In Defence of Parasites

Parasites have a bad reputation. These tiny creatures are responsible for some of the most visually horrifying diseases known. For example, a microscopic worm causes the grotesquely swollen limbs of elephantiasis, while a single-celled parasite, Leishmania, is capable of destroying a victim’s face. However, we humans often concentrate on the worst aspects of certain species – just ask your average wasp or spider – and there is much more to parasites than disease.  Many parasitic infections in fact cause little harm – if we die, so do they – and by concentrating solely on the diseases they cause we miss out on some fascinating underlying biology.

I remember arriving late for one of my first undergraduate parasitology lectures and being pleased to find the lecturer had not yet arrived either.  “Sorry I’m late,” he said when he appeared, “I was on the Tibetan plateau yesterday, looking for tapeworms in foxes”.  This lecture was my first glimpse into the ecology of parasites, and where the elegance and sheer complexity of their life cycles became apparent.  We all marvel at the epic journeys in nature, such as the great wildebeest migration across the Serengeti.  But compared to parasites, those TV regulars have it easy.  Strolling from one part of Africa to another, avoiding the occasional crocodile or lion? Simple.

The life cycles of parasites can be incredibly complex and quite ingenious.  These animals often need to jump between several host species to mature and reproduce and many have evolved amazing ways of completing these unlikely journeys.

Toxoplasma gondii is a single-celled parasite that can infect a number of mammals, but which ultimately needs to find its way into a cat to sexually reproduce.  The most common intermediate hosts for Toxoplasma are rodents and the parasite has evolved the remarkable ability to alter the behaviour of these animals in order to maximise its chances of finding a feline.  A mouse or rat that becomes infected with Toxoplasma not only loses its natural fear of cats but can even become actively drawn to their scent, deliberately seeking out catty environments and thereby increasing its chance of being eaten and the parasite’s chance of transmission.

The Lancet fluke, a flatworm that infects the liver of grazing animals, is also adept at brain washing its host.  This worm has two intermediate hosts, a snail and an ant.  Snails become infected by eating infected animal droppings, after which the parasites develop into cysts and are released in the snail’s slime.  Passing ants then swallow these cysts as they graze on the slime as a source of moisture.  This cycle alone is a beautiful example just how complex parasites’ life cycles can be, but the really clever part comes next…

Following infection of the ant, the Lancet fluke begins to exert its mind control.  The ant’s behaviour becomes peculiar and, like Toxoplasma and its stupidly brave rodents, this behaviour is due to the parasite’s attempts to increase its chance of transmission.  In the evening the ant leaves its colony members and travels to the top of some nearby vegetation.  Once there, it clamps its jaws on tightly and stays until dawn, a ruse by the parasite to increase the likelihood of the ant being accidentally swallowed by grazing cattle.  Clever stuff, but the parasite is cleverer still.  When day breaks, the fluke senses the rise in temperature and relinquishes its control over its insect host, allowing it to continue with its usual anty chores.  This prevents the ant dying in the daytime heat, which would also kill its parasitic puppeteer.

Even within a single host, parasites face extraordinary challenges.  A single individual may have to navigate blindly from the gut to the lungs, from the skin to the eye, or from the liver to the brain.  Tunnelling through organs and hitching a ride in our bloodstream, the travelling parasites must face a relentless barrage from our immune system.  Many have therefore evolved sophisticated ways of dampening down host immune responses to protect themselves.  They are in fact so proficient at this that many scientists believe the lack of parasitic infection in developed countries, and the loss of their calming influence on our immune system, has led to the observed increase in allergies and autoimmune diseases.  Indeed, deliberate infection with parasitic worms has actually been used to successfully treat many such disorders.

The complexity and elegance of parasites is often overlooked, but they are marvels of nature. Parasites can treat as well as cause disease, and can alter host physiology and behaviour; they form important parts of ecosystems and undertake journeys which, though microscopic, are unrivalled in nature.  A single individual must endure environments as diverse and hostile as the bottom of a pond, the gut of a snail and the tissues of a wildebeest. And yet who gets the David Attenborough treatment?

This post, by author Dr. Andy Turner, was kindly donated by the Scouse Science Alliance and the original text can be found here.