The Science of Parasites
Fri, 4th Jan 2008
How parasites and hosts live in scientific harmony...
We tend to think of parasites as evolutionary cheats, surreptitiously taking advantage of their hosts’ hard work while they sit back and enjoy an easy life. Take the common cuckoo for example, which, seemingly too lazy to rear its own young, lays its egg in another bird’s nest. When you look more closely it nonetheless turns out that, more than just a bunch of shameless blaggers, many parasites are just as assiduous and diligent as the hosts they exploit.
The cowbird for instance operates in a similar way to the cuckoo, but its deceit goes one step further. Young cowbirds grow up with an attentive ear in order to mimic the song of their foster parents, an impressive feat when you consider that the species parasitises over 200 different species.
And life is certainly no beach when you’re a parasitic worm larva less than half a millimetre in length living on a murky lake bottom. Especially when, to reproduce, you face the Herculean task of making your way into a bird’s bloodstream. Luckily this worm, known as an eye fluke, carries more than a few tricks up its sleeve.
The fluke first hitches a ride with a fish before pondering its next move. Tracking down and infecting a fish is essentially a matter of possessing a nifty sense of smell and good swimming skills, but how do you make the leap from a fish to a bird? You could remain optimistic and wait for your fishy host to fall prey to a gull. But research suggests that these worms are far from being the helpless victims of fate one might expect.
Instead, they appear to be doing everything possible in order to send their hosts swimming straight into the beaks of predators. Once they have wormed their way into a rainbow trout’s bloodstream, the fluke larvae lodge themselves in their host’s eye, causing a cataract to form. The trout is then left to fumble around blindly, vulnerable to birds in search of their next meal. And while this already seems like a step towards success for the worm, its manipulating schemes don’t end here.
Rainbow trout are known to adapt their skin colour to blend in with their surroundings, with the amount of light entering their eyes determining how dark they become. When a parasite-induced cataract clouds their vision, the fish are seemingly fooled into thinking they are in a dark environment, and adjust their skin tone accordingly.
A parasitized trout consequently stands out like a sore thumb in light environments, making it an easy target for birds, and thereby allowing the fluke to complete its life cycle as it passes into the bird’s bloodstream.
A leopard may not be able to change its spots but it seems that a parasitic worm can change a trout’s skin colour. And the eye fluke isn’t alone in its deceitful ways.
You might think that a parasitic fungus would be content with simply draining nutrients from its dungfly host’s bloodstream. The E. muscae fungus however continues to take advantage of its hostage even after its death, causing one biologist to nickname it an ‘enslaver’.
Healthy dungflies prefer to sit on low grasses close to the ground. But once parasitic infection has left a dungfly it on its last legs, instead it begins a perilous ascent to the top of a plant, where it dies at the top on the downwind side. From this high vantage point the infecting fungus releases its spores into the air and they are carried widely by the wind and towards their next victims.
Exactly how the fungus manages to control the behaviour of its host like this remains a mystery, but scientists think that it releases specific neurologically active biochemicals that can affect the hapless dungfly’s nervous system.
But sneaky as these parasites may be, their victims are always fighting back. As hosts wake up to the parasites’ dastardly ploys, they evolve to develop strategies of their own to avoid being made a fool of in future.
From generation to generation, bird species that commonly fall prey to the cuckoo’s tricks have become increasingly adept at spotting intruders' eggs amongst their own precious offspring. In response the cuckoo has evolved eggs that more closely resemble those of their victim. This battle of wit is known as an evolutionary arms race, with both sides genetically jostling to gain the upper hand. And whilst in many instances the cuckoo seems to be on top right now, the tables may well turn in future.
For the time being, it remains remarkable that the cuckoo continues to thrive on such an outrageous fib right under its victims’ beaks. So what the cuckoo lacks in chick rearing skills, it has undeniably made up for in sheer audacity!
Seppälä, O., Karvonen, A. and Tellervo Valtonen, E. (2005) Manipulation of fish host by eye flukes in reaction to cataract formation and parasite infectivity. Animal Behaviour, 70: 889 – 894.
Maitland, D.P. (1994) A parasitic fungus infecting yellow dungflies manipulates host perching behaviour. Proceedings of the Royal Society o London B: Biological Sciences, 258: 187 – 193.