The pandemic depicted in the superb The Last of Us is based on a very real fungus. Should we be worried about it?
Even if the prospect of seeing a new big name in video games slashed by Hollywood had thrilled some fans before the release of The Last of Us series, for once, we must admit that HBO has hit hard with a well-made, beautifully-acted series, and above all, true to the source material. A breath of fresh air for lovers of both arts.
For those who haven’t had the opportunity to play the game, the first episode was also an opportunity to discover the terrible infection that spread across America like wildfire. However, one of the things that make the strength of the story is that it is anchored in reality; it turns out that the devastating fungus is not entirely out of the imagination of the authors, and that there are already variants of it on the Earth today!
A mushroom rooted in reality
In The Last of Us, Joel, brilliantly portrayed by Pedro Pascal in the series, and his companions try to escape a disease called Cordyceps Brain Infection. The first term, Cordycepsdenotes a gender (in the taxonomic sense) whole of fungi, or more precisely of Ascomycete. There are lots of variations; more than 600 different species have already been described in the scientific literature. Although most of them are from Asia, they are found all over the world.
It doesn’t just grow in the ground. It is the best known representative of a large group of parasitic fungi, that is, they need to exploit a living host to complete their life cycle. The Cordycepsin particular, are entomopathogenic. This means that they are biologically armed to infect insects or arthropods, such as spiders (depending on the species).
Their main particularity is the turn taken by this famous life cycle. To ensure its descent, the common ancestor of these mushrooms developed a convoluted and nightmarish scheme that seems straight out of fiction.
How do the true Cordyceps ?
When a poor animal finds itself exposed to the spores of the fungus, it begins to germinate and emit what are called mycelial filaments. Functionally, they can be considered the “roots”, although this is a big misnomer.
Slowly, but surely, these filaments make their way to the nerve center. When they reach the gates of the brain, we witness the beginning of a true animal horror film.
Fungal cells then begin to accumulate around the brain. They form an absolutely fascinating three-dimensional network from a biological point of view. In practice, the fungus does not (yet) damage the nerve structure; he delicately wraps it around to take control of the animal, still technically alive at this point.
The next phase is still relatively poorly understood and varies depending on the precise species of Cordyceps and its target. But the strategy remains the same: the poor insect becomes a kind of zombie who is no longer in control of his movements. Instead, it’s this parasitic puppeteer who pulls the strings and entirely reprograms his natural instincts.
The fungus will then push its host (ant, dragonfly, cockroach, etc.) to move towards a very particular place. The site chosen is generally clearly visible, exposed, and located at a certain altitude (all things considered, since it is about insects).
Once the zombie puppet arrives at its destination, the Cordyceps engages in a final manipulation of the nervous system. It forces its host to cling firmly to its support by any means possible. For example, in the species concerned, the mandibles are often locked to serve as an anchor point. And unfortunately for the victim, she will never move again.
From puppet to spore factory
Because the mushroom, well established in its new resort, is now ready to complete its life cycle. It gradually digests the insect from the inside until the corpse is completely overflowing with conidia, veritable little spore factories.
It is at this stage that the strategic location of the zombie plays a decisive role. Since the insect is generally dead high up, these spores can be ejected by the wind… and beware of the other insects that will be affected by this deluge. Because if one of the spores manages to cling on, the cycle will start again!
For the record, other species have developed even more devious complementary strategies to achieve their ends. We can cite Entomophthora muscae. If it is not a Cordyceps strictly speaking, it is also a fungal parasite that attacks insects, in this case flies.
But to maximize its chances of success, at the sporulation stage, it also emits a powerful aphrodisiac. The victim, clinically dead and firmly clinging to his favorite surface, therefore becomes a formidable trap. Instead of waiting for his congeners to die, he forces them to grapple with the very source of the infection. This is only one isolated example, but it illustrates the fascinating variety of schemes employed by these parasites.
This fungus turns flies into zombies
Should we be worried about a possible pandemic?
There are indeed mushrooms whose spores can represent a danger for immunocompromised people. Mention may be made of Cryptococcus neoformans or Aspergillus fumigatis, which generate pulmonary infections whose symptoms are comparable to those of pneumonia.
But the researchers did not identify any fungus species, Cordyceps or otherwise, capable of inflicting such zombifying treatment on humans…until now.
Mushrooms are traditionally very sensitive to temperature. They can therefore quietly colonize poikilothermal organisms (ie “cold-blooded”, even if this is an abuse of language) such as insects. On the other hand, they do not tolerate the conditions of homeothermic (“warm-blooded”) organisms such as humans. In addition, the latter have a powerful and versatile immune system which is a formidable opponent for a fungus like Cordyceps.
But if we look for the little beast, we can still imagine that a pathogen of this kind could emerge. In the current context of global warming, the latter could eventually develop an increased resistance to temperature.
Above all, this global warming coupled with human activities (particularly animal husbandry, industrial-scale agriculture, etc.) tends to modify ecosystems in depth. This generates new selection pressures which will necessarily lead to the appearance of new strains of viruses, bacteria… and fungi, including certain pathogens. Among them, it is not excluded that a parasite of the type Cordyceps end up colonizing the human body.
Fortunately, these are only speculations at this time. Admittedly, it would be dishonest to say that a The Last of Us-like storyline is completely impossible; but he is extremely unlikely that a pandemic of this kind will occur in the short or medium term.
There is therefore no reason to panic about the imminent emergence of such a pandemic; no need to run to your doctor to ask him for a vaccine against the Cordyceps brain infection described in the Naughty Dog game. Especially since humanity will already have a lot to do in the coming years, with much more concrete threats looming over us – notably from viruses and bacteria, but also solar flares, volcanic mega-eruptions, and so on.
Will global warming cause a new pandemic?
