The Least Fun-Ghi: The Story of the Zombie-Ant Fungus

Real-life zombie-ant fungus and the portrayal of zombies from the HBO-hit The Last of Us: Getty Images from the New York Post

The Last of Us is a video game series that takes place in a post-apocalyptic world where a fungal infection turns humans into zombie-like aggressive creatures. The story of The Last of Us was heavily popularized by the HBO series, bringing a wider appreciation to the complex and caring relationship between Joel and Ellie. Both of them go through a whole lot to try and escape this brain-eating fungal disease, which really makes you appreciate the fact that it's just a show and brain-altering parasites like this don’t actually exist…WRONG! In this article, we will discuss several studies that look into how a specific type of fungus Ophiocordyceps unilateralis affects the central nervous system (CNS), and how these neural changes alter behavior output.

What Exactly is Ophiocordyceps unilateralis?

Lucky for us, cordyceps (genus of ascomycete fungi) only impact ants, hence the secondary name of the “zombie-ant fungus.” Ophiocordyceps unilateralis is a fungus most typically found in tropical environments. Carpenter ants in these environments contract this fungus via airborne spores that attach to the ants when they are foraging for food. Once the spores breach the ants’ exoskeleton, behaviors are performed that are completely new to the infected host. Some of these behaviors are leaving the nest at times not associated with foraging, as well as being completely unresponsive to other ants and other external stimuli. The most interesting thing we see in these zombified ants is right before the ant dies (for real this time). The fungus makes the ants climb up vegetation, orient their bodies upside down, and permanently “lock on” to it, where they will stay until the fungus runs its course. This “death grip” is accompanied by a fungal stalk that grows from the dead ant’s body, producing more infectious spores (Petruzello 2024). 

Carpenter ant engaged in the “death grip”; note the fungal stalks rising from the top: David P. Hughes from Live Science  

How does this fungus completely take over the ant like this? The change of behavior that is observed is simply an expression of the parasite’s genetic composition through the host itself. When the parasite expresses different genes, so will the host, eliciting a drastic behavior change. But what about the brain? 

Biological Changes

In a 2017 study, researchers found that in carpenter ants infected with this zombie-ant fungus, fungal cells were present throughout all regions of the host's body; furthermore, these cells were connected, forming a meshwork within the host's body. Even more freaky, most of the overall cells in the host’s body were fungal cells, showing how dominant this virus really is. However, no fungal cells were found in the brain at all. (Fredericksen et al., 2017).

Usually, our brain is the mastermind behind all of our behavior: walking, running, working out, you name it. However, the fact that these fungal cells aren’t expressed in the brain is peculiar. If the brain is responsible for our behavior, how is it not responsible in this case? The fungus, as we said before, forms an interconnected network of cells once inside the host’s body. This network surrounds most major components of the ant’s anatomy: the mandibles, legs, etc. This complex network of intruding fungal cells is the one landing the punches here, bringing the brain along for the ride, with the brain merely maintaining a survival role rather than controlling everything that goes on in the ants. In this case, the fungus controlling the legs and mandibles can be seen as the “brain” of the zombie ant, not the brain itself (Gill 2017).

In addition, this abnormal activation of the CNS causes the host to enter a constant starvation state, leading to behavioral changes that are directed toward food/nutrient goals. This has been found in several different studies where genes responsible for the coding of lipase, amylase, and insulin (which play a role in digestion and metabolism) were significantly altered. In a similar light, small secreted proteins that are excreted by the fungus correlate with an upregulation of biting behavior. While these molecules may have many downstream effects that lead to this change in behavior, it is one example of how fungi can introduce exogenous materials that influence insect function (de Bekker et al., 2021).

Moral of the story: the zombie-ant fungus isn’t really a fun-ghi. Ba dum tss.

Sources

De Bekker, C., Beckerson, W. C., & Elya, C. (2021). Mechanisms behind the Madness: How Do Zombie-Making Fungal Entomopathogens Affect Host Behavior To Increase Transmission? MBio, 12(5). https://doi.org/10.1128/mBio.01872-21 

Fredericksen, M. A., Zhang, Y., Hazen, M. L., Loreto, R. G., Mangold, C. A., Chen, D. Z., & Hughes, D. P. (2017). Three-dimensional visualization and a deep-learning model reveal complex fungal parasite networks in behaviorally manipulated ants. Proceedings of the National Academy of Sciences, 114(47), 12590-12595. https://doi.org/10.1073/pnas.1711673114 

Gill, C. (2017). “Zombie ant” brains left intact by fungal parasite. Penn State. https://www.psu.edu/news/research/story/zombie-ant-brains-left-intact-fungal-parasite/ 

Petruzzello, M. (2024, June 10). zombie-ant fungus. Encyclopedia Britannica. https://www.britannica.com/science/zombie-ant-fungus