Parasites: Weird Biology


I tapped out the following post because of two reasons: Parasites are my biological speciality; and I get to write the word penis a few times. The picture is a close up of the penis of the parasitic nematode Steinernema feltiae

In Itacoatiara, a village by the Amazon river in northern Brazil, a man steps up to the river bank and unzips his fly and begins to urinate into the water. While watching his urine mix with the brown waters of the Amazon, he becomes briefly curious about a sudden swirl where his urine hit the river. Then, something zips up his urine stream from the river at speed and forces itself into his penis. Looking down with unimaginable horror, he glimpses the tail of a small fish wriggle up inside his urethral opening and disappear. The fish quickly jams itself fast inside the man by springing four sharp spikes out of the side of its head, and blood pours. In agony, the man falls back to the ground, bent double, howling as he calls desperately for help. His companions coming to his aid are as confused as he is.

It’s a groin clenching story, and it is supposedly the most recent record of human urethral parasitism by this fish, occurring in October 1997. But it is mostly a complete fabrication. The only part that could be true is that a man may have indeed have relieved himself into the Amazon river. Yet I have heard stories of a parasite that shoots up male and female urinary tracts (and also anal openings) in Brazil for years, since I was a kid in fact. Early 19th century western explorers and biologists returning from the region were the first to bring back stories of the nefarious intentions of the parasite but on the face of it, they were unreliable messengers. First of all, they were strangers in what was to them a very strange and inhospitable land. And one could easily imagine that there would have been quite garbled communicative dynamics at play between white skinned visitors and natives speaking a mind-boggling array of languages and dialects. It would be very easy to jump to fantastical conclusions when half understanding tales about a fish and your penis and spikes and eggs, and something or other. Indeed, any story about penises and eggs and spikes would send any male listener into an imaginative tail spin.

The parasite of the stories does exist: it is a parasitic fish, candiru (Vandellia cirrhosa), but it’s target host is other larger fish, not mammals, and certainly not humans. Consider the principle glaring impossibility: how could a fish, or any animal, swim up a narrow urine stream at such speed and with such force? Such a feat belies the laws of fluid dynamics as we know them. Candiru is also way too large to fit into a male urethra. It would suffocate. The fish is supposedly attracted to urine to help find its host, but experiments have proved this fish shows no interest in urine or any of the components of urine, such as ammonia. The fish is said to reside in the penis once established, and lay eggs in the bladder. But if it could manage to survive there, it would have to be an amphibian air breather. There are numerous fish exhibiting this behaviour, yet candiru is not one of them. And among the 400 or so known species of air breathing fish (including the aquatic air breathers) none of them have ever demonstrated such parasitic behaviour. Candiru requires water in which to live and would clearly die if it rather inexplicably found itself inside an animal penis.

What these stories demonstrate however (and it is a pretty good story to hear while idling away a slow afternoon in a bar) is the great imaginative power that parasites hold over us. And they have done so for millennia. Few things are more cringe worthy than the thought of something living inside us eating away without paying rent. We do everything we can to be as far away from them as possible, but we are in fact their evolutionary travelling partners. We are even made up of them somewhat: molecular evidence is mounting that certain sections of our DNA originated from parasites at some point in our distant evolutionary past (Link).

They have benefited from leaps in human sociological behaviour and technological advancement. When we learnt to master the soil and grow more food with the development of irrigation some 9000 years ago, we freed a parasitic trematode worm, Schistosoma spp, to spread among exponentially growing Sapiens populations in the Middle East. Medical texts from the region, believed to have been written 4500 years ago, describe patients passing red urine, and blood in urine and stools is a key symptom of unchecked schistosomiasis. The agricultural revolution led to larger numbers of people living closer together. This empowered a wide range of parasites to proliferate and adapt to this newly abundant human biomass, especially fleas, ticks, lice, roundworms among many others, as well a whole range of other non-parasite diseases. We grew, and so did they, while we did all the work.

