Zombie Ants and the Last of Us

If your taste in horror entertainment runs to post-apocalyptic sagas peppered with cannibals, the collapse of society, and mushroom-headed zombies, you could probably do worse than HBO's adaptation of Craig Mazin and Neil Druckman's popular video game, The Last of Us. The story so far: In the early 2000s, a certain species of the fungal genus Cordyceps that normally invade the bodies of insects and alters their host's behavior in weird and wondrous ways…mutates into a form that invades human beings and alters their behavior in an equally entertaining fashion. Ramping up a few years to 2023, we're introduced to hard luck smuggler, Joel, who gets into a situation where he has to escort a 14-year-old orphan, Ellie, across the country while avoiding tyrannical government forces, rebel fighters with questionable intentions, bandits, cannibals and, of course, uber-charming mushroom-heads infected with the mutant Cordyceps. The plot turns on two points: (A.) The Cordyceps fungus not only seriously uglifies human anatomy, but also causes an "infected" to bite an uninfected person as a way of transmitting the fungus to a new victim, and (B.) Ellie may be the only person who is immune to the disease. From a biologist's perspective, the interesting question is whether a fungus or any other parasitic organism can change its host's behavior to the benefit of the parasite? And the answer is yes, there are lots of good examples—including many of the 100 or so known species of these fungi. But no, the basic premise of The Last of Us, that a mutant form of Cordyceps might somehow infect humans is more than just unlikely. It isn't going to happen. Mazin and Druckman have admitted that they were inspired by one of the segments of the 2006 BBC Planet Earth documentary that showed several gruesome examples of what different species of Cordyceps can do to the insects they parasitize, which is to take over their bodies and turn them into factories for producing more fungi. It's just a three-minute clip, but it's certainly worth a look. Get on YouTube and open the first video that pops up when you do a search for "Cordyceps: attack of the killer fungus." (You'll get the added benefit of hearing David Attenborough's smooth voice-over again.) A minor technical point is that recent genetic analyses have re-classified the Cordyceps fungi into a different genus, the Ophiocordyceps, and that's the name I'll use in the rest of this essay. But David's narrative may have been wrong on a much more interesting detail. Back when the documentary was filmed, everyone (including Attenborough and the creators of The Last of Us) thought the fungus in the video—wonderfully named the zombie ant fungus—works by infecting an ant's brain so as to cause it to climb up a plant stem, and latch onto it with its powerful mandibles. At that point, the fungus turns the ant's body into a fungus-reproduction facility, with other-worldly fruiting bodies sprouting out of its cuticle to rain spores down on the forest floor, infecting other ants. However, it may be the ant's brain is one of the only parts of the insect the fungus doesn't directly infect. At least, not until it's ready to kill the ant after it has attached itself to a stem. In 2017, Penn State researcher David Hughes and colleagues reported that strands of Ophiocordyceps fungus initially intertwine themselves among the muscles of the carpenter ants they studied, but leave the brain alone. Whether by manipulating the ant's muscles directly or by secreting chemicals that interfere with the insect's nervous system, one line of evidence suggests the ant's legs walk it up a plant stem and its mandibles crunch down on it, all without the brain having much say in the behavior. This remains an open area of investigation, however, and other data suggests the brain does play a role. But what's keeping the fungus from developing a Last of Us-style mutation, making the jump from an ant, beetle, grasshopper, or whatever to a human host, and triggering the end of civilization? For one thing, all of these very-well studied parasitic fungi are highly specific to just one or a very few species of closely related insect hosts. Furthermore, it's not as if they haven't had plenty of opportunities to try out humans as potential hosts. Biologists who study the fungi have breathed in their spores for decades without ill effect. And the fungi themselves have been used in traditional Asian medicines for centuries. Anyway, human immune systems and brains are far too highly evolved for some invasive organism to alter our behavior to its own benefit and our deficit. Right? Oh, really? Have you ever had a cold, respiratory flu, or Covid? What do you suppose all that coughing and sneezing is about?