The Last of Us IRL? : Nature’s most infamous “zombie” parasites

A minuscule parasite attaches to a host and cages it to their whims, before bursting out from its body ready to infect the next unsuspecting target. Though this might sound like something from The Last of Us series, some organisms have evolved to do just that as part of their natural life cycle.

To explore some of the most infamous parasites who take control of their hosts, The Ubyssey sat down with local mycologists and entomologists, including a UBC team who discovered one of these critters. From fungi to insects, these organisms may feel like the stuff of science fiction but for the experts, these organisms’ unique strategies for survival are both real and exciting.

Welcome to Zombieland

To ants of the species Camponotus leonardi, the parasitic fungus Ophiocordyceps unilateralis is the stuff of nightmares. This fungus, which infects individual ants and subverts their behaviour in drastic ways, has quite the infamous reputation in pop culture, having been the inspiration for the zombies in the cult video game The Last of Us.

The parasitic action of this fungus is a part of an intricate life cycle, according to Dr. Kent Brothers, a self-described amateur biologist, senior member of the Vancouver Mycological Society and former lecturer at UBC.

It all begins when the fungal spores find themselves on the body of a potential host. “The spores attach themselves when they fall onto an ant. It then proceeds to send out hyphae [extensions of the fungus] that penetrate the ant’s body,” he explained. The fungus can then “generate chemicals which affect the ant’s behaviour,” according to Brothers.

The ant then ventures away from its colony and climbs up to an elevated spot, before clamping its mandibles down on some vegetations to root itself in place. It is in this position that the ant soon dies.

This process, Brothers explained, fulfills a reproductive purpose for the fungus at the expense of its ant host. Once it has infected its host, “the Ophiocordyceps fungus proceeds to digest the innards of the ant,” he said. Ultimately, the fungus sends out a reproductive structure known as a sporophore which releases spores down onto the hapless ants below the original host, thus perpetuating the cycle of infection.

An ant having been infected by a parasitic fungus.
An ant having been infected by a parasitic fungus. Andreas Kay/Flickr

Brothers also said that every ant species is infected by a different species of Ophiocordyceps, demonstrating the high level of specificity involved in these interactions.

Ophiocordyceps can also engage in interactions that complicate how they may be classified by biologists. The genus Tolypocladium was initially included within the Ophiocordyceps group but has since been split off into its own. This genus is marked by fungi with the unique ability to parasitize other fungi rather than insects.

Brothers highlighted that much of what we know about these parasites is relatively new and “evolving very rapidly” as genetic analyses continue to emerge. For example, evidence supporting that each ant species is infected by a specific species of Ophiocordyceps is quite recent ­and there is still more work to be done.

“There is a lot that isn’t known in this field,” said Brothers.

UBC’s own parasitic wasp

Another parasite capable of transforming its host into a mindless automaton is the Zatypota wasp, first observed by entomologist and UBC alum Philippe Fernandez-Fournier, alongside a team of researchers that include PhD student Samantha Straus, UBC alum Dr. Ruth Sharpe and department of zoology professor Dr. Leticia Avilés.

Fernandez-Fournier is the lead author of the 2018 paper first describing this wasp and its parasitism of social spiders. Currently, he is a PhD candidate at Simon Fraser University.

Zatypota’s host, Anelosimus eximius is a species of social spider native to the Lesser Antilles and regions from Panama and Argentina.The social nature of these spiders in itself is “interesting” and distinct from typical spider behaviour, explained Fernandez-Fournier, as spiders are generally solitary creatures. In turn, Zatypota’s success as a parasite is rooted in subverting this aspect of the spider’s life style.

“This parasitoid wasp targets [the spider] and controls their behaviour [making] the spider not social anymore so they become solitary and leave the nest,” he said.

He described how the social spiders live together in the thousands in colonies ­— often leading to the emergence of what he described as “a whole tree full of spiders.”

Speaking to the wasp’s life cycle, Fernandez-Fournier described a process deeply intermingled with the spider host. “The wasp lays an egg on one of those spiders [and] the egg hatches and then becomes a larva,” he said. Once attached, “the larva pierces a hole in the abdomen of the spider to start sucking its [blood-like hemolymph] as food,” he said. He highlighted that “subadults” are often targeted, which are equivalent to “teenager” spiders.

The larva goes on to grow in size to cover the spider’s abdomen in just a few days and injects as-of-yet unknown hormones to control the spider’s behaviour.

The Zatypota larva feeds on the blood-like hemolymph of its spidery host.
The Zatypota larva feeds on the blood-like hemolymph of its spidery host. Courtesy Philippe Fernandez-Fournier

The usually gregarious and social spider soon becomes a recluse, skulking off to a distance of about a foot away from the colony and building a cocoon around itself that is basically “one big mesh of web.”

“So when it’s done building [the cocoon], the larva eats the spider and makes its own little cocoon inside the web that the spider built and is now protected from the elements.”

The cocoon acts as a safe environment for the larva to grow into an adult wasp and burst out to continue the cycle of parasitism on other many-legged victims.

Fernandez-Fournier emphasized the strangeness of the social spiders themselves, stating that the spiders “literally never leave the nest except if the conditions become really bad.” This makes the infected spider leaving the colony particularly notable. He also added that the immobility of the nests make it possible that the new wasps can come back to parasitize the spiders from that same colony over and over again.

The parasite’s strategy is not without risk for the parent wasp, however.

“If they get too close to the colony, they might get caught as prey … egg-laying is probably very risky,” said Fernandez-Fournier. “We believe they target spiders that are on the outside of the nest.”

The other spiders seem largely unperturbed by the appearance of a large parasite on the abdomen of one of their fellow residents, according to Feranadez-Fournier. The infected spiders “go about their normal business with a larva on their abdomen and nobody notices.”

The spider’s transformation is quite speedy, happening in a matter of around two weeks.

“I think why it’s interesting for everybody is because of how weird it is,” said Fernandez-Fournier.