They have pervaded our popular culture deeply, providing staple go to bio-forms for science fiction writers, film makers and comic book creators. This is especially true for a group of parasites known as parasitoids – parasites that must eventually kill their hosts. The movie franchise Alien, for instance, features a lethal fictional parasitoid from space that develops inside humans. When ready it bursts from the abdomen, killing its host in an orgy of blood and slop. It’s called a Xenomorph and it enables fantastically inventive cinema making. But in fact, a very straight-laced parasitologist can level the accusation that it’s pretty lazy work, imaginatively speaking. The xenomorph exhibits almost the same life patterns as any number of parasites in the insect world and below. In fact, often the natural world is far stranger than any movie.

There are small worms that invade caterpillars and rapidly kill and eat them wholesale, leaving nothing but a leathery skin husk; the carbon and nutrients that made up the original caterpillar is now transformed into thousands of tiny new parasitic worms that spill out into the soil to find and infect more larval insects. There are wasps that inject an egg into its host which is kept alive while the parasite larva eats it from the inside out. There are parasitoids that alter the behaviour of their hosts to maximise parasite survival. There are viruses that attach to a cell and inject its own DNA which hijacks the cellular apparatus, tricking it into producing many copies of new viruses. When there is enough of them they burst forth from the cell, killing it. Just like a toothy HR Giger alien.

To paraphrase the noted biologist, EO Wilson, parasitism is predation by a unit less than one.  Officially, it’s described as a relationship between two species where one species benefits the expense of the other. As opposed to what’s known as mutualism, where both parties of a two species relationship benefit, or commensalism, where one party benefits but other is unharmed, in parasitism the non-benefiting party is exactly that: they achieve nothing from the arrangement. In fact, more often than not, they are harmed by their freeloading hitchhikers, with symptoms ranging from a mild itch up to and including death. But within that definition there is an immense range of possibilities. Parasites are drawn from all major life groups, and includes a sometime mammalian parasite, the vampire bat. The term “sometime mammalian parasite” is used here as there isn’t a full scientific consensus that the vampire bat can be classed as a true parasite.

Ever heard the call of a cuckoo in a late summer meadow? That is a bird parasite that engages in what is known as brood parasitism, which in turn is a type of kleptoparasitism. It tricks another bird species into rearing its young which then steals the food foraged by the host. Cuckoo chicks are usually larger than the host young, grow far faster, and eat voraciously. Any of the hosts chicks still alive by the time the cuckoo chick has grown large enough will end up literally walking the plank.

Got bitten by bed bugs in your dingy flat? You were attacked by an ectoparasite, one that lives outside their hosts. In this case the parasites literally bite and run. Your new cute puppy just passed a knot of wriggling worms in the yard? It was infested with roundworm or nematode parasites known as endoparasites: those that live inside the host body. Easily treated nowadays indeed, but stomach churning. Ever notice a long thin red worm slithering about in your nice fresh fish fillet? Your fish supper has been infected with a cod worm, a mesoparasite that lives both inside and outside the body. In this case, the feeding parts, the head, burrows its way into the fish’s heart and feeds on the blood, while the tail end hangs out the fish exterior and scatters eggs.

There are so many forms of parasitism, it can make one dizzy. There are social parasites, where a species takes advantage of a social interaction, usually among social organisms such as ants or bees. An example is a tiny Alpine ant that lives exclusively on the backs of other ant species. There are adelpho-parasites where parasites are from the same taxonomic family or even the same genus of their target host. This would be like if humans could be parasitised by a bonobo.  There are sexual parasites where within the species one sex or another is wholly dependent on the host sex, giving nothing in return, except sperm when needed – yes the sexual parasite is usually the male. There are parasitic plants, parasitic bacteria, parasitic fungi (including types that spectacularly parasitise ants) Caught a cold? You have been infected with a virus; a genetic parasite. There are even parasites that parasitise parasites, knowns as hyperparasites. I kid you not one bit!

