Halloween Special: Terrifying Parasites

I sat down recently with my girlfriend to watch Alien - a movie that I hadn’t seen in a long time. As you might guess from my granny-murdering epic on the flight-or-fight response, when I see a good horror movie I think two words: blog article. The classic chest-bursting, cat-rescuing, android-killing sci-fi horror flick got me thinking about parasites and how there were some I know of that are freaky-as-hell. So, in my typical fashion I began to do some research on some of the most incredibly nasty ones I could find.

This article is, quite fittingly for the time of year, not for the faint of heart. Join me as we delve into some of the most stomach-turning little critters that nature has to offer…

Cymothoa exigua

The tongue-eating louse. You would think
the fish would notice... (source)

As you will no doubt be aware, us biology types are really not good at coming up with names for stuff. You might be thinking ‘That doesn’t sound like a very scary creature, Adam – you promised me an H.R. Geiger-esque grossfest!’. Perhaps if I gave it its English name, you might get a picture of just how nasty this wee bastard is – the tongue-eating louse.

‘Ewww’ I hear you say. And right you are. But don’t worry – it doesn’t chow down on human tongues (as far as we know *scary music*) – it likes to dine on fish. The louse enters the body of the fish through the gills and attaches itself to the base of the tongue. This adorable little critter then begins to drain the blood from the tongue through the claws at the front of its body. The tongue eventually shrivels up and the louse attaches itself to the remaining muscles where it actually becomes the tongue of the fish. Perhaps the strangest fact about this relationship is that the fish continues to use the louse as its tongue, suffering no ill effect whatsoever (other than losing their tongue and having it replaced with something out of a Ridley Scott movie). 

Scientists reckon that, despite its horrifying appearance and habits, this parasite is benign. Though I expect if one were to ask a fish suffering from this particular affliction, it might offer an alternate opinion. That somehow brings me on to a worm that can control the behaviour of the host it infects.

Dracunculus medinensis (Guinea worm)

The young guinea worm - I mercifully decided
not to include a photo of the worm hanging out of
some poor dude'sleg. (Source: CDC Public Health Library)

The scientific one for this one sounds somewhat less cuddly – unfortunately the plain English name makes it sound like a long and wriggly but furry and adorable house pet. Unlike the tongue eating louse, this little fella does parasitize humans. D. medinensis shares many physical and behavioural characteristics with most worms that like to be really friendly with humans, but the thing that makes it particularly scary is the way in which it spreads from host to host.

As with most parasites, part of the life cycle of this adorable little worm partly takes place in a second type of host. The parasite larvae are initially eaten by water fleas which are then ingested by humans drinking unfiltered water. The fleas are broken down in the stomach acids of the unfortunate host, leaving behind the Guinea worm larvae. The larvae escape into the stomach lining and intestine where they mature and develop into an adult worm measuring something like two or three feet in length (I know – the more you learn about this thing, the less you like it).

When it comes time for the young worm to ‘sow its wild oats’, it migrates to the surface of the skin – usually in the lower portion of the leg. This is where it gets weird(er). The worm creates a blister on the surface of the skin and induces a burning sensation which causes the host to seek the comfort of some nice cold water. When the blister in question is submerged in order to provide some relieve from the burning sensation, the worm (now poking its wormy little head out of the host) releases its larvae into the water – thus starting the whole unpleasant cycle all over again. 

Ophiocordyceps unilateralis – zombifying

fungus

A victim of O. unilateralis - yes, that
a mushroom growing out of it's head.
(Source: PLoS One)

This one is probably my favourite, is arguably the most disturbing and is definitely the most horror movie-esque.  That’s right folks – this is a parasitic fungus that turns its unsuspecting host into a zombie. Before you start to panic and build a bunker under your house or find a shotgun and team up with a band of rugged survivors, this fungus only works on ants. This is considerably less scary. Of course if they were giant zombie ants things would be quite different. 

Anyway- not much is known about the life cycle of this fungus, but scientists believe that the fungal spores enter the ant’s body and begin to develop, all the while consuming the insect’s soft tissues. During this horrific process, the fungus produces compounds which alter the ant’s behaviour, turning it into a ‘zombie’. When this happens, it begins to convulse and falls out of the tree it usually lives in. After it hits the ground, the ant finds a plant, climbs the stem and clamps itself to a leaf vein with force far in excess of its usual abilities. At this point, the ant dies- but the fungus continues its grizzly work.

The fungus continues to grow throughout the ant corpse, consuming the rest of its soft tissues and structurally reinforces the exoskeleton of the unfortunate victim. The fungus also sprouts outside of the ant where it completely anchors the ant to the plant (hey, look at that – I made a rhyme!). Finally we make it to the nastiest part – when the fungus is ready, a mushroom (or fruiting body) grows out of the dead ant’s head where it releases its spores into the environment.

