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Parasitic Wasp Reprograms Its Host Spider
Dan writes: "The New York Times has an article about a bizarre Costa Rican spider parasite. This tiny wasp larva forces its host, an orb spider, to do its bidding before killing it. Instead of building a normal round web, the spider spends its last night stringing together a frame. The larva then kills the spider and uses the frame to support its cocoon. The scientist who discovered the behavior still doesn't know how the parasite does it." Since this is an older article, there's probably some more recent information available about this critter.
Embrace and extend.
Some time ago, I saw an interesting documentary on TV about snail parasites (IIRC, the parasite was some kind of worm). The snail gets infected by eating the parasite's eggs which stick to edible plants. The eggs hatch inside the snail, and the parasitic larvae move to the snail's antennae where they start to grow. When they are mature, they somehow modify their hosts' behaviour - normally, the snail would hide from predators during the day. But now it exposes itself to birds, which mistake the swollen, bloated, parasite-containing snail antennae for yummy insects, rip them off and eat them. The snail doesn't survive this for long...
Inside the bird, the parasite lays its eggs and dies. The eggs get spread with the bird droppings, which hopefully fall on snail-edible plants, etc...
Cthulhu fhtagn!
One interesting example of host behavioral modification is in the single-celled protozoan Toxoplasma gondii.
This organism has two different hosts in it's life cycle, cats and rats. An infected cat will shed parasites in its feces which are then picked up by rats. The parasites take up residence in the rat until it is eaten by a cat, completing the life cycle.
The parasite takes up residence in various tissues of the rat, including the brain. Interestingly enough, infected rats show behavioral modifications. They become less cautious and more "curious", and may lose their normal aversion to the scent of cat urine -- thus making them easier prey.
Toxoplasma gondii also frequently infects humans, with some estimates suggesting up to half of the population having been exposed. It is dangerous to human fetuses and individuals with deficient immune systems (such as those with AIDS), but healthly carriers are usually asymptomatic.
It is uncertain whether or not the organism produces behavioral changes in humans, but there have been some suggestions that it might. Toxoplasma gondii link.