Parasites can invade animals, including humans, and control their behaviour from the inside. But this also means parasites may be able to teach us how to control our own brains and treat neurological diseases, a new documentary shows.

p-armata-ant-fungus

An ant from the rainforests of Brazil that has been overcome by a lethal fungus. The fungus grows a tall stalk out of the ant’s head so that its spores can be spread to the rest of the colony. (David Hughes/CBC)

"If a parasite can hijack a rat or a caterpillar because it knows what to do with the brain chemistry, or it knows what to target, it can do that to us too,” said David Wells, producer of Invasion of the Brain Snatchers, which aired on CBC-TV's The Nature of Things on Hallowe'en night.

"As we start to understand our brains as biochemical machines, and that we're not any different than other creatures, it is very unsettling to a lot of people."

Guided by scientists from North America and Europe, Wells portrays the effectiveness of animal mind-control by parasites.

In the coastal estuaries of California, a small flatworm forms cysts on the brain of a tiny killifish and forces the fish to turn its white belly up towards the sky in order to attract hungry birds. After all, the flatworm wants to end up in the bird so that it can finish its life cycle.

'We want the ick factor to trigger people's curiosity.'- David Wells, producer of Invasion of the Brain Snatchers

Ants in the Brazilian rainforest climb up the stalks of plants over their colony when they are infected by a fungus, latch on to the underside of leaves, and die. When the fungus matures, a large mushroom grows out the back of the ant’s head and spreads fungal spores over the ant colony.

A rat that loses its fear of cats on busy city streets puts its own life in great danger. Infection by Toxoplasma gondii causes a “fatal feline attraction” in rats, according to Joanne Webster at Imperial College in London, England. This parasite not only removes fear from rats, but they become strongly attracted to the smell of a cat.

The really surprising revelation is that Toxoplasma gondii can, and does, infect humans as well. 

“As many as a billion adults are (currently) infected with Toxoplasma,” said Wells.

This parasite may be causing people to be a little bit more impulsive or spontaneous, but its effects are hard to characterize.

The filmmakers push the boundaries of the ick factor in this film, said Wells, enticing you to keep watching and asking viewers, "You think that was something? Wait until you see what we've got next." 

Killifish-infected-with-flatworm

The black spots on the brain of a killifish are cysts that are caused by a parasitic flatworm. (Kelly Weinersmith/CBC)

The film isn't intended to be alarmist, he added.

"We wanted the 'ick factor' to trigger people's curiosity," said Wells. “These parasites are doing things (with the brain) that we just can't do."

This film reminds us that parasites have co-evolved with humans, and that they might already have the key to unlocking the secrets of the brain and the key to treatments for neurological disorders like depression.