I imagine we are all too grossed out at this point to enjoy a smart-ass comment, so I won’t make one. Moving on.

Leocochloridium Paradoxum – zombifying flatworm

An infected snail. No, it's not a snail who has seen an
attractive lady snail to it's left. Photo by
Thomas Hahmann

From zombie ants to zombie snails. That’s right – you heard me. Leocochloridium paradoxum is a flatworm more commonly known as a green-banded broodsac- and it’s fairly disgusting. Where this critter really wants to be (its definitive host) is in a bird. Unfortunately birds don’t really like to chow down on a tasty flatworm, creating a significant obstacle for the little fella to overcome.

The flatworm larva hangs about on the ground until it is hovered up by a passing snail – its intermediate host. The flatworm is now presented with a new problem – birds don’t really like to eat snails much either. One thing they do like to eat, however, is caterpillar (I am going somewhere with this, don’t worry).

Inside the snail the developing flatworm migrates to the eyestalks (weirdly, they seem to prefer the left one) where they grow into broodsacs. This process has a dramatic effect on the antenna of the snail – the broodsac causes the stalk to elongate, swell and turn bright green and yellow. That’s right – it makes it look like a caterpillar. Now for the clever part. The infection of the eye changes the snail’s sensitivity to light, causing it to mindlessly seek out light, open areas – which is kind of like lighting up an ‘ALL YOU CAN EAT BUFFET!’ sign for birds flying overhead. 

The poor zombified snail is then eaten by a bird which thinks it has a caterpillar stuck to its head. The flatworm then finishes its development in the bird with its larvae being excreted back to the earth, beginning the whole cycle  all over 

again.

Cotesia Glomerata – the real ‘Xenomorph’

I’ve saved the best for last – this horrible little creature is the real-life inspiration behind the iconic scene from Alien you can’t un-see: the chest-bursting scene. When the writers of Alien were coming up with the scene that would freak out a generation, they looked to Cotesia glomerata- a parasitic wasp. Now, wasps are scary at the best of times- but this one takes the cake.

Thankfully Cotesia doesn’t prey on humans, but is the bane of the white butterfly – giving it its common and vaguely cuddly name of the White Butterfly Parasite. Don’t let the name fool you – this is one nasty little bugger. 

The whole thing starts when the female wasp injects its eggs into the caterpillars of the white butterflies using a needle-like organ called an ovipositor. Over a 14-day period, the wasp embyros consume the caterpillar from within, developing into larvae inside the body of the wasp with the size of the brood reaching up to 50-60, with each larva being about the size of a rice grain. While the larvae develop within the caterpillar its behaviour is altered – it gorges itself, eating 1.5 times its usual food intake to nurture its unwanted passengers.

An interesting little side note here is how the eggs survive when initially injected into their host. Normally when a foreign body enters the circulation of a living organism it is usually neutralised and cleared by the host immune response. In order to overcome this threat, the White Butterfly Parasite eggs are coated in a virus which paralyses the insect’s immune response. The virus is actually genetically coded in the wasp’s DNA.

Anyway, back to the zombified-caterpillar-wasp-incubator. While the larvae are growing inside the caterpillar they are careful not to actually kill the host – only drinking its blood and never damaging the vital organs. This is a characteristic shared by the most effective parasites – they keep the host alive. When their two –week development is up, the larvae are ready to emerge and the scene from Alien begins.

Someone get Ellen Ripley... and tell her to bring a
pulse gun. (source)

The larvae release a toxin which paralyses the caterpillar before burrowing out of their poor victim with their razor-like teeth – just like poor Kane in Alien. Unbelievably, the larvae aren’t yet done with their former host. The larvae themselves begin to spin protective cocoons to safeguard the rest of their development, but that might not be enough to prevent them from being targeted by other parasitic wasps – so they recruit their poor adopted parent to provide an additional layer of protection by coating the brood in the silk that would have normally been reserved for forming its own cocoon. The zombie caterpillar then spends its remaining days defending its adopted brood from the same fate it originally suffered.

You might think it’s no walk in the park being a human being  - but if you imagine you could instead be a snail, an ant or caterpillar living in that park, things probably don’t seem that bad.  

Happy Halloween, internet folks! Remember to like the Facebook page to help support and receive updates from ScienceUnplugged!

As much as these parasites are interesting to read about, parasitic organisms are a huge problem in developing countries where they can be a leading cause of disease and death in humans. If you would like to know more about the issue or donate money to help combat tropical diseases please feel free to check out the Bill and Melinda Gates Foundation, UNICEF and the World Health Organisation.

As with every article I post here, this one has been thoroughly researched and a list of sources can be provided for anyone who is curious – just check out the contact page