There are parasites that eat only the reproductive tissues of their hosts, known as parasitic castrators. Why do they do this? It has to with energy conservation. A significant portion of an organisms’ energy supply is spent on reproduction. Sexual energy is spent not just on the development and maintenance of gonadal material, but also sexual competition, seeking and wooing mates, finding and maintaining nesting sites, developing eggs and caring for and feeding young. Liberate a host from the obligations of reproduction and evidence suggest that it lives longer. And this benefits the inhabiting parasite.

This example is ideal to highlight the most interesting element about the biological world of parasitism: that it is nothing short of an evolutionary marvel. For how long did it take for parasites to specialise in consuming gonadal tissues? And how did it start?  Biologists do not know, but there are theories. Firstly, back in the evolutionary timescale a hypothetical parasite ancestor was first either a predator or a scavenger. Eventually some began to colonise and feed on live hosts instead of killing them or scavenging. First the host may have died quickly, but certain individuals may have not killed the host right away, and thus lived longer and produced more parasite offspring. This may have allowed for a selective advantage, and the parasite may have gradually allowed its host to live longer and longer over time. Eventually different parasite strains may have emerged that specialised on feeding on different host tissues, and those feeding on gonadal tissues found themselves living in hosts that lived longer than the hosts of other strains that fed on muscle or indeed the hosts own food source. Now we have a fully formed parasitic castrator. Actually, scientists cannot ever say “fully formed” when speaking about evolution because we assume it is a continuous process. For the parasitologist thinking about the evolution of his charges, it is a wonderful fantasy to imagine zipping far into the future to see what weird and strange and wonderful forms may have evolved.

The complexities of their life cycles can be, what appears to our minds, unnecessarily complicated and based on coincidental encounters with different host vectors. To deal with this, many parasites species have developed ways to maximise host encounters. One of the most interesting and sinister strategies is influencing host behaviour. There is a worm that infects crickets. Post infection, the cricket feels an overwhelming desire to commit suicide; specifically, by jumping into nearby water. Blowing its little cricket brains out will not do. The water death suits the parasite perfectly as its in water where it must reproduce, which is what is gets right down to.

Another great example of parasite mediated behaviour modification occurs in the life cycle of the lancet liver fluke. This parasite must find and pass through three non-related hosts animals before it completes its lifecycle. It begins as an egg in the liver of ruminants (ungulates; cattle, sheep, goats) which is passed with the faeces into the grass. This egg must then be eaten by a snail in which the first larval stage develops. The snail has evolved to rid itself of the larvae by producing what are known as slime balls, which are produced in the respiratory chamber and expelled. The snail literally sneezes out the slime balls. The slime balls are then eaten by ants. The second stage larvae develop, and there could be hundreds of them in one slime ball, disperse thorough the body of the ant. The converge on the nerve centres of the brain to take control of the ant, but incredibly, like sperm rushing en mass to an egg, it is only one fluke larva that takes eventual control of the ant’s brain. The rest concede defeat and re-disperse though-out the body of the ant. Now in firm control of the ant, the larva begins to enact its master plan. On the onset of evening when the air cools and the light fades (I want to write that there is a full moon as well, but that would be unscientific) the ant is forced to leave the colony and climb up to the top of grass stalks. The ant will clamp its mandibles on the grass to ensure it remains there all night, held fast against wind or any other dispersal factors. If the ant is not eaten by daybreak, the ant is “allowed” to climb back down the stalk and re-join its colony exhibiting normal behaviour for the rest of the day. That is until night falls once again, where it must obey its parasite master and repeat the process once again. Night after night it must do this, until eaten by grazing ruminants. But, why does the fluke not simply leave the ant clamped up on the stalk of grass all day as well as night? Because the ant, left in an exposed position in daytime, would certainly die from the heat of the sun, killing all the flukes within with it. Pure evolutionary genius! And it is all completely real.

Photo Credit: Stephen Boyle/Carlow IT Ireland 2003.

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