Few people know that cats are the main carriers of a parasite that can cause serious health problems in humans, and not just any. And not just any: as several recent studies have shown, Toxoplasma gondii, the name of the parasite, can affect the brain and lead to changes in our behavior.
About half of the population is thought to have been infected with the parasite at some point in their lives, but our immune systems usually eliminate it without us even knowing it’s been hiding in our bodies.
Most people are aware of Toxoplasma gondii and the disease it causes, toxoplasmosis, when they are pregnant, because it is the human fetus that is really in danger: congenital toxoplasmosis can lead to mental retardation, deafness or retinal damage in newborns.
Many gynecologists advise women not to live with cats during pregnancy. But until recently, it was thought that, except in fetuses and people with compromised immune systems (such as AIDS patients), the parasite was not a problem. Should we really start to fear our pets?
The only carrier of the disease
Toxoplasma gondii has the ability to infect most warm-blooded animals, including humans, but it is only in felines that the parasite can complete its life cycle.
When a cat becomes infected with the parasite (usually because it has eaten a rodent with toxoplasmosis or contaminated raw meat), it invades the walls of its intestine, where it reproduces leaving millions of tiny oocysts: very resistant spores that carry the parasite until it finds a host and which the cat expels in its feces.
These oocysts can end up in our body or in the body of any other animal (including cats themselves) and, once inside, release the parasite into our bloodstream, where it acts until our immune system takes care of it.
Typically, the immune system forces the parasite to cluster in small cysts in the brain and muscles, and until then a healthy person only notices flu-like symptoms (although than less intense). However, in sick or old people, whose immune system is weaker, the parasite can be released and cause serious illness by attacking our organs, especially the brain and eyes.
Is your cat going to drive you crazy?
In 2012, a group of Swedish scientists discovered that Toxoplasma gondii has a mechanism by which it manages to manipulate our central nervous system. When the parasite infects dendritic cells – a type of immune system cell found only in mammals – they secrete a neurotransmitter called GABA which is normally produced in brain cells and which changes the way our brain functions. A sort of “Trojan horse”, as scientists say, which can lead to behavioral changes in infected people.
The extent to which the parasite can affect our psyche remains to be studied, but some scientists say its influence is much greater than we think. This is the case of Czech doctor Jaroslav Flegr, an outsider who has been speculating on this possibility since the 1990s and who is only beginning to be taken seriously by the scientific community.
According to Flegr, the parasite is capable of acting in its latent phase (from which, let us remember, half of the population “suffers”) and gradually causing changes in neuronal connections, modifying our reaction to frightening situations, our confidence in others and even our preference for certain smells. As Flegr explains in a controversial article in The Atlantic, the parasite could even trigger suicidal thoughts and mental disorders such as schizophrenia.
This all sounds far-fetched, but it has been shown, for example, that mice with toxoplasmosis are more easily preyed upon by cats, which could be a way for the parasite to ensure its survival. Is our brain in the hands of a parasite? This remains to be seen but, just in case, we must remember not to be happily contaminated.
A growing public health problem
As felines are the only natural hosts of the parasite, their increased presence in cities has also increased the prevalence of toxoplasmosis. According to a 2013 study published in the journal Trends in Parasitology, the presence of oocysts in children’s playgrounds or kindergartens should be considered a public health problem.
“It is estimated that one percent of cats are infected and produce oocysts constantly. The problem is that these spores can live for years and accumulate in very large numbers,” said Fuller Torrey, one of the study’s authors.
It should be emphasized that it is not cats that spread the disease, but their feces. It is entirely possible to live with a feline without ever contracting the parasite, especially if it does not leave the house: it is stray cats that carry the disease in greater numbers. Additionally, oocysts are only infectious 24 hours after defecation, so if you change your cat’s litter every day and use gloves to do so, you won’t have any problems. Even pregnant women don’t have to worry if they follow these rules.
Although felines are the only animals that can transmit the disease, humans generally do not catch the parasite through them, but through food. Since oocysts survive an average of 18 months and are ubiquitous in rural areas, it is not uncommon for livestock to become infected. Oocysts can also be found in fruits and vegetables. That is why pregnant women are asked not to eat raw meat under any circumstances and to wash fruits and vegetables thoroughly, but perhaps we should all take this advice to heart.
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