August 7, 2009

Strange BiologyDriven to the Edge by a Parasite

While the BioZine is on summer vacation, enjoy this article from the archives. This article was originally published in August 2008.

After forcing its ant-host to climb to the top of a plant, a parasitic fungus effectively kills the ant by growing out the top of its head.

Imagine what it would feel like if you were infected by a parasite that was able to control everything you do. You would no longer be in control of your central nervous system. You would move only when and where the parasite directed you to go. Sounds like the plot from a horror movie, right? Turns out, this sort of situation isn’t just the product of some Hollywood director’s imagination. Some parasites can and do take over their host’s bodies.

Bullet Ants and Parasitic Fungi

Bullet ants are a type of ant found in lowland rain forests from Nicaragua in Central America to Paraguay in South America. They are called “bullet ants” because their powerful sting is considered to be as painful as being shot with an actual bullet. In fact, in comparison to all other insect species, the bullet ant’s sting is considered to be the most painful and most incapacitating.

Among the bullet ant’s enemies are ants from other colonies and a species of parasitic flies. However, one of the bullet ant’s more sinister enemies is a type of parasitic fungus from the genus Cordyceps. The parasitic fungus infects bullet ants when spores enter the ant’s body through its spiracles. (Spiracles are holes on the body of an insect’s exoskeleton through which air can be taken in or released. Insects use spiracles to breathe.) Once inside the ant’s body, the fungal spores attach to the ant’s inner soft tissues and begin to grow. When the fungus matures, it begins to grow into the ant’s brain. At the same time, the fungus releases chemicals that change the way in which the ant senses pheromones (chemical signals that trigger certain behaviors). In this way, the fungus is able to affect the ant’s actions.

In effect, the fungus directs the ant to climb upward. This is in the best interest of the fungus, as the higher the ant climbs from the ground, the farther the fungus’s spores can be distributed once they are dispersed from the ant’s body. Once the ant has climbed high enough, the fungus eats the ant’s brain and shoots out from the top of the ant’s head. The fungus continues to grow for another three weeks before finally exploding, dispersing its spores into the air, and beginning the cycle again.

As a result of coevolution, bullet ants have the ability to detect an infected ant by both smell and chemical detection. The role of soldier ants is to protect the queen and her brood from enemies. When an infected ant is identified, it is prevented from entering the colony, killed, and dragged far away from the nest.

Like grasshoppers, crickets can also be infected by hairworms. In this image, a hairworm emerges from the posterior end of the drowned cricket.

Grasshoppers and the Parasitic Hairworm

Parasitic hairworms affect their grasshopper hosts in an slightly more dramatic way than the Cordyceps fungus affects its ant host. Scientists aren't entirely sure how the grasshoppers become infected by the hairworms. One hypothesis is that the grasshoppers become infected after drinking from a water source that is infested with hairworm larvae. Once a larva enters the grasshopper's body, it continues to grow into its adult form. At its full length, the hairworm occupies the grasshopper's entire body cavity, aside from its host's head and legs. Once inside the grasshopper's body, the hairworm releases proteins that affect its host's central nervous system and chemical signals in its brain. Once the hairworm has reached maturity, the changed chemical signals in the grasshopper's brain instruct it to seek out a source of water. Upon arrival, the grasshopper flings its body into the water, where it drowns. At the same time, the hairworm escapes out of the cricket's body and swims off to find a mate and reproduce, beginning the cycle anew.

Implications for Human Health

Some human pathogens—such as those that cause rabies, sleeping sickness, and toxoplasmosis—affect their human host's brain chemistry in a similar way as the parasites affect their hosts as described above. By studying more closely the biochemical changes that occur when parasitic species such as Cordyceps fungi and hairworms infect their hosts, scientists may one day be able to develop medical treatments for humans afflicted by similar central nervous-system altering infections.

More to Explore

• Photos of Insects Invaded by Parasitic Fungi
• Brainwashed by a Parasite
• Parasitic Mind Control [video]
• Suicide Grasshoppers Brainwashed by Parasitic Worms
• A Manipulative Parasite [video]
• Parasite Makes Ants Resemble Berries

Last updated August 7, 2009 | Comments (31) | Permalink | Strange Biology