‘Why I believe parasites are brilliant’

Neomorphs: In the new Alien movie out this month they burst out of human chests and necks.
by Andrew Turner

Recently a research associate in Evolution, Ecology and Behaviour at the University of Liverpool he is an expert on the parasite systems of the field vole.

As the new horror movie “Alien: Covenant” gets rave reviews for its disgusting acid-dripping space parasites, the author mounts a vigorous defence of these often-maligned organisms.

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.

We often overlook the complexity and elegance of parasites, but they are marvels of nature.

I remember arriving late for an undergraduate parasitology lectures and finding that 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 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 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.

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.

We often overlook the complexity and elegance of parasites, 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?

You Decide

  1. Do you find the idea of parasites fascinating, disgusting, or both?

Activities

  1. Research a particularly interesting parasite and give a five minute presentation about it to your class.

Word Watch

Diseases
By far the most widespread deadly parasitic disease is malaria, which kills around 800,000 people every year. Others include Cryptosporidiosis, Amoebiasis and Chagas disease.
Leishmania
The leishmania parasite is spread by the bite of sandflies, causing the disease Leishmaniasis. Its primary hosts are vertebrates. Leishmania commonly infects rodents, dogs and humans.
Serengeti
A geographical region in Africa, mainly located in Tanzania, famous for hosting the largest mammal migration in the world.

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