The reason? A parasite lurking in the pets' poop.
Cats can shed a parasite called Toxoplasma gondii in their feces, and if a pregnant woman accidentally inhales or eats the parasite and becomes infected, there are serious health risks. In 20% to 50% of cases
(PDF), the fetus also becomes infected.
Only a very small number of cats are estimated to be shedding these parasites at any time -- about 1%, by some estimates
-- but if a woman is infected within her first trimester, it could lead to microcephaly and other birth defects, as well as an increased risk of mental disability and blindness later in the child's life.
People with weakened immune systems, such as those who have HIV or are undergoing chemotherapy, can develop neurological problems including headaches and seizures from the infection.
What is the risk to the general public?
Other than the groups who were at greater risk, experts had generally thought toxoplasma, or toxo, was of little consequence. But that view started to change in the past decade as reports claimed
that the parasite could influence a person's behavior and even increase the risk of schizophrenia.
Once a person is infected, toxo lies dormant in their body throughout their life. A recent study found between 10% and 15%
of people in the United States to be latently infected with the parasite.
"The consequences of infection with toxo not during pregnancy is all new and not well understood," said Dr. E. Fuller Torrey, a research psychiatrist at the Stanley Medical Research Institute, a nonprofit organization that supports research on schizophrenia and bipolar disorder.
There have been so many new studies into the link between toxo infection and schizophrenia that Torrey created a web page
to keep track of them all. "I'm adding studies almost every month," he said.
In a 2007 review
, Torrey found that people with schizophrenia were 2.7 times more likely to have antibodies against the parasite -- which indicate that someone has been infected -- than healthy people. This increase in risk would mean that the rate of schizophrenia would increase from 1 in 100 in the general adult population in the United States to 2.7 in 100 among people who have a toxo infection.
A large study
by Torrey and his colleagues found that adults who had schizophrenia were more likely to have grown up in homes that had cats, compared with healthy controls. Parents of young children may therefore want to be careful about bringing a new cat into the home.
Although most of the attention has been on schizophrenia, toxo infection has also been linked to aggressive or suicidal behavior, Torrey said. A study published in March
found that people with an impulse-control disorder, which is marked by sudden aggressive behavior, were more likely to have toxo antibodies. Higher prevalence of toxo infections has also been seen among people who attempted suicide in studies in the United States
, as well as people in Europe
who completed suicide.
went so far as to speculate that toxo "could easily take malaria down from its throne" of being the most deadly parasite in the world, especially if more evidence shows that lifelong latent infection increases the risk of suicide, as well as other incidents resulting from aggressive behavior.
Is toxo really the risk factor?
Despite the evidence found to date, it is still not possible to say whether toxo infections cause these illnesses, said William Sullivan, professor of pharmacology and toxicology, microbiology and immunology at the Indiana University School of Medicine.
Research has merely suggested that people with psychiatric disorders happen to be more likely to have been exposed to toxo. "Correlation plus correlation plus correlation does not equal causation," Sullivan said.
Sullivan suspects that toxo is not enough on its own to bring on the illness and that another, as yet unknown, risk factor could also be involved.
A good way to find out whether toxo truly causes mental health disorders and changes in behavior would be to give people with schizophrenia a drug that clears their dormant toxo infection and see whether their symptoms improve, Sullivan said. But no such drug exists.
There are medications for toxo
, but they only stop the parasite from replicating inside the body -- to prevent the infection from spreading -- and cannot cure latent infections. These drugs are recommended for pregnant women, to reduce their chances of passing the parasite on to their babies, as well as people with weakened immunity.
Cats aren't the only ones to blame
According to Sullivan, there are other ways to become infected with toxo, including eating raw meat, which can harbor the parasite, and gardening or working with soil, where toxo can survive for several years.
A study by Torrey
found that there can be 100 toxo spores, or oocysts, under a gardener's fingernail. Those with a green thumb can take protective steps, such as wearing gloves and a face mask.
People can also become infected by eating the raw meat of animals harboring the parasite. To prevent this, Sullivan advises that people make sure to cook meat thoroughly or else to freeze the meat before eating it.
How can you avoid getting toxo from your cat?
If you are a doting cat owner and worried about your risk of toxo, here are a few facts that might ease your mind:
• Experts think that cats excrete the parasite in their poop only for a period of two or three weeks when they're first infected.
• There is little toxo risk from just being around a cat. The parasite does not linger in their fur or mouth.
• Older cats are less likely to carry toxo, as cats become infected by eating rodents, birds or other small animals infected with the parasite. It is mainly kittens or young cats that hunt these animals.
• Keeping your cat from going outside, where they could kill and eat rodents, can reduce their chances of infection.
• Feeding your cat canned or dry food reduces their risk of infection by preventing them consuming mice, chicken and other animals that can harbor the parasite in their muscles. Humans can also become infected this way.
• Cleaning the litter box every day can help protect you from infection, because the parasites need about a day to form spores and become infectious.
Even with all these precautions, pregnant women should avoid changing litter. "The consequences can become catastrophic," Sullivan said.
However, if no one else can do it, the Centers for Disease Control and Prevention
recommends that pregnant women wear gloves and wash their hands with soap and water right after -- advice that Sullivan also offers to everyone in contact with cats.
Wearing a face mask could also be a good idea to make sure you don't breathe the spores, Sullivan said.
He believes that until we know more, it would be "extreme" to recommend that people, even pregnant women, give up their cats. "Cats and pets bestow upon us very advantageous things, such as companionship," said Sullivan.
The best bet remains to simply steer clear of the litter box, or maybe change it more regularly.