Intruders

No organ in the body is better protected than the brain: The skull is tough, and the Blood-brain barrier nearly impenetrable. And that’s a good thing – after all, the brain is the center of our existence: we inhabit it. But the world is a dark place, and everything that lives serves as food, shelter, or a host for someone or something else. This isn’t limited to viruses or bacteria – nor is it limited to the skin and intestines: some neurotropic viruses, parasites, amoebas, and even worms find their way into the brain, as well.

Insects are a popular vehicle – a stop along the way to the final host’s stomach. To achieve this, the parasite alters the intermediate host’s behavior, with the goal of inducing its suicide. This is what the larva of the small liver fluke does, for example, by causing ants to bite ithe dust. Toxoplasma gondii operates in a similar way: the parasite removes the mouse’s fear of cats so that it can, precisely, more easily enter the cat’s stomach. Or from a chimpanzee into a leopard – the species doesn’t really matter.

Rabies manifests itself in the emotional brain structures of mammals; microorganisms travel along the olfactory nerve and eat their way through the brain. The same is true of misfolded protein – they are behind mad cow disease in cattle and scrapie in sheep.
They became known in West Papua through kuru. There, people still ate the brains of their deceased loved ones for ritual reasons as late as the 20th century. Even neurodegenerative diseases like Parkinson’s and Alzheimer’s might begin in the gut. Conversely – to say something positive for a change – the right fecal bacteria can apparently reduce anxiety and depression.

But overall, we’re dealing with some pretty creepy stuff here. That includes fine particulate matter from air pollution. And Long COVID. As said: The world is a dark place. And this topic isn’t for the faint of heart.

If you’re brave enough, start with Nora Schultz’s essay on ▸ Foreign Creatures in the Head.