Thursday, 12 February 2015

Parasite paradise: zombification and mind control

Paralysed and helpless, twitching periodically and uncontrollably, a zombie and a slave – this is the story of a ladybird beetle that’s been infected by a parasite.

Four weeks ago, the ladybird was attacked. A wasp laid an egg inside its belly, but life went on. Under the surface of its belly the wasp egg hatched and the larva grew and fed on its internal organs.

Still, life went on.

Finally, a week ago, the larva grew big enough to burst out from the belly of the beetle. But the ladybird beetle’s job wasn’t done. The larva started to make a cocoon between its legs. That’s when the paralysis set in. That’s when the zombification started.

For an entire week, the ladybird stood guard involuntarily – twitching and shaking to ward predators away from the defenceless cocoon. The matured wasp then leaves, looking for another ladybird to infect anew. Our ladybird stands stoic.

[caption id="attachment_292" align="aligncenter" width="640"]Muuuu Forced to watch and protect - a ladybird beetle is paralysed over the wasp larva cocoon. Image credit: Nutmeg, CC2.0.[/caption]

One question that has bugged scientists for years about this process was how the infected ladybird beetle becomes zombified at just the right time when the larva comes out?

The answer, published this week, lies in the teamwork between the wasp larva and a virus. The newly discovered virus infects the brain of the ladybird beetle and is transmitted from the wasp during the egg-laying process. The virus incorporates itself into the brain of the beetle, causing the zombification so the larva can use the ladybird as its home until it matures.

The parasitic wasp larva is just one example of a “zombification” parasite. In the animal kingdom, parasites often cause a fatal change in the behaviour of their prey to further their own ends.

Toxoplasmosis

Toxoplasmosis is a disease caused by the parasite Toxoplasma gondii. It’s a single-celled organism with a complex life cycle. While it can live in many animals (hosts), it can only sexually reproduce in one “definitive” host – cats. In its life cycle, T. gondii may be eaten by cats, rats, birds, mice, and even humans.

In hosts other than cats, it forms cysts – sometimes in the brain – and has been linked to behavioural changes in some animals. For instance, infected rats lose their sense of fear. They approach areas even when the scent of cat urine is present. This leads to many infected rats being eaten by cats – where T. gondii can reproduce.

In humans, there have been reported cases where people lost their aversion to cat urine. A study in the Czech Republic also found that those suffering from toxoplasmosis – having T. gondii in their system – were more likely to experience traffic accidents. We might not be immune from the effects of mind-altering parasites.

Suicide fish

The, perhaps aptly named, killifish can play host to a parasite that reproduces in shorebirds. The parasite in this instance is a fluke called Euhaplorchis californiensis (EUHA for short). The birds are the definitive host for EUHA, and it lays its eggs in their gut. The eggs are released in droppings where they mature in horn snails, until the larvae venture out to find the unsuspecting killifish.

Once a larva finds a killifish, it enters through the gills and makes its way to the brain cavity. Here it influences the behaviour of the killifish, causing it to shimmer and shake near the surface of the water where – you probably guessed it – it’s noticed by birds and eaten.

Zombie ants

One of the more famous instances of parasitic zombification is the infection of ants by the fungus Ophiocordyceps unilateralis. Spores of the fungus rain down on the ant from the air and burrow their way through its tough carapace. Once inside, the spores germinate and infect the ant’s body.

The ant then behaves strangely, with jolting motions and twitching. It then climbs to the top of a tall plant nearby and clings to a leaf. (Incidentally, this “clinging” makes a distinctive marking. In plant fossils, the same markings can be found dating back 48 million years!)

Now sitting at the top of a plant, high up from the ant colony, the fungus will break through the ant’s skin and spread its spores – raining down on a new generation of unsuspecting ants.

Back to the ladybird beetle

Does this behaviour ring a bell? Fortunately for our ladybird beetle friend, after the wasp is gone the virus stops paralysing him. Like a quarter of all ladybirds that get infected, not enough damage was done to kill him. He twitches and moves under his own control now. He may even get infected again!

Life goes on.

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