Subversion

This is a Biology (specifically Life, Form and Function) essay. Those of you who abhor science in any form ought to stop reading now. Those of you who hate creepy crawlies should also go somewhere else, STAT.

Still with me? Good. So, biologists are known for their interest in (or at least indifference to) the macabre and parasites are just about as macabre a topic that can be discussed outside of a B-movie.

We tend to dismiss parasites. The ones that are familiar to us Homo sapiens have names like Plasmodium falciparum (Malaria), Human Immuno-deficiency Virus (AIDS), Guinea worms (Dracunculiasis) and Hookworms (Ankylostomiasis). They’re painful, disgusting and debilitating and can be fatal but lack this certain x-factor that would make them well and truly terrifying.

I’m not trying to belittle death here. Contracting AIDS is hardly a laughing matter and Malaria is still one of the leading causes of death in the under-developed world with an estimated 1 million fatalities annually. However, human parasites seem to have quite the straightforward game plan: find a way into a human host, make lots of babies and find a way to spread said babies to other hosts. Thus, people in general tend to pooh-pooh human parasites as unsophisticated life-forms that often don’t even have the self-control to prevent themselves from killing their hosts (if host lives longer -> parasite nourished longer -> more chance to spread progeny).

What makes a truly horrifying parasite then? Consider the standard fare of B-movies: Zombies whose bites can transform humans into ravening beasts. Vampires whose seduction can turn a loved one into the corrupt undead. Tentacled/Slug-like/Floating-Brain-like monsters who through some means gain control of human minds and turns the world against itself.

One of the things that seems to horrify the human psyche is the total and utter loss of control to a malignant entity. Some people even get off on fantasies of this sort, but we digress.

Parasites of this sort do exist, with life-cycles that involve the control of another entity to serve their reproductive needs. Let’s look at a few examples:

First up is the Emerald Cockroach Wasp (Ampulex compressa). Wasps as a family (Vespidae) are known for behavior that, if displayed by humans, would be considered pretty damn sadistic and the Emerald Cockroach Wasp is no different. This wasp parasities cockroaches by injecting a venom that blocks the action of octopamine (similar to dopamine in humans) that prevents the cockroach from initiating higher-level actions like walking of its own accord.

The wasp apparently stings its prey twice: once for general paralysis and once again right smack in the roach’s head ganglia (analogous to the human brain). The effects of the venom lead to the roach being unable to walk (and hence escape) of its own accord and the wasp subsequently trims the antennae of the roach before guiding the roach to a nesting site by pulling on its antennae like a leash:

Once the roach is comfortably sequestered in the nest, the wasp lays a single egg on the roach’s abdomen and walls it in. After 3 days, the larvae hatches and begins to consume the roach for nutrition. The larvae burrows into the roach and eats the roach’s internal organs in a specific order that maximises the life-span of the roach so that the roach can survive (and thus stays fresh) till the larvae metamorphosises into an adult and bursts out of its body, finally killing the poor bugger. How’s that for freaky?

Like I mentioned earlier, wasps are known for this sort of grisly behaviour. The Ichneumon Wasps (family Ichneumonoidea), for example, caused Darwin to have a crisis of faith that lead him down the road to developing the Theory of Evolution - these wasps have long ovipositors which deposit their eggs into caterpillars which are then consumed from the inside when the ravenous larvae hatch and begin to feed. Darwin wrote that he could not persuade himself that “a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a Cat should play with Mice.”

Let’s look at barnacles next. Barnacles are commonly thought of as nuisances to ships (one ship was almost chased down by Somalian pirates because the clusters of barnacles clinging to its hull slowed it down that much) and the most sinister aspect of them that most of us will ever encounter is that it is incredibly painful to walk on them while at the beach. Pity the poor swimmer who doesn’t look where he’s going and rams into a rock or pier support covered with them at high tide; he’ll need plenty of stitches.

