Sara Moore
Science Editor
While human zombies do not walk among us, there are “zombies” who exist in nature. Ophiocordyceps unilateralis, more commonly known as the zombie-ant fungus, is an insect-pathogenic fungus that targets members of the tribe Camponotini — most often carpenter ants. While most other fungi focus on decomposing plant material or forming symbiotic relationships with plant roots, the zombie-ant fungus targets living organisms. The fungus is found around the world in tropical and temperate forests where the humidity and temperature supports their growth.
The “zombie” name is a bit of a misnomer, as ants are not compelled to eat each others’ brains when infected with the fungus. Instead, the ant becomes a host to the fungus, putting the parasite’s needs ahead of its own. The fungus infects the ant by releasing its spores into the air, which then settle on the ant and penetrate its exoskeleton. The fungus compels the ant to leave its nest and head to a warmer microclimate that favors the fungi’s growth. Once the ant finds a suitable spot off the ground, it locks its mandibles into a leaf vein in what is known as a death grip. The fungus then feeds on the ant’s innards, killing the ant. Finally, the fungus sends a fruiting body through the base of the ant’s head, allowing the fungus to spore and begin the cycle again.
Contrary to its name, the zombie-ant fungus does not infect the host’s brain. A study done at Pennsylvania State University found no trace of the fungus in the brains of the infected ants, despite the fruiting body growing out of their heads. The fungus instead invades the host’s muscle fibers, creating a new network that encircles the muscles. The fungus is then able to send signals to the ant’s brain through this new network. This allows O. unilateralis to influence the ant’s movements while remaining undetected by the other ants.
Unlike other diseases and parasites, O. unilateralis can incubate in the ant’s body for a period of time before full infection occurs. This is beneficial for the fungus, as social insects like carpenter ants protect their colony through social immunity. If an ant shows signs of illness, they are removed from the colony in order to prevent the spread of disease to the rest of the ants. However, this is not necessary for the zombie-ant fungus, as the infected individual will remove themselves from the colony on their own. Since the mortality rate for infected individuals is 100%, O. unilateralis cannot infect every individual in the colony in order to maintain population balance and have continued access to living hosts. The fungus acts as a form of population control for the ants, but it only needs to infect a few individuals at a time in order to maintain its own population.
Beyond O. unilateralis, there are over 200 species in the genus Ophiocordyceps which can infect hosts from over 10 insect orders as well as spiders. These fungi do not impact their hosts in the same way O. unilateralis does as they do not always kill their host or alter their behavior. For example, the Ophiocordyceps fungus which infects Japanese cicadas, has replaced symbiotic bacteria in aiding their hosts in processing nutrients from tree sap. Other forms of the fungi are used in medicine, such as O. sinensis, which infects ghost moth caterpillars. These infected insects are prized in traditional Chinese and Tibetan medicine as an immune booster, cancer treatment and an aphrodisiac.
Ophiocordyceps unilateralis gained popularity in recent years after it was featured in the video game and television show “The Last of Us.” In this fictional world, the fungus evolved to infect humans as an adaptation to climate change. Scientists are not concerned with this becoming a reality, as humans and insects are biologically very different, only belonging to the same kingdom, Animalia. As Ilan Schwartz, a scientist at Duke University who studies invasive fungal species told Vulture, “there’s lots of serious concerns in the world, but this isn’t one of them.”
