Toxoplasmosis gondii isn't your garden variety parasite.  Well, actually it is since the one place it is found with very high frequency is in soil, but I digress.  Toxoplasmosis, like all parasites, has a life-cycle it tries its darndest to complete, and that includes finding the right host.  In this case, that ideal host would be the cat, and more precisely the cat’s intestine. This is the one place where the parasite can undergo proper reproduction and create offspring, ensuring its genes survive at least one more generation.  However, it's the disturbing manner in which Toxoplasmosis facilitates this task that has caused some to dub it the “Zombie parasite".    

Anything contaminated by the feces of an infected cat can be the first step on the parasite’s journey back to a new cat host, in particular, any nearby water or surrounding soil.  A very frequent visitor to these types of areas is the mouse, and more often than not it is the mouse that gets infected next.  What better way to increase your chance of being consumed by a cat than to be inside a mouse? I would guess at this point it seems like a statistical inevitability that Toxoplasmosis will either end up in the stomach of a cat or a mouse trap in my attic.  With odds like this, you would think that our parasite would be content rolling the dice, but for Toxoplasmosis, these increased odds are apparently still not good enough.  In a cunning way of drastically increasing the odds of its rodent-host meeting its feline-demise, Toxoplasmosis begins to alter some code in the mouse's brain and ultimately makes the mouse no longer afraid of cats.  In fact, infected mice have actually been shown to be drawn to the scent of cat urine. By altering the mouse's behaviour, and inducing it to engage in high-risk activities almost guarantees that its temporary host will succumb to the teeth of a cat in the very near future, essentially completing its lifecycle.  


This "mind control"  that Toxoplasmosis imparts on its hosts is where it gets its zombie reputation from, and it's a bit disconcerting to know that any animal, even the lowly mouse,  can have its behaviour altered so drastically by a parasite.  But what's far more terrifyingly is that these zombie-like effects aren't exclusive to rodents, and numerous studies have shown them to be present in infected humans.  Specifically, humans infected with Toxoplasmosis  have been shown to engage in more high-risk activities, be more suspicious and jealous, more likely to be schizophrenic and/or bipolar, more likely to commit suicide, and be 2.65 times more likely to be in a car accident.  And to top it all off,  30-50% of the global population is estimated to be infected with Toxoplasmosis.  The Walking Dead doesn't seem so far-fetched now, does it?   

It's debatable why Toxoplasmosis would have these effects on humans, since altering its human host's behaviour in all these high risks ways will increase the risk of the human dying, but not necessarily increase the parasite’s chance of completing its goal and being consumed by a cat. Some theorize that although they don't advance the parasite any further along its path, by increasing the mortality rate of the human host gives the parasite another chance of infecting a cat, likely via a mouse.  Others claim that perhaps prehistoric humans were hunted by large cats, and from Toxoplasmosis’ perspective, a human and a mouse where at one point in time both short steps away from a cat's dinner table.  This idea seems to be supported by the fact that infected humans don't dislike the smell of cat urine as much as their non-infected contemporaries.

... 30-50% of the global population is estimated to be infected with Toxoplasmosis

Either way, Toxoplasmosis definitely infects humans, with astounding rates worldwide.  Since it is so prevalent in soil, in addition to lots of animal tissues, it's very important to wash fresh produce and cook meat (especially pork) to high enough temperatures to ensure destruction of any parasite.  If you do own a cat, its cat-litter needs to be changed almost daily to prevent any Toxoplasmosis in the area from having a chance to begin its journey.  

A retina damaged by Toxoplasmosis, leading to blindness.  The whitish yellow lesion in the centre of the photo is scar tissue related to a Toxoplasmosis cyst.  Copyright Dr. Robert Burke, 2017

The majority of people, if affected, will experience flu like symptoms, but a healthy immune system will back the parasite into a corner, where it will form a small cyst and essentially lie dormant.  Immunocompromised individuals and pregnant women, though, need to be particularly careful, as the parasite can find its way into the brain and/or eyes, where it can cause a significant amount of damage before it's done.  The area in the eye that Toxoplasmosis has a predilection for is the retina, where it has the ability to destroy large swaths of tissue, leading to significant vision loss, and even blindness.  During an eye examination, retinal tissue damage by Toxoplasmosis looks white and excavated, with these white areas representing the cysts of the now dormant parasite.  The majority of patients seen in our clinic with retinal damage from Toxoplasmosis have been infected from birth, being passed to them from their mother when they were a fetus.  This is why pregnant mothers need to be careful around cat litter, and need to use gardening gloves when working with soil.

There is still a fair amount of debate in the scientific community regarding the apparent correlation between those who test seropositive for Toxoplasmosis, and any apparent behavioral changes.  In fact, a recent study that attempted to control for confounding variables did not support the hypothesis that Toxoplasmosis can alter human behaviour, and ultimately concluded that if anything, more studies are needed.  So maybe you don't have to start stock piling your Zombie apocalypse shelter just yet.


Dr. Burke is an optometrist practicing at Calgary Vision Centre.  He does his best to interrupt the Toxoplasmosis lifecyle with mouse traps in his attic.  Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health