Alert! "Cats can literally drive you crazy." Wait! "Cats do not cause mental illness." News headlines are as alarming as they are contradictory. All refer to Toxoplasma gondii, a cerebral parasite carried by our feline companions that infects about one in three people.
Scientists have long hypothesized that T. gondii plays a role in mental illness, including schizophrenia. But although more than 100 studies have found a correlation, none has shown that the parasite actually causes mental illness. So what's really going on? Here's what you need to know:
How are humans infected?
T. gondii it is not a bacterium or a virus, but a unicellular microscopic organism distantly related to the parasite responsible for malaria. Cats get T. gondii and the disease it causes, toxoplasmosis, by eating rodents, birds and other infected animals. Estimates suggest that about 40% of cats in the United States are infected; most have no symptoms, but can develop jaundice or blindness and experience personality changes if the parasite spreads to the liver or nervous system.
In the first few weeks after infection, a cat can throw millions of sturdy egg eggs, called oocysts, into bedding every day. Although some people contract toxoplasmosis through direct contact with domestic cats and their excrement, many more are infected when oocysts released by cats enter the soil and water, where they can survive for a year or more .
Only about 11% of people infected with the virus T. gondii in the United States, although rates are much higher in areas where people eat more raw meat or where sanitation is poor; for example, infection rates exceed 90% in some parts of Europe and South America. In healthy people, toxoplasmosis often causes flu illness or no symptoms. But it can sometimes be dangerous – even fatal – in people whose immune system is weakened. Antibiotics can treat the infection, although the drugs can not completely ban the parasite.
Why do scientists think that toxoplasmosis could cause mental illness?
Most of the evidence comes from rodents, who develop bizarre behaviors in case of infection with the virus. T. gondii. They lose their fear of the smell of cat urine and sometimes fall into the jaws of felines waiting. Scientists think T. gondii Alters brain function by forming cysts in areas that deal with fear and decision-making. Cysts can also affect the behavior by increasing the levels of dopamine, a neurotransmitter involved in reward and risk taking. There is evidence that T. gondii can permanently rewire the brain, which makes the mice little fear of cats even long after the elimination of the parasite.
T. gondii also forms cysts inside human neurons. In people with HIV or other immunodeficient diseases, cysts can develop and reproduce, causing deadly inflammation of the brain, dementia, and psychosis. Although scientists have long believed that cysts are benign in healthy people, a growing body of evidence suggests T. gondii the infection can alter the personality and increase the risk of developing schizophrenia and other mental illnesses. Even without directly infecting the brain, a chronic disease T. gondii The infection can worsen the inflammation and the inflammation has been linked to mental disorders such as schizophrenia, autism and Alzheimer's disease.
How strong is the evidence that this happens in people?
This is where things get complicated. According to Karen Sugden, a geneticist at Duke University in Durham, North Carolina, the hypothesis that toxoplasmosis could be at the root of a mental illness is extremely compelling, but it is extremely difficult to test it.
In a 2016 study, Sugden revealed that 200 New Zealanders infected with the virus T. gondii had no significantly higher chances of schizophrenia or any other mental disorder. But she says the study does not prove that the parasite has no connection with mental illness. Schizophrenia usually does not appear before the end of adolescence or the twenties.
To find out if toxoplasmosis the causes Schizophrenia, Sugden says that it would be necessary to know if the participants were exposed to T. gondii as children or adolescents – before they develop a mental illness. But his study only tested the parasite at the age of 38, too late to tell whether the infection or schizophrenia was the first. Many correlational studies, including that of Sugden, do not have access to this kind of information, she notes.
The Sugden study, like others, was also based on small samples. Schizophrenia is a rare disease, usually affecting about 1% of the population. For reliable statistical results, researchers need to track tens or even hundreds of thousands of people over long periods of time by testing T. gondii and mental illness periodically to determine what happened first, she says.
Have studies been conducted on the timing of T. gondii and the onset of mental illness?
Last month, scientists published the first study of more than 80,000 Danish blood donors. Yet even in this large group, the number of diagnoses of schizophrenia was relatively low: 151 people. The study found that people exposed to T. gondii 47% chance of being diagnosed with schizophrenia. When the researchers looked at the problem of timing, reducing their analysis to 28 people diagnosed with schizophrenia for the first time after a positive test for schizophrenia. T. gondii exposure – they found that these people were 2.5 times more likely to develop the disease after exposure.
This number is comparable to that of other large correlational studies, which also revealed a 2.5-fold increased probability of diagnosing schizophrenia in infected individuals, says Robert Yolken, a virologist at the Faculty of Medicine of the United States. Johns Hopkins University in Baltimore, Maryland, and one of the authors of the Danish study. Since the overall rate of diagnoses of schizophrenia is rare, however, the infection only slightly increases the odds – for example, one chance in 100 to two to three out of 100 a possibility of schizophrenia diagnosis.
Yolken and other researchers suspect that T. gondii may not cause mental illness per se, but interact with genetic variants that make some people more vulnerable. This adds T. gondii to the list of environmental factors that increase the risk of schizophrenia by a small but measurable amount, such as prenatal infection and socio-economic status, he says.
So should you worry?
Even if you become one of about three people who carry a latent virus T. gondii Infection, current research suggests that the risk of developing schizophrenia directly related to infection with toxoplasmosis is low.
At what point? It would be premature to say a number, but it seems to be at the same level as other risk factors for schizophrenia that you probably do not worry about, such as living in the city.
"The advice to prevent infection with toxoplasmosis has been around for a very long time," says Sugden. This includes keeping cats indoors where they can not chase infected animals, evacuate litter daily, cook well and other recommendations from US Centers for Disease Control and Prevention.
Yolken, who has two cats, does not want people to worry about their pets, but enough concern to support the search for a toxoplasmosis vaccine for cats and better treatments for cats and dogs. humans. He thinks that it will not be possible to really pin down T. gondii on mental health once it is possible to prevent and treat the parasite. The need is most urgent in countries where infection rates are high, he notes. "The question is, what could people do better if we could get rid of toxoplasmosis?" He said. "The risk may be low, but we could reduce it further."