Let me introduce you then to barnacles of the genus Sacculina. These creatures begin life just like all other barnacles - as tiny larvae that look like a teardrop with a row of legs attached. Unlike their ship and rock loving brethren, though, Sacculina larvae seek out crabs. When they find one, they walk on the crab’s shell till they find a joint. The larvae then injects part of itself into the crab, discarding the rest of its body as a husk. The injected portion moves around inside the crab and eventually takes up residence in the crab’s abdomen, forming a fleshy sac called an externa:

The barnacle then begins to grow root-like fibres that penetrate deep into the crab’s body, siphoning food from the crab’s circulatory system while actively modifying the crab’s physiology to suit its needs. Crabs of both genders are rendered infertile through hormonal changes and male crabs are so thrown out of whack that they begin to change gender into females - the male crab’s abdomen widens and flattens out and it begins to show behaviour that is distinctive to female crabs. The crabs also lose the ability to molt (and hence grow larger) and to regenerate lost limbs. This apparently benefits the barnacle since either of those processes would drain resources that the barnacle could otherwise exploit.

The crab has been turned into a zombified barnacle-nanny. From Discover magazine:

Despite having been castrated, the crab doesn’t lose its urge to nurture. It simply directs its affection toward the parasite. A healthy female crab carries her fertilized eggs in a brood pouch on her underside, and as her eggs mature she carefully grooms the pouch, scraping away algae and fungi. When the crab larvae hatch and need to escape, their mother finds a high rock on which to stand, then bobs up and down to release them from the pouch into the ocean current, waving her claws to stir up more flow. The knob that Sacculina forms sits exactly where the crab’s brood pouch would be, and the crab treats the parasite knob as such. She strokes it clean as the larvae grow, and when they are ready to emerge she forces them out in pulses, shooting out heavy clouds of parasites. As they spray out from her body, she waves her claws to help them on their way.

Higher organisms, by dint of their more complicated neural structures, seem to have no parasites capable of controlling them to this extent. However, there do exist parasites that are notably able to directly influence action.

Take the fluke Euhaplorchis californiensis for example:

Birds release the fluke’s eggs in their droppings, which are eaten by horn snails. The eggs hatch, and the resulting flukes castrate the snail and produce offspring, which come swimming out of their host and begin exploring the marsh for their next host, the California killifish. Latching onto the fish’s gills, the flukes work their way through fine blood vessels to a nerve, which they crawl along to the brain. They don’t actually penetrate the killifish’s brain but form a thin carpet on top of it, looking like a layer of caviar. There the parasites wait for the fish to be eaten by a shorebird. When the fish reaches the bird’s stomach, the flukes break out of the fish’s head and move into the bird’s gut, stealing its food from within and sowing eggs in its droppings to be spread into marshes and ponds.

The thin carpet of flukes on the killifish’s brain insidiously causes the fish to suface and spasm four times more than normal. Both these actions make it easier for the fish to be caught by birds because both directly expose the fish to sight (the belly of the fish is white and so attracts a lot of attention when it flashes upwards during a spasm). Ergo, the parasite was directly increasing the chance of the its fish host being caught and eaten by a predatory bird, which is in the parasite’s best interest since its life cycle only completes in avian intestines.

This happens to us too. Ever consider why certain diseases give us diahorrea? The bacteria species that cause these diseases directly cause diahorrea by excreting toxins which interfere with ion balances in our gut (Cholera) or by causing the overstimulation of our posterior pituitary gland and the anti-diuretic hormone surge that follows (Dysentery). This is in the parasite’s best interest since it spreads to other hosts when they ingest water and diahorrea is a sure-fire way to get the host to eject the parasite’s progeny into local water sources.

The Guinea worm was mentioned earlier. This is one of the most disgusting human parasites I’ve ever studied. This nasty bugger enters humans through contaminated water supplies as larvae inside infected water fleas. The water fleas are digested within our alimentary canal and the larvae are released into our gut where they mature and mate. The female then burrows out of our intestines into our body cavity or through the connective tissue and muscles into the limbs of the body and begins the process of growing. These nasty buggers can grow up to a meter in length and as thick as a spaghetti noodle.

Approximately a year after infection, the female will attempt to leave the human body by burrowing directly out of the body, usually creating a very painful blister that erupts in 3 days to reveal the white and wriggling end of the worm (ewwwwww). This eruption causes an intense burning sensation which usually causes the human host to immerse the affected area in the nearest available body of water for relief. Once the worm detects that it’s under water, it immediately sprays hundreds of thousands of larvae into the water, contaminating it and starting the cycle anew. The most horrible part is that there’s no cure for the disease. Treatment simply involves winding the worm around a stick as it continues to emerge from the body,millimeter by millimeter, a process that can take up to a month. If the worm breaks during the process, the host is almost sure to suffer from an infection of some sort as the rest of the worm decomposes inside him.

Hope all that didn’t spoil your appetite.

-Marc