In recent years, there have been some rather controversial headlines in the news about domestic cats. According to the first, pets are capable of driving a person crazy, and in this case it was not about the joy of watching the next video. Experts blamed the Toxoplasma gondii parasite for the development of serious mental illnesses, such as schizophrenia. T. gondii is carried by cats.
However, in other articles the opposite was stated.
Let us explain that T. gondii causes the infectious disease toxoplasmosis. It is estimated that about 65% of people on the planet are carriers of Toxoplasma, although the data vary greatly from country to country.
Toxoplasmosis in most cases is asymptomatic or can manifest itself as infection with the common influenza virus. However, for people with suppressed immunity and for a fetus growing in the womb of the mother, the disease can give serious complications, including death. Antibiotics can cure the infection, but they cannot completely "expel" the parasite from the body.
Scientists have long hypothesized that T. gondii parasites play a role in the development of mental illness, including schizophrenia. And earlier, experts have carried out more than a hundred works in which such a correlation was really shown.
But there is one "but": no research has shown that the parasite actually causes this disease.
Actually, in connection with this, a completely natural question arises: how is it really the case?
First, let's explain how the ill-fated T. gondii can end up in the human body.
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The mustachioed and tailed stars of the Internet catch intruders and toxoplasmosis by eating infected rodents, birds or other animals.
According to some reports, in the United States alone, approximately 40% of cats are infected. Moreover, they may not show any symptoms, although in some cases jaundice or blindness develops.
In the first few weeks after infection, a cat can shed millions of hardy oocysts into its litter box every day.
Some people can contract toxoplasmosis through direct contact with pets and cat feces. Most become infected when the oocysts left by cats enter soil and water, where they can survive for a year or longer.
In other words, Toxoplasma can enter the human body through contact with cat feces, along with poorly cooked meat or contaminated water.
The parasite does not make itself felt, and after infection, people do not begin to feel bad. But he can form cysts in the brain, where he can stay until the end of a person's life.
Why do some scientists seriously believe that toxoplasmosis can cause mental illness?
Much of the evidence came from studies in rodents, which showed strange behavior after infection with T. gondii. For example, they lost all fear, smelling the smell of cat urine (usually animals freeze from fear or hide). Moreover, in some cases, the rodents stubbornly walked towards their enemy.
Experts believe T. gondii alters brain function by forming cysts in areas responsible for fear and decision-making.
In addition, cysts can also influence behavior by increasing levels of dopamine, a neurotransmitter involved in reward and risk taking.
In addition, there is growing evidence that T. gondii infection can change a person's personality and increase the likelihood of developing schizophrenia and other mental illnesses.
Even aside from direct infection of the brain (and the formation of cysts), chronic T. gondii infection can lead to inflammation, which, as previously suggested, is also associated with the same mental disorders such as schizophrenia, as well as autism and Alzheimer's disease. …
However, if with animals everything is more or less clear, then with people it is much more difficult, writes Sciencemag.
While the hypothesis that toxoplasmosis can cause mental illness is compelling, it is extremely difficult to test it in humans, says Duke University geneticist Karen Sugden.
In 2016, Sugden found that 200 New Zealanders infected with T. gondii were not significantly more likely to develop schizophrenia or any other mental disorder.
But, in her opinion, this study does not prove that the parasite is not associated with mental illness. Schizophrenia usually does not appear until late adolescence or 20 years of age.
To find out for sure whether toxoplasmosis causes schizophrenia or not, you need to know if patients were exposed to T. gondii as children or adolescents, that is, before they developed a mental illness, the scientist explains.
Meanwhile, her study tested people as young as 38, so it's hard to say which came first: schizophrenia or a parasite.
Many correlation studies, including the aforementioned work of Sugden, lack the necessary information.
In addition, the Sugden study, like other studies, used a small sample. Schizophrenia is a fairly rare condition, usually affecting about one percent of the population.
And to get reliable statistical results, researchers need to study tens or even hundreds of thousands of people over long periods of time, periodically testing them for T. gondii and mental illness, Sugden says.
Interestingly, in January 2019, scientists published a paper on the analysis of more than 80 thousand Danish blood donors. The number of diagnoses of schizophrenia was quite small - 151 people.
According to a study, people who were exposed to T. gondii had a 47% increased chance of developing a mental disorder.
When the researchers narrowed their analysis down to 28 people who were first diagnosed with schizophrenia after testing positively for T. gondii, they found that these people were 2.5 times more likely to develop the disease after being exposed.
The findings are consistent with other large correlation studies that have also found an approximately 2.5-fold increase in the likelihood of schizophrenia in infected people, says virologist Robert Yolken of Johns Hopkins University School of Medicine (he co-authored the Danish research).
Because the overall frequency of diagnoses of schizophrenia is rare, infection slightly increases the chances of developing an illness, from one in 100 to two to three in 100.
As Yolken and colleagues suggest, T. gondii by itself cannot cause mental illness, but it can interact with genetic variants that make some people more susceptible to developing the disease. If so, T. gondii could be added to the list of environmental factors that do not significantly increase the risk of schizophrenia.
According to experts, even if a person is a carrier of latent T. gondii infection, the chances of getting schizophrenia as a result of toxoplasmosis are small (this is how modern works show).
So far, experts are in no hurry to talk about exact numbers, but they believe that the infection is on a par with other risk factors for the onset of the disease, about which a person most likely has little concern (for example, living in the city).
As for tips on preventing toxoplasmosis infection, everything is pretty commonplace here. Keep cats indoors where they cannot hunt infected animals, dispose of pet feces daily, prepare food properly, and follow other guidelines.