The fungus that inspired HBO’s and Naughty Dog’s “The Last of Us”

This week we are looking at the incredible fungus, Orphiocorydeps unilateralis, that started an amazing horror series. The fungus is commonly referred to as the zombie ant fungus and ants have evolved to deal with it in surprisingly similar ways to the people living in the world of "The Last of Us"

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mell-ø 

 

Transcript: 

 

Welcome to episode 29 of insects for dummies! I’m your host Mitchel Logan and today we’ll be talking about the crazy fungus that inspired a fantastic horror series. 

 

If you aren’t familiar with the series “The Last of Us” it is a critically acclaimed horror video game for Playstation that got turned into an equally stunning HBO television series. The idea behind the series is that in the future there is an infectious fungus that takes over a human’s body and turns them into a mindless zombie. The greater the host is infected the more monstrous and mushroomlike the host looks. If you’re a fan of horror I highly recommend you give either the games or the television series a shot. I’ve played both games and thoroughly enjoyed them. But what if I told you the fungus this series was based around actually exists in real life or that fungi and humans aren’t that different? We’re going to get into all that and more with today’s episode. 

 

In the insect world, a fungus known as Orphiocordyceps can infect a host and then use its body to grow until it breaks out and releases spores. The specific species which got the attention from the creators of Last of Us is known as Orphiocorydeps unilateralis or the Zombie ant fungus. This fungus is found in Brazil, Australia, Africa, and Thailand as well as temperate forests in Japan, South Carolina, and Florida. It’s specific to Formicine ants and causes them to behave abnormally until they can’t move at all, at which point the fungus grows out of the ant's head to release its spores. The shape of the fungus is actually dependent on the geographic location leading to the further classification of subspecies. 

 

Here’s a breakdown of the entire process. First, an unsuspecting ant gets exposed to the fungus by walking over it or having spores land on its body from above. At this point, the fungus itself is relatively harmless until it uses an enzyme to breach the surface of the exoskeleton. Once inside the body, cells of this fungus float around in the ants hemolymph which is essentially the blood, insects don’t have real blood because they’re arthropods. 

 

The fungal cells now slowly begin connecting to each other using tubes to share nutrients. At this point, the fungus starts infecting and controlling muscle cells within the ant. It’s actually a misconception that the ant gets mind controlled because the brain of an infected ant remains untouched through the entire process. Think of the ant as a puppet at this point where the fungus is the puppeteer pulling all the strings from within using the ants' muscles. At this stage, infected ants will begin behaving abnormally, twitching, having strange jolted movements, and leaving the nest for seemingly no reason. Fortunately, because ants are social insects they can pick up on these cues and either kill the infected ant, preventing the virus from spreading in the colony, or leading it very far away to die (last of us quote). There are actually ant graveyards filled with ants that have succumbed to this zombie fungi and in some countries, particularly in the Indo-Australian region, a species of ant known as Colobopsis leonardi has evolved to keep their nests up in trees, and only sends a select few individuals down to forage in the forest floor which minimizes their contact with fungal spores. 

 

Once an infected ant no longer has any control over its body is when the fungus leads it away from the nest and makes the ant crawl up a nearby plant to reach an optimal humidity and temperature for reproduction. The ant then clamps down and remains motionless while the fungus proceeds to kill and then grow out of the body releasing its spores onto an unsuspecting ant colony or worse, ant trails. This behavior evolved with the fungi for two reasons. One, to relocate the ant before a colony member kills it or leads it too far away and the second is to get the ant in an optimal position for fungal development and spreading spores. Wind can carry the spores and has a greater chance of infecting other colonies. The goal is not to kill a colony though because then the fungus dies too! It simply needs a few more hosts to keep itself alive. 

 

A similar fungus known as Ophicordyceps siensis infects ghost moth caterpillars in the upper altitudes of the Tibetan plateau and the infected caterpillars share a similar fate to the ants mentioned earlier. Something different about this fungus though is that it is actually used for medicinal purposes and is very expensive. This fungus is so popular that it's actually worth 3x its weight in gold and many Tibetans rely on the harvesting of infected caterpillars for their livelihood. For example, one pound of this fungi can cost 63,000 dollars and one pound of gold is 22,000. The important must though is for the caterpillar to be intact and connected to the fungus. If the fungus breaks from the caterpillar the price takes a cut.

 

The fungus infects caterpillars during the summer while they’re underground feeding on root systems of host plants, and the fungus slowly grows through the caterpillar during the fall and winter months.

 

 In spring the caterpillar is led towards the surface and a long, brown, spore-filled stalk proceeds to grow out of the head and through the surface of the dirt. I recently saw these being used in a survival television series. They were given as a gift to the survivalist in the show by local Tibetan people. The properties of these caterpillar treats are supposed to provide an all-around boost to your health but most notably to the immune system and your energy levels.

 

As you’ve probably guessed by now the fungi responsible for these kinds of insect deaths cannot affect humans otherwise we wouldn’t be eating them. This is not to say there are no fungi out there which can infect a human, because we, of course, do get fungal infections, but fortunately, the vast majority of fungi are self-reliant and don’t require a host for their survival. Out of the millions of Fungi in the world there are only a few hundred which can affect a human, and much fewer are capable of infecting a healthy person. Fungi usually require much lower temperatures than the human body and they usually get killed off really fast by our immune system. Believe it or not, we actually have beneficial fungi inside that work alongside beneficial bacteria to keep us in good health. However, people who are immunocompromised are more susceptible to having this equilibrium fall out of balance and or getting infected with fungal infections that otherwise don’t have a chance of surviving. This is also possible for people receiving organ transplants because everyone has a different cocktail of bacteria and fungi, and unfortunately an internal fungi infection is not an easily treatable condition because as it turns out, fungi and people share a lot of cellular similarities. Animals and Fungi are actually sister groups and because of this, we have to be careful with how we treat fungal infections.