AP/THE CONVERSATION – “Is a zombie apocalypse caused by fungi, like the Cordyceps from The Last of Us, something that could realistically happen?” asked 15-year-old Jupiter of New York.
Zombies strike fear into our hearts – and if they’re persistent, eventually they get inside our heads. Animals taken over by zombies no longer control their own bodies or behaviours. Instead, they serve the interests of a master, whether it’s a virus, fungus or some other harmful agent.
The term “zombi” comes from Vodou, a religion that evolved in the Caribbean nation of Haiti. But the idea of armies of undead, brain-eating human zombies comes from movies, such as Night of the Living Dead, television shows like The Walking Dead and video games like Resident Evil.
Those all are fictional. Nature is where we can find real examples of zombification – one organism controlling another organism’s behaviour.
I study fungi, a huge biological kingdom that includes molds, mildews, yeasts, mushrooms and zombifying fungi. Don’t worry – these “brain-eating organisms” tend to target insects.
INSECT BODY SNATCHERS
One of the most famous examples is the zombie ant fungus, Ophiocordyceps unilateralis, which is part of a larger group known as Cordyceps fungi. This fungus inspired the video game and HBO series The Last of Us, in which a widespread fungal infection turns people into zombie-like creatures and causes society to collapse.
In the real world, ants usually comes into contact with this fungus when spores – pollen-size reproductive particles that the fungus makes – fall onto the ant from a tree or plant overhead. The spores penetrate the ant’s body without killing it.
Once inside, the fungus spreads in the form of a yeast. The ant stops communicating with nestmates and staggers around aimlessly. Eventually it becomes hyperactive.
Finally, the fungus causes the ant to climb up a plant and lock onto a leaf or a stem with its jaws – a behaviour called summiting. The fungus changes into a new phase and consumes the ant’s organs, including its brain.
A stalk erupts from the dead insect’s head and produces spores, which fall onto healthy ants below, starting the cycle again.
Scientists have described countless species of Ophiocordyceps. Each one is tiny, with a very specialised lifestyle. Some live only in specific areas: for example, Ophiocordyceps salganeicola, a parasite of social cockroaches, is found only in Japan’s Ryukyu Islands. I expect that there are many more species around the world awaiting discovery. The zombie cicada fungus, Massospora cicadina, has also received a lot of attention in recent years. It infects and controls periodical cicadas, which are cicadas that live underground and emerge briefly to mate on 13- or 17-year cycles.
The fungus keeps the cicadas energised and flying around, even as it consumes and replaces their rear ends and abdomens. This prolonged “active host” behaviour is rare in fungi that invade insects. Massospora has family members that target flies, moths, millipedes and soldier beetles, but they cause their hosts to summit and die, like ants affected by Ophiocordyceps.
THE REAL FUNGAL THREATS
These diverse morbid partnerships – relationships that lead to death – were formed and refined over millions of years of evolutionary time. A fungus that specialises in infecting and controlling ants or cicadas would have to evolve vastly new tools over millions more years to be able to infect even another insect, even one that’s closely related, let alone a human.
In my research, I’ve collected and handled hundreds of living and dead zombie cicadas, as well as countless fungus-infected insects, spiders and millipedes.
I’ve dissected hundreds of specimens and uncovered fascinating aspects of their biology. Despite this prolonged exposure, I still control my own behaviour.
Some fungi do threaten human health. Examples include Aspergillus fumigatus and Cryptococcus neoformans, both of which can invade people’s lungs and cause serious pneumonia-like symptoms.
Cryptococcus neoformans can spread outside the lungs into the central nervous system and cause symptoms such as neck stiffness, vomiting and sensitivity to light.
Invasive fungal diseases are on the rise worldwide. So are common fungal infections, such as athlete’s foot – a rash between your toes – and ringworm, a rash that despite its name is caused by a fungus.
Fungi thrive in perpetually warm and wet environments. You can protect yourself against many of them by showering after you get sweaty or dirty. – Matt Kasson
