Sunday, February 23, 2014

Is cat poop making us crazy?

When a woman who owns cats finds out she is pregnant, she will probably be warned to stop cleaning out the litter box. This is because cat feces can harbor a parasite called Toxoplasma gondii, which can cross the placenta and infect an unborn fetus. The infection can increase the risk of premature birth and low birth weight, and is associated with anemia, deafness, hydrocephalus, and mental retardation in the child after birth. Sometimes, if these problems aren't apparent at birth, they can develop later in life.

T. Gondii is not only found in cat owners, however. Up to a ⅓ of the population in the developed world might harbor the parasite. And for a parasite that seems to possess some potent mind control capabilities, that’s a scary thought.

T. Gondii is a protozoan parasite that likes to live inside the intestines of cats. In the world of parasites, the host where the parasite is able to thrive and reproduce is known as its definitive host; cats are the definitive host for T. Gondii. T. Gondii infection in American cats has been found to range from 16-80%, and worldwide it’s thought to be around the same as what is seen in humans: 30-40%. 
Often parasites need help getting into their definitive host, and so they will utilize what is known as an intermediate host to get that help. Cats infected with T. Gondii will shed oocysts (sort of a protozoan parasite version of an embryo) in their feces. The parasite can then infect other cats if they come in contact with an infected cat’s feces.


But rodents also often come in contact with cat feces. In fact, they have a habit of digging undigested food out of cat and dog feces to make a sort of recycled dinner for themselves. Thus, they often end up ingesting T. Gondii, and that’s where things start to get interesting.


It is believed that T. gondii has evolved a way to manipulate the behavior of the rodents that it infects. After being ingested, T. gondii makes its way to their brains. Then, through a mechanism that isn’t yet understood, it does something remarkable.

Rats are born afraid of the smell of cat urine. Even if you take a laboratory rat that has never been outside of his cage--a rat that comes from hundreds of generations of other laboratory rats that never saw or smelled a cat before--and expose him to cat urine, he will exhibit a defensive and aversive response.


T. gondii, however, has found a way to influence the behavior of a rat so as to essentially erase that inborn fear. T. gondii-infected rats are much less averse to cat urine, and don’t avoid it any more than they do the urine of other rats. This change in behavior doesn’t appear to be due to a loss of smell, and it seems to be specific for cat urine.

How does this work in T. gondii’s favor? A rat that is not afraid of cat urine is more likely to put itself in situations where a cat might be nearby. Thus, it is more likely to be eaten by a cat (one of its natural predators), allowing T. gondii to end up right where it wants to be: in a cat’s intestines. Many thus believe that this is an example of T. gondii manipulating rat behavior to find its way back into its definitive host.


Rats, however, are perhaps not the only mammals whose behavior can be manipulated by T. gondii. Mounting evidence suggests that T. gondii may also affect human behavior.

In the middle of the twentieth century, some studies started to link T. Gondii infection to schizophrenia. Since then, a number of studies have also found this association. One study that used magnetic resonance imaging (MRI) to look at the brains of schizophrenic patients found that reductions in gray matter (which are a recognized, but not understood, hallmark of schizophrenia) were only seen in patients who also tested positive with T. Gondii.


What the link between T. Gondii infection and schizophrenia means isn’t clear. But a number of studies have suggested links between T. Gondii infection and other psychiatric morbidities as well. For example, there are indications that T. Gondii infection is associated with suicide. In a Danish cohort of almost 46,000 women followed over a period of more than 10 years, researchers found a 1.5-times increased risk of self-directed violence in infected mothers. There are indications of a T. Gondii association with traffic accidents and homicides as well. Curiously, one study even found that T. Gondii infected men rated the odor of cat urine as more pleasant than non-infected men (although in women, the opposite relationship was seen).


How and why T. Gondii would influence human behavior is unclear. It’s possible that T. Gondii just treats rodent and human brains similarly, and any effect on human behavior is simply an unintentional byproduct of the similarities our brains have with other mammals. We have just begun to scratch the surface of T. Gondii’s effects on humans, however. If this infection truly is influencing human behavior, then perhaps we have to begin considering microbial influence as a significant factor in behavior, especially abnormal psychiatric behavior. 


Flegr J (2013). How and why Toxoplasma makes us crazy. Trends in parasitology, 29 (4), 156-63 PMID: 23433494

3 comments :

Rachel Kroupa said...

I find it so crazy that a small little parasite can have such an impact on our brain. The fact that it can control our cerebral cortex without us even really knowing is fascinating. And the fact that we can get this not only by cats but also by cows and pork makes me not want to touch another animal. Our brain is so sensitive and important and the fact that something so small can change our lives so much scares me.

Anonymous said...

To be fair, you should point out that there are as many correlations between T. gondii exposure in humans and arguably desirable variations in personality and cognition (e.g., increased intelligence in women) as between exposure and arguably undesirable variations. It could be considered as much a symbiont as a parasite; there are nations where the large majority of the population are infected and they don't all turn to mindless mush.

Second, the scare-fact that 16 to 80 percent of American cats are "infected" means nothing. As the linked study notes in the very abstract, a cat is infectious only once in its life, for a couple of weeks shortly after infection, and never again. (Further, the feces during that short period have to sit around for a couple of days for the oocytes to become infectious.) If your cat's been an indoor cat for more than a month or two, she's not infectious. You are many times more likely to acquire T. gondii from exposure to raw meat or soil - and yet, there's some evidence that even from a purely healthist value system, exposure to dirt is "good for us."

cc_ctc said...

Microbes rule. Although not entirely convinced that car accidents are caused by T. gondii, it is apparent there can be some influences. For example it has been shown that apetite is caused by hormones sent to the brain from the stomach, and we all know what else is in the tummy. No doubt "we are what we eat" is probably not far from the truth,and probably due to microbes. There is a lot to learn about how they interact with the world.