If you thought the only thing ants had to worry about was anteaters, you might want to click off this page now before things get too grisly for you.
Deep in the undergrowth of the world’s tropical forests, a rather frightening yet fascinating fungus is lurking. While its name may not trip off the tongue, Ophiocordyceps unilateralis has a modus operandi that is absolutely unforgettable.
From the Cordyceps fungus family, this parasite has the capacity to do very nasty things indeed to ants. Once an ant walks through some of its fungal spores, the fungus bores its way into the insect and eventually takes over its nervous system and its brain. Then things get really weird.
The fungus makes the ant climb up a tree and bite into a leaf. The ant soon dies and a long stalk grows from out of the back of its head, which then drops fungal spores on to the colony below so the cycle can begin again.
‘What’s rather spectacular in the case of the zombie ant fungi is that they are able to precisely control the behaviour of the host before it dies,’ said disease biologist David Hughes, assistant professor of entomology and biology at Pennsylvania State University, who has followed these intriguing organisms around the world.
They can be found in the forests of countries such as Thailand, China, Australia and Brazil.
He described what happens when a spore first attaches itself to an ant on the forest floor.
‘It does a rather interesting thing – it sticks tight so it can’t be pulled off and then it uses enzymes and pressure to blow a hole through the ant’s body.’
After two weeks growing inside the ant, the fungus is able to control its behaviour and produces chemicals which tell the ant to leave the rest of the colony. Dr Hughes is currently preparing a report which identifies how exactly these chemicals work.
‘The infected ant goes and latches itself on to a leaf in the understory vegetation of a tropical rainforest and this provides a perfect platform for when the spore-producing structure grows out of the back of the ant’s head,’ he added.
‘It’s one of the most complex examples of parasites controlling animal behaviour that we know about.’
And it is behaviour that has been going on for 48m years. In 2010, Dr Hughes was part of a team which found fossilised evidence in Germany of the practice.
This example of nature at its most brilliantly and disgustingly devious sounds like something more suitable to science-fiction. It is the perfect premise, then, for a horror video-game.
The Last of Us, developed by Naughty Dog and exclusively available on PlayStation 3 from Friday, takes the Cordyceps concept and pushes it to its limit. The game is set 20 years after a fungal parasite has made its way through most of humanity. Players take the role of a survivor battling to survive in a world of grotesque, zombiesque, fungus-faced creatures.
Dr Hughes, who pointed out that fungi are more closely related to animals than they are to plants, worked as a consultant to the game-makers.
‘They gravitated towards the idea of growth and they liked the grossness of it,’ he revealed.
‘It reflects the increasing sophistication of the audience. They love stories but they like it when there’s more detail. You start off with Danny Boyle and 28 Days Later and then a few years later you have these games which have a lot of sophisticated biology in them. That’s what the audience are demanding.’
‘It’s a flight of fancy to think one of those parasites will be a specialised fungus that controls ant behaviour but the history of medicine shows us that there’s lots and lots of parasites jumping over from animals into humans and then having crazy effects,’ said Dr Hughes.
‘We constantly inhale billions of spores of fungi every day and our immune system is very well set up to prevent these infections. We do occasionally see fungi jumping the species barrier, going from one animal into humans.’
He said Aids sufferers in South East Asia have died from fungal infections contracted from small mammals.
In parts of the US, the fungal disease coccidiodomycosis – or Valley Fever – kills hundreds of people a year after it is contracted from spores swept into the air from soil.
‘It’s foolish to to think we’re living in a sanitised world protected from mass outbreaks,’ said Dr Hughes, before listing examples such as influenza, bird flu, SARS and Aids.
However, he believes such an outbreak is more likely to come from a virus than a fungus. And he said that fungi are also a positive force of nature.
‘Fungi are remarkably good at producing compounds that affect the physiology and the neurobiology of mammals,’ he explained. ‘Quite often they’re accidental effects – natural selection hasn’t designed them for such things – but we take advantage of these, that’s the very foundation of our entire antibiotic industry.
‘Penicillin, which has revolutionised humanity, is from a fungus producing a compound. The fungi are constantly at war with other organisms and we exploit that.’
At the forefront of that exploitation is an attempt to use one of the compounds of the Cordyceps family to fight cancer.
‘We have found that cordycepin affects the expression of specific genes related to cell proliferation and to inflammation,’ said lead researcher Dr Cornelia de Moor.
‘Our research indicates that cordycepin could treat inflammatory diseases such as asthma and arthritis and cancer, which is a condition of uncontrolled cell proliferation.’
She said studies indicate that cordycepin could prevent metastasis, the spread of cancer through the body.
Tests on animals have yet to be carried out. If successful, human trials could begin in three to five years, said Dr de Moor.
Back on the tropical forest floor, things remain tough for the ant, but he has some help from a rather unexpected source. A second parasitic fungus – a hyperparasite – has been uncovered which attacks the first, ‘ant zombifying’ one.
‘There’s always somebody going to take a free lunch from somebody else,’ said Dr Hughes.
And so the circle of life – albeit a life with mind control and things growing out of dead beings’ heads – goes on.
‘Half of life on Earth is parasitic,’ said Dr Hughes. ‘Half of the organisms that have evolved by natural selection, they have evolved an ability to get their lunch from eating another organism.’
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