the parasite can sexually reproduce.

How do you get it?

This parasite has a complicated life cycle. Reproduction in the feline host results in oocysts, the latent zygote that is shed through the cat's feces. Shedding of oocysts only occurs for one to two weeks but within one to five days the oocysts in cat feces become infectious. It's at this point that a cat owner can conceivably pick up the parasite from their pet, most likely while cleaning out a litter box or from touching contaminated surfaces, and eventually ingest the cysts by accident. If that does happen, the parasite undergoes asexual replication, first transforming to tachyzoites, then into tissue cyst, called bradyzoites, which enter the walls of the stomach and small intestines that invade muscle and brain tissue, and eventually spread through the host's bloodstream.

The good news is you can keep your cat around, according to the U.S. Centers for Disease Control and Prevention. "People are probably more likely to get toxoplasmosis from gardening or eating raw meat than from having a pet cat," the agency writes on its website.

There are also reports of acquiring the parasite through blood transfusion and organ transplantation.

How many people have toxoplasmosis?

Approximately 30 to 50 percent of the world's human population plays host to this parasite.

According to the CDC, more than 60 million adults and children in the U.S. carry t. gondii, but very few people actually have symptoms of toxoplasmosis since a normal, healthy immune system can keep it under control.

Geography appears to matter as well. In the U.S., t. gondii is more common in the Northeast than it is in the West. The parasite is also common in countries such as France, where raw or undercooked meat is part of a standard diet, and tropical areas in South America and sub-Saharan Africa, where cats are common and warm weather is favorable to the parasite's survival, according to a study published in the American Journal of Epidemiology.

Who is most at risk for toxoplasmosis?

As with most parasitic and bacterial infections, t. gondii poses a health threat to the elderly, children, women who are pregnant or nursing and people with existing health conditions that already compromise their immune systems, such as HIV or while in chemotherapy. Numerous studies have found pregnant women can transmit the virus to their fetus, which may result in birth defects, miscarriage or stillbirth.

What are the symptoms of infection?

Most people will not have any symptoms, but those who do may report a flu-like feeling that includes swollen glands, muscle aches and pains that can last a month or more. Severe toxoplasmosis causes damage to the brain, eyes and other organs. In particular, eyes may be affected with chronic problems such as poor vision, blurriness, pain, redness and sometimes tearing, depending on the location of the eye lesions.

What are the effects of toxoplasmosis on the brain?

Numerous studies link chronic, not acute, toxoplasmosis to a number of psychiatric conditions, including bipolar disorder and schizophrenia. The new retrospective analysis in Acta Psychiatrica Scandinavica reviewed a total of 50 studies on the subject that analyzed antibody counts. The researchers found toxoplasmosis may have played a role in conditions that include schizophrenia, bipolar disorder, obsessive-compulsive disorder and addiction but most likely isn't linked to depression.

The connection was most clearly defined with schizophrenia, where patients with the psychiatric condition were twice as likely to have t. gondii antibodies, which the immune system produces with an infection from this parasite. Another study suggests that exposure to the parasite during pregnancy can place an offspring at higher risk for developing schizophrenia as an adult.

Analysis of existing research published in the Iranian Journal of Parasitology suggests a chronic infection can impact behavior and personality, from impulsivity to prolonged feelings of guilt and excessive worry.

Other research published in 2009 in the journal BMC Infectious Diseases suggests that the disease may also impact a person's natural reflexes. The researchers found that a higher prevalence of traffic accidents occurred in people with toxoplasmosis, compared with the general population.

What can you do to protect yourself?

Experts say that owning a cat doesn't necessarily put a person at higher risk for acquiring the parasite, though living in an environment with a litter of kittens may contribute to a higher risk.

However, cat owners should still take precaution by cleaning litter boxes on a routine basis since it takes a few days for cysts to become infectious. Pregnant women should consider deferring litter box duties to a domestic partners.