Random Thoughts

Parasite Influence on Host Behavior - Part 1

When I read an article in the science section of the NYTimes about this, I did some research and reviewed Richard Dawkins' book 'The Extended Phenotype" and below is a list of what I compiled. According to the theory of selfish genes, the parasite genes are likely to modify the behavior of the host to ensure the survival and propagation of the parasite genes even at the cost of destruction of the same of the host. 

Toxoplasma is a single celled parasite that reproduces sexually in the gut of a cat and the resultant cysts are excreted in cat feces. Rats or other warm blooded mammals foraging in garbage and trash ingest the cysts and toxoplasma proliferates asexually by simple division. To complete its life cycle the dividing parasites must reach the intestines of the cat from the body of the intermediate host.

Cats are by nature hunting predators and don't eat dead animals, so it wouldn't help the parasite to kill the intermediate host. Any warm blooded animal in theory could forage in the trash, but as you may have guessed the most likely ones are rats. Now the parasite's problem is how to get the infected rat into a cat's stomach.

Toxoplasma can and does infect even humans. In animals other than a cat, it divides prolifically and invades many cells including brain cells. In rats, it affects a part of the rhinocortex (smell brain) in such a way that the infected rat loses its inherent fear of the smell of a cat. The infected rat when exposed to cat odor or a living cat, behaves nonchalantly instead of running away, and is killed and consumed by a cat to allow the parasite to complete and restart its life cycle by having sex with another parasite.

The thorny headed worm uses a pill bug as its intermediate host but eventually needs to enter the digestive tract of a bird to restart its life cycle. The parasite infects the dark colored pill bug and grows within it but has time constraints to reach a bird's intestine. Normal pill bugs are shy creatures averse to light and hide under or within some equally dark detritus, tree stump or other suitable shelter during sunlight when many birds are foraging. The parasite worm once again alters the bug's behavior and it cavorts in bright daylight with a preference for light colored background where it stands out conveying a message 'eat me' to any bird in the vicinity.

The saga of the horsehair worm and an infected host like a cricket or grasshopper is equally strange. This parasite must reach water to breed and the above insects are normally averse to water environments. The infected host just dives into a body of water and the parasite bursts out from it like Narsinha from a pillar, while the host dies. Another worm larva of the nematode family infects bees and as an adult lives in water. The infected bee on seeing water takes a kamikaze dive in it and the worm bursts out of it killing the bee.

Parasites manipulate the behavior of intermediate hosts in such a way as to facilitate their being eaten by the parasite's final host. Another example is flukes of the genus Leuchochloridium whose intermediate hosts are snails and final hosts birds. The fluke invades a snail and migrates into the snail's horn below its eye stalk. Normal snails are light averse but infected snails seek light and thus remain exposed. As though this were not enough, the parasite makes the snail's horn transparent and then pulsates like a neon sign advertising to any nearby bird to attack and eat the pulsating tentacle.
Polymorphus paradoxus another worm infects fresh water amphipod Gammarus lacustris (shrimp like). The definitive hosts are ducks. Normal shrimps are again averse to light and avoid the surface of the water. The infected shrimps stay close to the surface and stick to surface plants. The shrimps move under water to feed but instead of taking their food to the bottom and eating it, they rise to the water surface to eat it and are easy prey for ducks in the vicinity.

The brain-worm fluke Dicrocoelium dendriticum passes through snails and ants on its way to a sheep stomach. It burrows into the ant's brain (suboesophageal ganglion) and changes the ant's behavior. The infected ant will retreat from heat which is lethal to the parasite but when it gets cold instead of retiring to its nest, it climbs to the top of a grass stem and clamps its jaw to the stem waiting for a passing sheep to eat it. A single ant is often infected by fifty cercariae (parasites) and only one of them burrows into the ant's brain and dies to ensure the survival of his remaining brothers.

Next week I will give examples of how viruses and bacteria modify plant, animal and human behavior.

By the way do readers think that President Bush should have a medical exam to diagnose if any parasites are causing his self-destructive behavior like commuting Scooter Libby's sentence and thus further sabotaging the US justice system already on its last legs due to the new unjust Supreme Court decisions like denying justice in the sex discrimination case against Goodyear and the strange verdict against the Kentucky and Seattle school districts trying to remedy segregated schools. I don't think so, I think his brain malady is developmental and not parasitogenic.


More by :  Gaurang Bhatt, MD

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