How the tiny parasite Toxoplasma alters its host’s behavior

Scanning electron micrograph of a Toxoplasma gondii tissue cyst in the brain of an infected mouse. Credit: David Ferguson /

Scanning electron micrograph of a Toxoplasma gondii tissue cyst in the brain of an infected mouse. Credit: David Ferguson /

It’s been a while since I received my PhD, but I finally got around to writing an article about the parasite I spent so many years studying in graduate school. That would be Toxoplasma gondii, a tiny intracellular parasite that infects just about any warm-blooded animal, including humans.

My PhD thesis focused on how Toxo invades human cells and makes itself at home, but this parasite is probably more famous for its ability to make rats and mice lose their instinctive fear of cats. That’s a pretty handy trick, as that could in theory make Toxo-infected rodents easy prey for cats. And cats are the only host where Toxo reproduces sexually, so getting into felines allows the parasite to complete its life cycle.

How exactly does a tiny intracellular parasite influence its host’s behavior? Figuring that out has been a challenge. Every time researchers think they have the answer, Toxo seems to confound their expectations.

What makes this particularly interesting is that Toxo is estimated to infect about a third of all humans, and has been associated with various disorders in humans as well, from schizophrenia and suicide attempts to traffic accidents and cultural differences. To be fair, a lot of the human data isn’t particularly robust or convincing…part of my motivation to finally write about Toxo is the fact that there are a lot of misleading or overly speculative articles about what Toxo does in humans, and the evidence for a lot of those effects is pretty weak.

Still, in theory if we understand what Toxo does in rodents, that might help us figure out how it affects us.

We may finally be getting closer to understanding how exactly Toxo manipulates mammalian behavior, and there are some pretty compelling new theories. You can check out my new feature in Ars Technica for more details.

There were definitely pros and cons to writing about a parasite I know so well, and spent so much time thinking about and working on for so many years. On the one hand, I was excited to finally write about Toxo, and I knew a lot about the parasite before I even started my reporting. On the other hand, I maybe knew too many details… I had to work pretty hard to keep the article interesting for a general audience, rather than delving into minutiae that only Toxo researchers would care about.

I also initially included a lot more details about the research in humans, but as I mentioned, that research is pretty messy and inconclusive, and doesn’t really relate very well to the research in rodents, so I decided it was better left to a future article.

Anyway, check out the feature here for a deep dive into how Toxo manipulates rodent behavior.

Also published on Medium.

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