The pathogens mantra “keep going”
In these blog posts, we might have discussed how amazingly pathogens manage to transmit themselves. And if we have not discussed it, this type of information can easily be extracted from the many wildlife disease stories I have told you. At the end of the day, seizing the opportunity to transmit a pathogen is what is called “spillover”, literally the evolutionary jump from one species to another. That is not the only evolutionary strategy, though. If a pathogen is, for example, highly specialized in one species, it would have to make sure that it reproduces along, that it “keeps going”. From infection to reproduction to infection again. This happens when a pathogen necessitates a host for its reproduction, a sort of host-mediated cycle, which is most often the case. A shocking one comes from a genus of endoparasitic fungi, including about 600 different species: the cordyceps.
Cordyceps: it’s not a horror movie
The cordyceps life cycle is what made me drop my jaw and open my eyes VERY wide while sitting in a parasitology classroom during my undergraduate course. Most cordyceps are parasitic, ranging from infecting insects to infecting other fungi. One species, in particular, Ophiocordyceps unilateralis, infects ants in a very unique manner. The elongated fruit body of this fungus releases infectious spores, which attach to the exoskeleton (the external skeleton) of ants living in the canopy. These spores slowly but surely break down and enter the exoskeleton, making their way into the ant body and musculoskeletal system. Quite normal so far, just pathogens entering the hosts.
However… something else happens. Ants that have been infected by the spores just start acting strange. At this point, the fungus is growing its filaments inside the ant body and commanding its behavior. Towards the infection peak, the fungus filaments will “command” the ant to climb up a stem, clamp their jaws on it, dry out and die there. If you think this is horror enough, wait for what comes next. The new adult fungus that has slowly established its filaments through the ant systems slowly grows out of the insect’s head, from which it will release more spores. Evolution has led the ant population to somehow adapt to this fungus attack. So much so that ants can sometimes be seen kicking out ants that “act strangely”, to protect the colony from a potential cordyceps spore release happening nearby. Quite crazy, I know. If you don’t believe me, you might believe a much more well-renowned scientist, David Attenborough, in his documentary video depiction of this terrifying yet incredibly fascinating event below.
A terrifying but natural cycle
Why am I telling you this? Yes, it is a cool story but what does it have to do with One Health? Well, we always talk about links between different realms and this, even though a scary one, is nevertheless a connection. However more importantly, I have found no information on this pathogen being a threat to carpenter ant population survival and conservation. Actually, fossil records suggest this interaction was already occurring 48 million years ago. Parasitic cycles are harmonious in themselves and happen throughout evolution to balance out populations and ecosystems. The real danger comes when such cycles are artificially disrupted. Examples at the hand of parasitic fungi are Chytridiomycosis of amphibians and White Nose Syndrome of bats, both threatening species with extinction most likely exacerbated by anthropogenic activities. Trouble is when there is a disruption to the natural ecosystems. Being aware and learning about every delicate cycle is extremely important, which is what the One Health philosophy teaches us.
By Costanza Manes, Research Fellow
References
- Hughes DP, Wappler T, Labandeira CC (February 2011) [18 August 2010]. “Ancient death-grip leaf scars reveal ant-fungal parasitism”. Biology Letters. 7 (1): 67–70. doi:10.1098/rsbl.2010.0521. PMC 3030878. PMID 20719770.
- https://www.theatlantic.com/science/archive/2018/06/how-to-tame-a-zombie-fungus/562544/
- https://en.wikipedia.org/wiki/Ophiocordyceps_unilateralis#Life_cycle
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